| .. XXX: reference/datamodel and this have quite a few overlaps! |
| |
| |
| .. _bltin-types: |
| |
| ************** |
| Built-in Types |
| ************** |
| |
| The following sections describe the standard types that are built into the |
| interpreter. |
| |
| .. index:: pair: built-in; types |
| |
| The principal built-in types are numerics, sequences, mappings, classes, |
| instances and exceptions. |
| |
| Some collection classes are mutable. The methods that add, subtract, or |
| rearrange their members in place, and don't return a specific item, never return |
| the collection instance itself but ``None``. |
| |
| Some operations are supported by several object types; in particular, |
| practically all objects can be compared, tested for truth value, and converted |
| to a string (with the :func:`repr` function or the slightly different |
| :func:`str` function). The latter function is implicitly used when an object is |
| written by the :func:`print` function. |
| |
| |
| .. _truth: |
| |
| Truth Value Testing |
| =================== |
| |
| .. index:: |
| statement: if |
| statement: while |
| pair: truth; value |
| pair: Boolean; operations |
| single: false |
| |
| Any object can be tested for truth value, for use in an :keyword:`if` or |
| :keyword:`while` condition or as operand of the Boolean operations below. The |
| following values are considered false: |
| |
| .. index:: single: None (Built-in object) |
| |
| * ``None`` |
| |
| .. index:: single: False (Built-in object) |
| |
| * ``False`` |
| |
| * zero of any numeric type, for example, ``0``, ``0.0``, ``0j``. |
| |
| * any empty sequence, for example, ``''``, ``()``, ``[]``. |
| |
| * any empty mapping, for example, ``{}``. |
| |
| * instances of user-defined classes, if the class defines a :meth:`__bool__` or |
| :meth:`__len__` method, when that method returns the integer zero or |
| :class:`bool` value ``False``. [1]_ |
| |
| .. index:: single: true |
| |
| All other values are considered true --- so objects of many types are always |
| true. |
| |
| .. index:: |
| operator: or |
| operator: and |
| single: False |
| single: True |
| |
| Operations and built-in functions that have a Boolean result always return ``0`` |
| or ``False`` for false and ``1`` or ``True`` for true, unless otherwise stated. |
| (Important exception: the Boolean operations ``or`` and ``and`` always return |
| one of their operands.) |
| |
| |
| .. _boolean: |
| |
| Boolean Operations --- :keyword:`and`, :keyword:`or`, :keyword:`not` |
| ==================================================================== |
| |
| .. index:: pair: Boolean; operations |
| |
| These are the Boolean operations, ordered by ascending priority: |
| |
| +-------------+---------------------------------+-------+ |
| | Operation | Result | Notes | |
| +=============+=================================+=======+ |
| | ``x or y`` | if *x* is false, then *y*, else | \(1) | |
| | | *x* | | |
| +-------------+---------------------------------+-------+ |
| | ``x and y`` | if *x* is false, then *x*, else | \(2) | |
| | | *y* | | |
| +-------------+---------------------------------+-------+ |
| | ``not x`` | if *x* is false, then ``True``, | \(3) | |
| | | else ``False`` | | |
| +-------------+---------------------------------+-------+ |
| |
| .. index:: |
| operator: and |
| operator: or |
| operator: not |
| |
| Notes: |
| |
| (1) |
| This is a short-circuit operator, so it only evaluates the second |
| argument if the first one is :const:`False`. |
| |
| (2) |
| This is a short-circuit operator, so it only evaluates the second |
| argument if the first one is :const:`True`. |
| |
| (3) |
| ``not`` has a lower priority than non-Boolean operators, so ``not a == b`` is |
| interpreted as ``not (a == b)``, and ``a == not b`` is a syntax error. |
| |
| |
| .. _stdcomparisons: |
| |
| Comparisons |
| =========== |
| |
| .. index:: |
| pair: chaining; comparisons |
| pair: operator; comparison |
| operator: == |
| operator: < |
| operator: <= |
| operator: > |
| operator: >= |
| operator: != |
| operator: is |
| operator: is not |
| |
| There are eight comparison operations in Python. They all have the same |
| priority (which is higher than that of the Boolean operations). Comparisons can |
| be chained arbitrarily; for example, ``x < y <= z`` is equivalent to ``x < y and |
| y <= z``, except that *y* is evaluated only once (but in both cases *z* is not |
| evaluated at all when ``x < y`` is found to be false). |
| |
| This table summarizes the comparison operations: |
| |
| +------------+-------------------------+ |
| | Operation | Meaning | |
| +============+=========================+ |
| | ``<`` | strictly less than | |
| +------------+-------------------------+ |
| | ``<=`` | less than or equal | |
| +------------+-------------------------+ |
| | ``>`` | strictly greater than | |
| +------------+-------------------------+ |
| | ``>=`` | greater than or equal | |
| +------------+-------------------------+ |
| | ``==`` | equal | |
| +------------+-------------------------+ |
| | ``!=`` | not equal | |
| +------------+-------------------------+ |
| | ``is`` | object identity | |
| +------------+-------------------------+ |
| | ``is not`` | negated object identity | |
| +------------+-------------------------+ |
| |
| .. index:: |
| pair: object; numeric |
| pair: objects; comparing |
| |
| Objects of different types, except different numeric types, never compare equal. |
| Furthermore, some types (for example, function objects) support only a degenerate |
| notion of comparison where any two objects of that type are unequal. The ``<``, |
| ``<=``, ``>`` and ``>=`` operators will raise a :exc:`TypeError` exception when |
| comparing a complex number with another built-in numeric type, when the objects |
| are of different types that cannot be compared, or in other cases where there is |
| no defined ordering. |
| |
| .. index:: |
| single: __eq__() (instance method) |
| single: __ne__() (instance method) |
| single: __lt__() (instance method) |
| single: __le__() (instance method) |
| single: __gt__() (instance method) |
| single: __ge__() (instance method) |
| |
| Non-identical instances of a class normally compare as non-equal unless the |
| class defines the :meth:`__eq__` method. |
| |
| Instances of a class cannot be ordered with respect to other instances of the |
| same class, or other types of object, unless the class defines enough of the |
| methods :meth:`__lt__`, :meth:`__le__`, :meth:`__gt__`, and :meth:`__ge__` (in |
| general, :meth:`__lt__` and :meth:`__eq__` are sufficient, if you want the |
| conventional meanings of the comparison operators). |
| |
| The behavior of the :keyword:`is` and :keyword:`is not` operators cannot be |
| customized; also they can be applied to any two objects and never raise an |
| exception. |
| |
| .. index:: |
| operator: in |
| operator: not in |
| |
| Two more operations with the same syntactic priority, :keyword:`in` and |
| :keyword:`not in`, are supported only by sequence types (below). |
| |
| |
| .. _typesnumeric: |
| |
| Numeric Types --- :class:`int`, :class:`float`, :class:`complex` |
| ================================================================ |
| |
| .. index:: |
| object: numeric |
| object: Boolean |
| object: integer |
| object: floating point |
| object: complex number |
| pair: C; language |
| |
| There are three distinct numeric types: :dfn:`integers`, :dfn:`floating |
| point numbers`, and :dfn:`complex numbers`. In addition, Booleans are a |
| subtype of integers. Integers have unlimited precision. Floating point |
| numbers are usually implemented using :c:type:`double` in C; information |
| about the precision and internal representation of floating point |
| numbers for the machine on which your program is running is available |
| in :data:`sys.float_info`. Complex numbers have a real and imaginary |
| part, which are each a floating point number. To extract these parts |
| from a complex number *z*, use ``z.real`` and ``z.imag``. (The standard |
| library includes additional numeric types, :mod:`fractions` that hold |
| rationals, and :mod:`decimal` that hold floating-point numbers with |
| user-definable precision.) |
| |
| .. index:: |
| pair: numeric; literals |
| pair: integer; literals |
| pair: floating point; literals |
| pair: complex number; literals |
| pair: hexadecimal; literals |
| pair: octal; literals |
| pair: binary; literals |
| |
| Numbers are created by numeric literals or as the result of built-in functions |
| and operators. Unadorned integer literals (including hex, octal and binary |
| numbers) yield integers. Numeric literals containing a decimal point or an |
| exponent sign yield floating point numbers. Appending ``'j'`` or ``'J'`` to a |
| numeric literal yields an imaginary number (a complex number with a zero real |
| part) which you can add to an integer or float to get a complex number with real |
| and imaginary parts. |
| |
| .. index:: |
| single: arithmetic |
| builtin: int |
| builtin: float |
| builtin: complex |
| operator: + |
| operator: - |
| operator: * |
| operator: / |
| operator: // |
| operator: % |
| operator: ** |
| |
| Python fully supports mixed arithmetic: when a binary arithmetic operator has |
| operands of different numeric types, the operand with the "narrower" type is |
| widened to that of the other, where integer is narrower than floating point, |
| which is narrower than complex. Comparisons between numbers of mixed type use |
| the same rule. [2]_ The constructors :func:`int`, :func:`float`, and |
| :func:`complex` can be used to produce numbers of a specific type. |
| |
| All numeric types (except complex) support the following operations, sorted by |
| ascending priority (operations in the same box have the same priority; all |
| numeric operations have a higher priority than comparison operations): |
| |
| +---------------------+---------------------------------+---------+--------------------+ |
| | Operation | Result | Notes | Full documentation | |
| +=====================+=================================+=========+====================+ |
| | ``x + y`` | sum of *x* and *y* | | | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``x - y`` | difference of *x* and *y* | | | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``x * y`` | product of *x* and *y* | | | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``x / y`` | quotient of *x* and *y* | | | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``x // y`` | floored quotient of *x* and | \(1) | | |
| | | *y* | | | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``x % y`` | remainder of ``x / y`` | \(2) | | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``-x`` | *x* negated | | | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``+x`` | *x* unchanged | | | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``abs(x)`` | absolute value or magnitude of | | :func:`abs` | |
| | | *x* | | | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``int(x)`` | *x* converted to integer | \(3)\(6)| :func:`int` | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``float(x)`` | *x* converted to floating point | \(4)\(6)| :func:`float` | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``complex(re, im)`` | a complex number with real part | \(6) | :func:`complex` | |
| | | *re*, imaginary part *im*. | | | |
| | | *im* defaults to zero. | | | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``c.conjugate()`` | conjugate of the complex number | | | |
| | | *c* | | | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``divmod(x, y)`` | the pair ``(x // y, x % y)`` | \(2) | :func:`divmod` | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``pow(x, y)`` | *x* to the power *y* | \(5) | :func:`pow` | |
| +---------------------+---------------------------------+---------+--------------------+ |
| | ``x ** y`` | *x* to the power *y* | \(5) | | |
| +---------------------+---------------------------------+---------+--------------------+ |
| |
| .. index:: |
| triple: operations on; numeric; types |
| single: conjugate() (complex number method) |
| |
| Notes: |
| |
| (1) |
| Also referred to as integer division. The resultant value is a whole |
| integer, though the result's type is not necessarily int. The result is |
| always rounded towards minus infinity: ``1//2`` is ``0``, ``(-1)//2`` is |
| ``-1``, ``1//(-2)`` is ``-1``, and ``(-1)//(-2)`` is ``0``. |
| |
| (2) |
| Not for complex numbers. Instead convert to floats using :func:`abs` if |
| appropriate. |
| |
| (3) |
| .. index:: |
| module: math |
| single: floor() (in module math) |
| single: ceil() (in module math) |
| single: trunc() (in module math) |
| pair: numeric; conversions |
| pair: C; language |
| |
| Conversion from floating point to integer may round or truncate |
| as in C; see functions :func:`floor` and :func:`ceil` in the :mod:`math` module |
| for well-defined conversions. |
| |
| (4) |
| float also accepts the strings "nan" and "inf" with an optional prefix "+" |
| or "-" for Not a Number (NaN) and positive or negative infinity. |
| |
| (5) |
| Python defines ``pow(0, 0)`` and ``0 ** 0`` to be ``1``, as is common for |
| programming languages. |
| |
| (6) |
| The numeric literals accepted include the digits ``0`` to ``9`` or any |
| Unicode equivalent (code points with the ``Nd`` property). |
| |
| See http://www.unicode.org/Public/6.0.0/ucd/extracted/DerivedNumericType.txt |
| for a complete list of code points with the ``Nd`` property. |
| |
| |
| All :class:`numbers.Real` types (:class:`int` and :class:`float`) also include |
| the following operations: |
| |
| +--------------------+------------------------------------+--------+ |
| | Operation | Result | Notes | |
| +====================+====================================+========+ |
| | ``math.trunc(x)`` | *x* truncated to Integral | | |
| +--------------------+------------------------------------+--------+ |
| | ``round(x[, n])`` | *x* rounded to n digits, | | |
| | | rounding half to even. If n is | | |
| | | omitted, it defaults to 0. | | |
| +--------------------+------------------------------------+--------+ |
| | ``math.floor(x)`` | the greatest integral float <= *x* | | |
| +--------------------+------------------------------------+--------+ |
| | ``math.ceil(x)`` | the least integral float >= *x* | | |
| +--------------------+------------------------------------+--------+ |
| |
| For additional numeric operations see the :mod:`math` and :mod:`cmath` |
| modules. |
| |
| .. XXXJH exceptions: overflow (when? what operations?) zerodivision |
| |
| |
| .. _bitstring-ops: |
| |
| Bitwise Operations on Integer Types |
| -------------------------------------- |
| |
| .. index:: |
| triple: operations on; integer; types |
| pair: bitwise; operations |
| pair: shifting; operations |
| pair: masking; operations |
| operator: ^ |
| operator: & |
| operator: << |
| operator: >> |
| |
| Bitwise operations only make sense for integers. Negative numbers are treated |
| as their 2's complement value (this assumes a sufficiently large number of bits |
| that no overflow occurs during the operation). |
| |
| The priorities of the binary bitwise operations are all lower than the numeric |
| operations and higher than the comparisons; the unary operation ``~`` has the |
| same priority as the other unary numeric operations (``+`` and ``-``). |
| |
| This table lists the bitwise operations sorted in ascending priority |
| (operations in the same box have the same priority): |
| |
| +------------+--------------------------------+----------+ |
| | Operation | Result | Notes | |
| +============+================================+==========+ |
| | ``x | y`` | bitwise :dfn:`or` of *x* and | | |
| | | *y* | | |
| +------------+--------------------------------+----------+ |
| | ``x ^ y`` | bitwise :dfn:`exclusive or` of | | |
| | | *x* and *y* | | |
| +------------+--------------------------------+----------+ |
| | ``x & y`` | bitwise :dfn:`and` of *x* and | | |
| | | *y* | | |
| +------------+--------------------------------+----------+ |
| | ``x << n`` | *x* shifted left by *n* bits | (1)(2) | |
| +------------+--------------------------------+----------+ |
| | ``x >> n`` | *x* shifted right by *n* bits | (1)(3) | |
| +------------+--------------------------------+----------+ |
| | ``~x`` | the bits of *x* inverted | | |
| +------------+--------------------------------+----------+ |
| |
| Notes: |
| |
| (1) |
| Negative shift counts are illegal and cause a :exc:`ValueError` to be raised. |
| |
| (2) |
| A left shift by *n* bits is equivalent to multiplication by ``pow(2, n)`` |
| without overflow check. |
| |
| (3) |
| A right shift by *n* bits is equivalent to division by ``pow(2, n)`` without |
| overflow check. |
| |
| |
| Additional Methods on Integer Types |
| ----------------------------------- |
| |
| The int type implements the :class:`numbers.Integral` :term:`abstract base |
| class`. In addition, it provides one more method: |
| |
| .. method:: int.bit_length() |
| |
| Return the number of bits necessary to represent an integer in binary, |
| excluding the sign and leading zeros:: |
| |
| >>> n = -37 |
| >>> bin(n) |
| '-0b100101' |
| >>> n.bit_length() |
| 6 |
| |
| More precisely, if ``x`` is nonzero, then ``x.bit_length()`` is the |
| unique positive integer ``k`` such that ``2**(k-1) <= abs(x) < 2**k``. |
| Equivalently, when ``abs(x)`` is small enough to have a correctly |
| rounded logarithm, then ``k = 1 + int(log(abs(x), 2))``. |
| If ``x`` is zero, then ``x.bit_length()`` returns ``0``. |
| |
| Equivalent to:: |
| |
| def bit_length(self): |
| s = bin(self) # binary representation: bin(-37) --> '-0b100101' |
| s = s.lstrip('-0b') # remove leading zeros and minus sign |
| return len(s) # len('100101') --> 6 |
| |
| .. versionadded:: 3.1 |
| |
| .. method:: int.to_bytes(length, byteorder, \*, signed=False) |
| |
| Return an array of bytes representing an integer. |
| |
| >>> (1024).to_bytes(2, byteorder='big') |
| b'\x04\x00' |
| >>> (1024).to_bytes(10, byteorder='big') |
| b'\x00\x00\x00\x00\x00\x00\x00\x00\x04\x00' |
| >>> (-1024).to_bytes(10, byteorder='big', signed=True) |
| b'\xff\xff\xff\xff\xff\xff\xff\xff\xfc\x00' |
| >>> x = 1000 |
| >>> x.to_bytes((x.bit_length() // 8) + 1, byteorder='little') |
| b'\xe8\x03' |
| |
| The integer is represented using *length* bytes. An :exc:`OverflowError` |
| is raised if the integer is not representable with the given number of |
| bytes. |
| |
| The *byteorder* argument determines the byte order used to represent the |
| integer. If *byteorder* is ``"big"``, the most significant byte is at the |
| beginning of the byte array. If *byteorder* is ``"little"``, the most |
| significant byte is at the end of the byte array. To request the native |
| byte order of the host system, use :data:`sys.byteorder` as the byte order |
| value. |
| |
| The *signed* argument determines whether two's complement is used to |
| represent the integer. If *signed* is ``False`` and a negative integer is |
| given, an :exc:`OverflowError` is raised. The default value for *signed* |
| is ``False``. |
| |
| .. versionadded:: 3.2 |
| |
| .. classmethod:: int.from_bytes(bytes, byteorder, \*, signed=False) |
| |
| Return the integer represented by the given array of bytes. |
| |
| >>> int.from_bytes(b'\x00\x10', byteorder='big') |
| 16 |
| >>> int.from_bytes(b'\x00\x10', byteorder='little') |
| 4096 |
| >>> int.from_bytes(b'\xfc\x00', byteorder='big', signed=True) |
| -1024 |
| >>> int.from_bytes(b'\xfc\x00', byteorder='big', signed=False) |
| 64512 |
| >>> int.from_bytes([255, 0, 0], byteorder='big') |
| 16711680 |
| |
| The argument *bytes* must either support the buffer protocol or be an |
| iterable producing bytes. :class:`bytes` and :class:`bytearray` are |
| examples of built-in objects that support the buffer protocol. |
| |
| The *byteorder* argument determines the byte order used to represent the |
| integer. If *byteorder* is ``"big"``, the most significant byte is at the |
| beginning of the byte array. If *byteorder* is ``"little"``, the most |
| significant byte is at the end of the byte array. To request the native |
| byte order of the host system, use :data:`sys.byteorder` as the byte order |
| value. |
| |
| The *signed* argument indicates whether two's complement is used to |
| represent the integer. |
| |
| .. versionadded:: 3.2 |
| |
| |
| Additional Methods on Float |
| --------------------------- |
| |
| The float type implements the :class:`numbers.Real` :term:`abstract base |
| class`. float also has the following additional methods. |
| |
| .. method:: float.as_integer_ratio() |
| |
| Return a pair of integers whose ratio is exactly equal to the |
| original float and with a positive denominator. Raises |
| :exc:`OverflowError` on infinities and a :exc:`ValueError` on |
| NaNs. |
| |
| .. method:: float.is_integer() |
| |
| Return ``True`` if the float instance is finite with integral |
| value, and ``False`` otherwise:: |
| |
| >>> (-2.0).is_integer() |
| True |
| >>> (3.2).is_integer() |
| False |
| |
| Two methods support conversion to |
| and from hexadecimal strings. Since Python's floats are stored |
| internally as binary numbers, converting a float to or from a |
| *decimal* string usually involves a small rounding error. In |
| contrast, hexadecimal strings allow exact representation and |
| specification of floating-point numbers. This can be useful when |
| debugging, and in numerical work. |
| |
| |
| .. method:: float.hex() |
| |
| Return a representation of a floating-point number as a hexadecimal |
| string. For finite floating-point numbers, this representation |
| will always include a leading ``0x`` and a trailing ``p`` and |
| exponent. |
| |
| |
| .. classmethod:: float.fromhex(s) |
| |
| Class method to return the float represented by a hexadecimal |
| string *s*. The string *s* may have leading and trailing |
| whitespace. |
| |
| |
| Note that :meth:`float.hex` is an instance method, while |
| :meth:`float.fromhex` is a class method. |
| |
| A hexadecimal string takes the form:: |
| |
| [sign] ['0x'] integer ['.' fraction] ['p' exponent] |
| |
| where the optional ``sign`` may by either ``+`` or ``-``, ``integer`` |
| and ``fraction`` are strings of hexadecimal digits, and ``exponent`` |
| is a decimal integer with an optional leading sign. Case is not |
| significant, and there must be at least one hexadecimal digit in |
| either the integer or the fraction. This syntax is similar to the |
| syntax specified in section 6.4.4.2 of the C99 standard, and also to |
| the syntax used in Java 1.5 onwards. In particular, the output of |
| :meth:`float.hex` is usable as a hexadecimal floating-point literal in |
| C or Java code, and hexadecimal strings produced by C's ``%a`` format |
| character or Java's ``Double.toHexString`` are accepted by |
| :meth:`float.fromhex`. |
| |
| |
| Note that the exponent is written in decimal rather than hexadecimal, |
| and that it gives the power of 2 by which to multiply the coefficient. |
| For example, the hexadecimal string ``0x3.a7p10`` represents the |
| floating-point number ``(3 + 10./16 + 7./16**2) * 2.0**10``, or |
| ``3740.0``:: |
| |
| >>> float.fromhex('0x3.a7p10') |
| 3740.0 |
| |
| |
| Applying the reverse conversion to ``3740.0`` gives a different |
| hexadecimal string representing the same number:: |
| |
| >>> float.hex(3740.0) |
| '0x1.d380000000000p+11' |
| |
| |
| .. _numeric-hash: |
| |
| Hashing of numeric types |
| ------------------------ |
| |
| For numbers ``x`` and ``y``, possibly of different types, it's a requirement |
| that ``hash(x) == hash(y)`` whenever ``x == y`` (see the :meth:`__hash__` |
| method documentation for more details). For ease of implementation and |
| efficiency across a variety of numeric types (including :class:`int`, |
| :class:`float`, :class:`decimal.Decimal` and :class:`fractions.Fraction`) |
| Python's hash for numeric types is based on a single mathematical function |
| that's defined for any rational number, and hence applies to all instances of |
| :class:`int` and :class:`fraction.Fraction`, and all finite instances of |
| :class:`float` and :class:`decimal.Decimal`. Essentially, this function is |
| given by reduction modulo ``P`` for a fixed prime ``P``. The value of ``P`` is |
| made available to Python as the :attr:`modulus` attribute of |
| :data:`sys.hash_info`. |
| |
| .. impl-detail:: |
| |
| Currently, the prime used is ``P = 2**31 - 1`` on machines with 32-bit C |
| longs and ``P = 2**61 - 1`` on machines with 64-bit C longs. |
| |
| Here are the rules in detail: |
| |
| - If ``x = m / n`` is a nonnegative rational number and ``n`` is not divisible |
| by ``P``, define ``hash(x)`` as ``m * invmod(n, P) % P``, where ``invmod(n, |
| P)`` gives the inverse of ``n`` modulo ``P``. |
| |
| - If ``x = m / n`` is a nonnegative rational number and ``n`` is |
| divisible by ``P`` (but ``m`` is not) then ``n`` has no inverse |
| modulo ``P`` and the rule above doesn't apply; in this case define |
| ``hash(x)`` to be the constant value ``sys.hash_info.inf``. |
| |
| - If ``x = m / n`` is a negative rational number define ``hash(x)`` |
| as ``-hash(-x)``. If the resulting hash is ``-1``, replace it with |
| ``-2``. |
| |
| - The particular values ``sys.hash_info.inf``, ``-sys.hash_info.inf`` |
| and ``sys.hash_info.nan`` are used as hash values for positive |
| infinity, negative infinity, or nans (respectively). (All hashable |
| nans have the same hash value.) |
| |
| - For a :class:`complex` number ``z``, the hash values of the real |
| and imaginary parts are combined by computing ``hash(z.real) + |
| sys.hash_info.imag * hash(z.imag)``, reduced modulo |
| ``2**sys.hash_info.width`` so that it lies in |
| ``range(-2**(sys.hash_info.width - 1), 2**(sys.hash_info.width - |
| 1))``. Again, if the result is ``-1``, it's replaced with ``-2``. |
| |
| |
| To clarify the above rules, here's some example Python code, |
| equivalent to the builtin hash, for computing the hash of a rational |
| number, :class:`float`, or :class:`complex`:: |
| |
| |
| import sys, math |
| |
| def hash_fraction(m, n): |
| """Compute the hash of a rational number m / n. |
| |
| Assumes m and n are integers, with n positive. |
| Equivalent to hash(fractions.Fraction(m, n)). |
| |
| """ |
| P = sys.hash_info.modulus |
| # Remove common factors of P. (Unnecessary if m and n already coprime.) |
| while m % P == n % P == 0: |
| m, n = m // P, n // P |
| |
| if n % P == 0: |
| hash_ = sys.hash_info.inf |
| else: |
| # Fermat's Little Theorem: pow(n, P-1, P) is 1, so |
| # pow(n, P-2, P) gives the inverse of n modulo P. |
| hash_ = (abs(m) % P) * pow(n, P - 2, P) % P |
| if m < 0: |
| hash_ = -hash_ |
| if hash_ == -1: |
| hash_ = -2 |
| return hash_ |
| |
| def hash_float(x): |
| """Compute the hash of a float x.""" |
| |
| if math.isnan(x): |
| return sys.hash_info.nan |
| elif math.isinf(x): |
| return sys.hash_info.inf if x > 0 else -sys.hash_info.inf |
| else: |
| return hash_fraction(*x.as_integer_ratio()) |
| |
| def hash_complex(z): |
| """Compute the hash of a complex number z.""" |
| |
| hash_ = hash_float(z.real) + sys.hash_info.imag * hash_float(z.imag) |
| # do a signed reduction modulo 2**sys.hash_info.width |
| M = 2**(sys.hash_info.width - 1) |
| hash_ = (hash_ & (M - 1)) - (hash & M) |
| if hash_ == -1: |
| hash_ == -2 |
| return hash_ |
| |
| .. _typeiter: |
| |
| Iterator Types |
| ============== |
| |
| .. index:: |
| single: iterator protocol |
| single: protocol; iterator |
| single: sequence; iteration |
| single: container; iteration over |
| |
| Python supports a concept of iteration over containers. This is implemented |
| using two distinct methods; these are used to allow user-defined classes to |
| support iteration. Sequences, described below in more detail, always support |
| the iteration methods. |
| |
| One method needs to be defined for container objects to provide iteration |
| support: |
| |
| .. XXX duplicated in reference/datamodel! |
| |
| .. method:: container.__iter__() |
| |
| Return an iterator object. The object is required to support the iterator |
| protocol described below. If a container supports different types of |
| iteration, additional methods can be provided to specifically request |
| iterators for those iteration types. (An example of an object supporting |
| multiple forms of iteration would be a tree structure which supports both |
| breadth-first and depth-first traversal.) This method corresponds to the |
| :attr:`tp_iter` slot of the type structure for Python objects in the Python/C |
| API. |
| |
| The iterator objects themselves are required to support the following two |
| methods, which together form the :dfn:`iterator protocol`: |
| |
| |
| .. method:: iterator.__iter__() |
| |
| Return the iterator object itself. This is required to allow both containers |
| and iterators to be used with the :keyword:`for` and :keyword:`in` statements. |
| This method corresponds to the :attr:`tp_iter` slot of the type structure for |
| Python objects in the Python/C API. |
| |
| |
| .. method:: iterator.__next__() |
| |
| Return the next item from the container. If there are no further items, raise |
| the :exc:`StopIteration` exception. This method corresponds to the |
| :attr:`tp_iternext` slot of the type structure for Python objects in the |
| Python/C API. |
| |
| Python defines several iterator objects to support iteration over general and |
| specific sequence types, dictionaries, and other more specialized forms. The |
| specific types are not important beyond their implementation of the iterator |
| protocol. |
| |
| Once an iterator's :meth:`__next__` method raises :exc:`StopIteration`, it must |
| continue to do so on subsequent calls. Implementations that do not obey this |
| property are deemed broken. |
| |
| |
| .. _generator-types: |
| |
| Generator Types |
| --------------- |
| |
| Python's :term:`generator`\s provide a convenient way to implement the iterator |
| protocol. If a container object's :meth:`__iter__` method is implemented as a |
| generator, it will automatically return an iterator object (technically, a |
| generator object) supplying the :meth:`__iter__` and :meth:`__next__` methods. |
| More information about generators can be found in :ref:`the documentation for |
| the yield expression <yieldexpr>`. |
| |
| |
| .. _typesseq: |
| |
| Sequence Types --- :class:`str`, :class:`bytes`, :class:`bytearray`, :class:`list`, :class:`tuple`, :class:`range` |
| ================================================================================================================== |
| |
| There are six sequence types: strings, byte sequences (:class:`bytes` objects), |
| byte arrays (:class:`bytearray` objects), lists, tuples, and range objects. For |
| other containers see the built in :class:`dict` and :class:`set` classes, and |
| the :mod:`collections` module. |
| |
| |
| .. index:: |
| object: sequence |
| object: string |
| object: bytes |
| object: bytearray |
| object: tuple |
| object: list |
| object: range |
| |
| Strings contain Unicode characters. Their literals are written in single or |
| double quotes: ``'xyzzy'``, ``"frobozz"``. See :ref:`strings` for more about |
| string literals. In addition to the functionality described here, there are |
| also string-specific methods described in the :ref:`string-methods` section. |
| |
| Bytes and bytearray objects contain single bytes -- the former is immutable |
| while the latter is a mutable sequence. Bytes objects can be constructed the |
| constructor, :func:`bytes`, and from literals; use a ``b`` prefix with normal |
| string syntax: ``b'xyzzy'``. To construct byte arrays, use the |
| :func:`bytearray` function. |
| |
| While string objects are sequences of characters (represented by strings of |
| length 1), bytes and bytearray objects are sequences of *integers* (between 0 |
| and 255), representing the ASCII value of single bytes. That means that for |
| a bytes or bytearray object *b*, ``b[0]`` will be an integer, while |
| ``b[0:1]`` will be a bytes or bytearray object of length 1. The |
| representation of bytes objects uses the literal format (``b'...'``) since it |
| is generally more useful than e.g. ``bytes([50, 19, 100])``. You can always |
| convert a bytes object into a list of integers using ``list(b)``. |
| |
| Also, while in previous Python versions, byte strings and Unicode strings |
| could be exchanged for each other rather freely (barring encoding issues), |
| strings and bytes are now completely separate concepts. There's no implicit |
| en-/decoding if you pass an object of the wrong type. A string always |
| compares unequal to a bytes or bytearray object. |
| |
| Lists are constructed with square brackets, separating items with commas: ``[a, |
| b, c]``. Tuples are constructed by the comma operator (not within square |
| brackets), with or without enclosing parentheses, but an empty tuple must have |
| the enclosing parentheses, such as ``a, b, c`` or ``()``. A single item tuple |
| must have a trailing comma, such as ``(d,)``. |
| |
| Objects of type range are created using the :func:`range` function. They don't |
| support concatenation or repetition, and using :func:`min` or :func:`max` on |
| them is inefficient. |
| |
| Most sequence types support the following operations. The ``in`` and ``not in`` |
| operations have the same priorities as the comparison operations. The ``+`` and |
| ``*`` operations have the same priority as the corresponding numeric operations. |
| [3]_ Additional methods are provided for :ref:`typesseq-mutable`. |
| |
| This table lists the sequence operations sorted in ascending priority |
| (operations in the same box have the same priority). In the table, *s* and *t* |
| are sequences of the same type; *n*, *i*, *j* and *k* are integers. |
| |
| +------------------+--------------------------------+----------+ |
| | Operation | Result | Notes | |
| +==================+================================+==========+ |
| | ``x in s`` | ``True`` if an item of *s* is | \(1) | |
| | | equal to *x*, else ``False`` | | |
| +------------------+--------------------------------+----------+ |
| | ``x not in s`` | ``False`` if an item of *s* is | \(1) | |
| | | equal to *x*, else ``True`` | | |
| +------------------+--------------------------------+----------+ |
| | ``s + t`` | the concatenation of *s* and | \(6) | |
| | | *t* | | |
| +------------------+--------------------------------+----------+ |
| | ``s * n, n * s`` | *n* shallow copies of *s* | \(2) | |
| | | concatenated | | |
| +------------------+--------------------------------+----------+ |
| | ``s[i]`` | *i*\ th item of *s*, origin 0 | \(3) | |
| +------------------+--------------------------------+----------+ |
| | ``s[i:j]`` | slice of *s* from *i* to *j* | (3)(4) | |
| +------------------+--------------------------------+----------+ |
| | ``s[i:j:k]`` | slice of *s* from *i* to *j* | (3)(5) | |
| | | with step *k* | | |
| +------------------+--------------------------------+----------+ |
| | ``len(s)`` | length of *s* | | |
| +------------------+--------------------------------+----------+ |
| | ``min(s)`` | smallest item of *s* | | |
| +------------------+--------------------------------+----------+ |
| | ``max(s)`` | largest item of *s* | | |
| +------------------+--------------------------------+----------+ |
| | ``s.index(i)`` | index of the first occurence | | |
| | | of *i* in *s* | | |
| +------------------+--------------------------------+----------+ |
| | ``s.count(i)`` | total number of occurences of | | |
| | | *i* in *s* | | |
| +------------------+--------------------------------+----------+ |
| |
| Sequence types also support comparisons. In particular, tuples and lists are |
| compared lexicographically by comparing corresponding elements. This means that |
| to compare equal, every element must compare equal and the two sequences must be |
| of the same type and have the same length. (For full details see |
| :ref:`comparisons` in the language reference.) |
| |
| .. index:: |
| triple: operations on; sequence; types |
| builtin: len |
| builtin: min |
| builtin: max |
| pair: concatenation; operation |
| pair: repetition; operation |
| pair: subscript; operation |
| pair: slice; operation |
| operator: in |
| operator: not in |
| |
| Notes: |
| |
| (1) |
| When *s* is a string object, the ``in`` and ``not in`` operations act like a |
| substring test. |
| |
| (2) |
| Values of *n* less than ``0`` are treated as ``0`` (which yields an empty |
| sequence of the same type as *s*). Note also that the copies are shallow; |
| nested structures are not copied. This often haunts new Python programmers; |
| consider: |
| |
| >>> lists = [[]] * 3 |
| >>> lists |
| [[], [], []] |
| >>> lists[0].append(3) |
| >>> lists |
| [[3], [3], [3]] |
| |
| What has happened is that ``[[]]`` is a one-element list containing an empty |
| list, so all three elements of ``[[]] * 3`` are (pointers to) this single empty |
| list. Modifying any of the elements of ``lists`` modifies this single list. |
| You can create a list of different lists this way: |
| |
| >>> lists = [[] for i in range(3)] |
| >>> lists[0].append(3) |
| >>> lists[1].append(5) |
| >>> lists[2].append(7) |
| >>> lists |
| [[3], [5], [7]] |
| |
| (3) |
| If *i* or *j* is negative, the index is relative to the end of the string: |
| ``len(s) + i`` or ``len(s) + j`` is substituted. But note that ``-0`` is |
| still ``0``. |
| |
| (4) |
| The slice of *s* from *i* to *j* is defined as the sequence of items with index |
| *k* such that ``i <= k < j``. If *i* or *j* is greater than ``len(s)``, use |
| ``len(s)``. If *i* is omitted or ``None``, use ``0``. If *j* is omitted or |
| ``None``, use ``len(s)``. If *i* is greater than or equal to *j*, the slice is |
| empty. |
| |
| (5) |
| The slice of *s* from *i* to *j* with step *k* is defined as the sequence of |
| items with index ``x = i + n*k`` such that ``0 <= n < (j-i)/k``. In other words, |
| the indices are ``i``, ``i+k``, ``i+2*k``, ``i+3*k`` and so on, stopping when |
| *j* is reached (but never including *j*). If *i* or *j* is greater than |
| ``len(s)``, use ``len(s)``. If *i* or *j* are omitted or ``None``, they become |
| "end" values (which end depends on the sign of *k*). Note, *k* cannot be zero. |
| If *k* is ``None``, it is treated like ``1``. |
| |
| (6) |
| Concatenating immutable strings always results in a new object. This means |
| that building up a string by repeated concatenation will have a quadratic |
| runtime cost in the total string length. To get a linear runtime cost, |
| you must switch to one of the alternatives below: |
| |
| * if concatenating :class:`str` objects, you can build a list and use |
| :meth:`str.join` at the end; |
| |
| * if concatenating :class:`bytes` objects, you can similarly use |
| :meth:`bytes.join`, or you can do in-place concatenation with a |
| :class:`bytearray` object. :class:`bytearray` objects are mutable and |
| have an efficient overallocation mechanism. |
| |
| |
| .. _string-methods: |
| |
| String Methods |
| -------------- |
| |
| .. index:: pair: string; methods |
| |
| String objects support the methods listed below. |
| |
| In addition, Python's strings support the sequence type methods described in the |
| :ref:`typesseq` section. To output formatted strings, see the |
| :ref:`string-formatting` section. Also, see the :mod:`re` module for string |
| functions based on regular expressions. |
| |
| .. method:: str.capitalize() |
| |
| Return a copy of the string with its first character capitalized and the |
| rest lowercased. |
| |
| |
| .. method:: str.casefold() |
| |
| Return a casefolded copy of the string. Casefolded strings may be used for |
| caseless matching. |
| |
| Casefolding is similar to lowercasing but more aggressive because it is |
| intended to remove all case distinctions in a string. For example, the German |
| lowercase letter ``'ß'`` is equivalent to ``"ss"``. Since it is already |
| lowercase, :meth:`lower` would do nothing to ``'ß'``; :meth:`casefold` |
| converts it to ``"ss"``. |
| |
| The casefolding algorithm is described in section 3.13 of the Unicode |
| Standard. |
| |
| .. versionadded:: 3.3 |
| |
| |
| .. method:: str.center(width[, fillchar]) |
| |
| Return centered in a string of length *width*. Padding is done using the |
| specified *fillchar* (default is a space). |
| |
| |
| .. method:: str.count(sub[, start[, end]]) |
| |
| Return the number of non-overlapping occurrences of substring *sub* in the |
| range [*start*, *end*]. Optional arguments *start* and *end* are |
| interpreted as in slice notation. |
| |
| |
| .. method:: str.encode(encoding="utf-8", errors="strict") |
| |
| Return an encoded version of the string as a bytes object. Default encoding |
| is ``'utf-8'``. *errors* may be given to set a different error handling scheme. |
| The default for *errors* is ``'strict'``, meaning that encoding errors raise |
| a :exc:`UnicodeError`. Other possible |
| values are ``'ignore'``, ``'replace'``, ``'xmlcharrefreplace'``, |
| ``'backslashreplace'`` and any other name registered via |
| :func:`codecs.register_error`, see section :ref:`codec-base-classes`. For a |
| list of possible encodings, see section :ref:`standard-encodings`. |
| |
| .. versionchanged:: 3.1 |
| Support for keyword arguments added. |
| |
| |
| .. method:: str.endswith(suffix[, start[, end]]) |
| |
| Return ``True`` if the string ends with the specified *suffix*, otherwise return |
| ``False``. *suffix* can also be a tuple of suffixes to look for. With optional |
| *start*, test beginning at that position. With optional *end*, stop comparing |
| at that position. |
| |
| |
| .. method:: str.expandtabs([tabsize]) |
| |
| Return a copy of the string where all tab characters are replaced by zero or |
| more spaces, depending on the current column and the given tab size. The |
| column number is reset to zero after each newline occurring in the string. |
| If *tabsize* is not given, a tab size of ``8`` characters is assumed. This |
| doesn't understand other non-printing characters or escape sequences. |
| |
| |
| .. method:: str.find(sub[, start[, end]]) |
| |
| Return the lowest index in the string where substring *sub* is found, such |
| that *sub* is contained in the slice ``s[start:end]``. Optional arguments |
| *start* and *end* are interpreted as in slice notation. Return ``-1`` if |
| *sub* is not found. |
| |
| .. note:: |
| |
| The :meth:`~str.find` method should be used only if you need to know the |
| position of *sub*. To check if *sub* is a substring or not, use the |
| :keyword:`in` operator:: |
| |
| >>> 'Py' in 'Python' |
| True |
| |
| |
| .. method:: str.format(*args, **kwargs) |
| |
| Perform a string formatting operation. The string on which this method is |
| called can contain literal text or replacement fields delimited by braces |
| ``{}``. Each replacement field contains either the numeric index of a |
| positional argument, or the name of a keyword argument. Returns a copy of |
| the string where each replacement field is replaced with the string value of |
| the corresponding argument. |
| |
| >>> "The sum of 1 + 2 is {0}".format(1+2) |
| 'The sum of 1 + 2 is 3' |
| |
| See :ref:`formatstrings` for a description of the various formatting options |
| that can be specified in format strings. |
| |
| |
| .. method:: str.format_map(mapping) |
| |
| Similar to ``str.format(**mapping)``, except that ``mapping`` is |
| used directly and not copied to a :class:`dict` . This is useful |
| if for example ``mapping`` is a dict subclass: |
| |
| >>> class Default(dict): |
| ... def __missing__(self, key): |
| ... return key |
| ... |
| >>> '{name} was born in {country}'.format_map(Default(name='Guido')) |
| 'Guido was born in country' |
| |
| .. versionadded:: 3.2 |
| |
| |
| .. method:: str.index(sub[, start[, end]]) |
| |
| Like :meth:`find`, but raise :exc:`ValueError` when the substring is not found. |
| |
| |
| .. method:: str.isalnum() |
| |
| Return true if all characters in the string are alphanumeric and there is at |
| least one character, false otherwise. A character ``c`` is alphanumeric if one |
| of the following returns ``True``: ``c.isalpha()``, ``c.isdecimal()``, |
| ``c.isdigit()``, or ``c.isnumeric()``. |
| |
| |
| .. method:: str.isalpha() |
| |
| Return true if all characters in the string are alphabetic and there is at least |
| one character, false otherwise. Alphabetic characters are those characters defined |
| in the Unicode character database as "Letter", i.e., those with general category |
| property being one of "Lm", "Lt", "Lu", "Ll", or "Lo". Note that this is different |
| from the "Alphabetic" property defined in the Unicode Standard. |
| |
| |
| .. method:: str.isdecimal() |
| |
| Return true if all characters in the string are decimal |
| characters and there is at least one character, false |
| otherwise. Decimal characters are those from general category "Nd". This category |
| includes digit characters, and all characters |
| that can be used to form decimal-radix numbers, e.g. U+0660, |
| ARABIC-INDIC DIGIT ZERO. |
| |
| |
| .. method:: str.isdigit() |
| |
| Return true if all characters in the string are digits and there is at least one |
| character, false otherwise. Digits include decimal characters and digits that need |
| special handling, such as the compatibility superscript digits. Formally, a digit |
| is a character that has the property value Numeric_Type=Digit or Numeric_Type=Decimal. |
| |
| |
| .. method:: str.isidentifier() |
| |
| Return true if the string is a valid identifier according to the language |
| definition, section :ref:`identifiers`. |
| |
| |
| .. method:: str.islower() |
| |
| Return true if all cased characters [4]_ in the string are lowercase and |
| there is at least one cased character, false otherwise. |
| |
| |
| .. method:: str.isnumeric() |
| |
| Return true if all characters in the string are numeric |
| characters, and there is at least one character, false |
| otherwise. Numeric characters include digit characters, and all characters |
| that have the Unicode numeric value property, e.g. U+2155, |
| VULGAR FRACTION ONE FIFTH. Formally, numeric characters are those with the property |
| value Numeric_Type=Digit, Numeric_Type=Decimal or Numeric_Type=Numeric. |
| |
| |
| .. method:: str.isprintable() |
| |
| Return true if all characters in the string are printable or the string is |
| empty, false otherwise. Nonprintable characters are those characters defined |
| in the Unicode character database as "Other" or "Separator", excepting the |
| ASCII space (0x20) which is considered printable. (Note that printable |
| characters in this context are those which should not be escaped when |
| :func:`repr` is invoked on a string. It has no bearing on the handling of |
| strings written to :data:`sys.stdout` or :data:`sys.stderr`.) |
| |
| |
| .. method:: str.isspace() |
| |
| Return true if there are only whitespace characters in the string and there is |
| at least one character, false otherwise. Whitespace characters are those |
| characters defined in the Unicode character database as "Other" or "Separator" |
| and those with bidirectional property being one of "WS", "B", or "S". |
| |
| .. method:: str.istitle() |
| |
| Return true if the string is a titlecased string and there is at least one |
| character, for example uppercase characters may only follow uncased characters |
| and lowercase characters only cased ones. Return false otherwise. |
| |
| |
| .. method:: str.isupper() |
| |
| Return true if all cased characters [4]_ in the string are uppercase and |
| there is at least one cased character, false otherwise. |
| |
| |
| .. method:: str.join(iterable) |
| |
| Return a string which is the concatenation of the strings in the |
| :term:`iterable` *iterable*. A :exc:`TypeError` will be raised if there are |
| any non-string values in *iterable*, including :class:`bytes` objects. The |
| separator between elements is the string providing this method. |
| |
| |
| .. method:: str.ljust(width[, fillchar]) |
| |
| Return the string left justified in a string of length *width*. Padding is done |
| using the specified *fillchar* (default is a space). The original string is |
| returned if *width* is less than or equal to ``len(s)``. |
| |
| |
| .. method:: str.lower() |
| |
| Return a copy of the string with all the cased characters [4]_ converted to |
| lowercase. |
| |
| The lowercasing algorithm used is described in section 3.13 of the Unicode |
| Standard. |
| |
| |
| .. method:: str.lstrip([chars]) |
| |
| Return a copy of the string with leading characters removed. The *chars* |
| argument is a string specifying the set of characters to be removed. If omitted |
| or ``None``, the *chars* argument defaults to removing whitespace. The *chars* |
| argument is not a prefix; rather, all combinations of its values are stripped: |
| |
| >>> ' spacious '.lstrip() |
| 'spacious ' |
| >>> 'www.example.com'.lstrip('cmowz.') |
| 'example.com' |
| |
| |
| .. staticmethod:: str.maketrans(x[, y[, z]]) |
| |
| This static method returns a translation table usable for :meth:`str.translate`. |
| |
| If there is only one argument, it must be a dictionary mapping Unicode |
| ordinals (integers) or characters (strings of length 1) to Unicode ordinals, |
| strings (of arbitrary lengths) or None. Character keys will then be |
| converted to ordinals. |
| |
| If there are two arguments, they must be strings of equal length, and in the |
| resulting dictionary, each character in x will be mapped to the character at |
| the same position in y. If there is a third argument, it must be a string, |
| whose characters will be mapped to None in the result. |
| |
| |
| .. method:: str.partition(sep) |
| |
| Split the string at the first occurrence of *sep*, and return a 3-tuple |
| containing the part before the separator, the separator itself, and the part |
| after the separator. If the separator is not found, return a 3-tuple containing |
| the string itself, followed by two empty strings. |
| |
| |
| .. method:: str.replace(old, new[, count]) |
| |
| Return a copy of the string with all occurrences of substring *old* replaced by |
| *new*. If the optional argument *count* is given, only the first *count* |
| occurrences are replaced. |
| |
| |
| .. method:: str.rfind(sub[, start[, end]]) |
| |
| Return the highest index in the string where substring *sub* is found, such |
| that *sub* is contained within ``s[start:end]``. Optional arguments *start* |
| and *end* are interpreted as in slice notation. Return ``-1`` on failure. |
| |
| |
| .. method:: str.rindex(sub[, start[, end]]) |
| |
| Like :meth:`rfind` but raises :exc:`ValueError` when the substring *sub* is not |
| found. |
| |
| |
| .. method:: str.rjust(width[, fillchar]) |
| |
| Return the string right justified in a string of length *width*. Padding is done |
| using the specified *fillchar* (default is a space). The original string is |
| returned if *width* is less than or equal to ``len(s)``. |
| |
| |
| .. method:: str.rpartition(sep) |
| |
| Split the string at the last occurrence of *sep*, and return a 3-tuple |
| containing the part before the separator, the separator itself, and the part |
| after the separator. If the separator is not found, return a 3-tuple containing |
| two empty strings, followed by the string itself. |
| |
| |
| .. method:: str.rsplit(sep=None, maxsplit=-1) |
| |
| Return a list of the words in the string, using *sep* as the delimiter string. |
| If *maxsplit* is given, at most *maxsplit* splits are done, the *rightmost* |
| ones. If *sep* is not specified or ``None``, any whitespace string is a |
| separator. Except for splitting from the right, :meth:`rsplit` behaves like |
| :meth:`split` which is described in detail below. |
| |
| |
| .. method:: str.rstrip([chars]) |
| |
| Return a copy of the string with trailing characters removed. The *chars* |
| argument is a string specifying the set of characters to be removed. If omitted |
| or ``None``, the *chars* argument defaults to removing whitespace. The *chars* |
| argument is not a suffix; rather, all combinations of its values are stripped: |
| |
| >>> ' spacious '.rstrip() |
| ' spacious' |
| >>> 'mississippi'.rstrip('ipz') |
| 'mississ' |
| |
| |
| .. method:: str.split(sep=None, maxsplit=-1) |
| |
| Return a list of the words in the string, using *sep* as the delimiter |
| string. If *maxsplit* is given, at most *maxsplit* splits are done (thus, |
| the list will have at most ``maxsplit+1`` elements). If *maxsplit* is not |
| specified, then there is no limit on the number of splits (all possible |
| splits are made). |
| |
| If *sep* is given, consecutive delimiters are not grouped together and are |
| deemed to delimit empty strings (for example, ``'1,,2'.split(',')`` returns |
| ``['1', '', '2']``). The *sep* argument may consist of multiple characters |
| (for example, ``'1<>2<>3'.split('<>')`` returns ``['1', '2', '3']``). |
| Splitting an empty string with a specified separator returns ``['']``. |
| |
| If *sep* is not specified or is ``None``, a different splitting algorithm is |
| applied: runs of consecutive whitespace are regarded as a single separator, |
| and the result will contain no empty strings at the start or end if the |
| string has leading or trailing whitespace. Consequently, splitting an empty |
| string or a string consisting of just whitespace with a ``None`` separator |
| returns ``[]``. |
| |
| For example, ``' 1 2 3 '.split()`` returns ``['1', '2', '3']``, and |
| ``' 1 2 3 '.split(None, 1)`` returns ``['1', '2 3 ']``. |
| |
| |
| .. method:: str.splitlines([keepends]) |
| |
| Return a list of the lines in the string, breaking at line boundaries. Line |
| breaks are not included in the resulting list unless *keepends* is given and |
| true. |
| |
| |
| .. method:: str.startswith(prefix[, start[, end]]) |
| |
| Return ``True`` if string starts with the *prefix*, otherwise return ``False``. |
| *prefix* can also be a tuple of prefixes to look for. With optional *start*, |
| test string beginning at that position. With optional *end*, stop comparing |
| string at that position. |
| |
| |
| .. method:: str.strip([chars]) |
| |
| Return a copy of the string with the leading and trailing characters removed. |
| The *chars* argument is a string specifying the set of characters to be removed. |
| If omitted or ``None``, the *chars* argument defaults to removing whitespace. |
| The *chars* argument is not a prefix or suffix; rather, all combinations of its |
| values are stripped: |
| |
| >>> ' spacious '.strip() |
| 'spacious' |
| >>> 'www.example.com'.strip('cmowz.') |
| 'example' |
| |
| |
| .. method:: str.swapcase() |
| |
| Return a copy of the string with uppercase characters converted to lowercase and |
| vice versa. Note that it is not necessarily true that |
| ``s.swapcase().swapcase() == s``. |
| |
| |
| .. method:: str.title() |
| |
| Return a titlecased version of the string where words start with an uppercase |
| character and the remaining characters are lowercase. |
| |
| The algorithm uses a simple language-independent definition of a word as |
| groups of consecutive letters. The definition works in many contexts but |
| it means that apostrophes in contractions and possessives form word |
| boundaries, which may not be the desired result:: |
| |
| >>> "they're bill's friends from the UK".title() |
| "They'Re Bill'S Friends From The Uk" |
| |
| A workaround for apostrophes can be constructed using regular expressions:: |
| |
| >>> import re |
| >>> def titlecase(s): |
| return re.sub(r"[A-Za-z]+('[A-Za-z]+)?", |
| lambda mo: mo.group(0)[0].upper() + |
| mo.group(0)[1:].lower(), |
| s) |
| |
| >>> titlecase("they're bill's friends.") |
| "They're Bill's Friends." |
| |
| |
| .. method:: str.translate(map) |
| |
| Return a copy of the *s* where all characters have been mapped through the |
| *map* which must be a dictionary of Unicode ordinals (integers) to Unicode |
| ordinals, strings or ``None``. Unmapped characters are left untouched. |
| Characters mapped to ``None`` are deleted. |
| |
| You can use :meth:`str.maketrans` to create a translation map from |
| character-to-character mappings in different formats. |
| |
| .. note:: |
| |
| An even more flexible approach is to create a custom character mapping |
| codec using the :mod:`codecs` module (see :mod:`encodings.cp1251` for an |
| example). |
| |
| |
| .. method:: str.upper() |
| |
| Return a copy of the string with all the cased characters [4]_ converted to |
| uppercase. Note that ``str.upper().isupper()`` might be ``False`` if ``s`` |
| contains uncased characters or if the Unicode category of the resulting |
| character(s) is not "Lu" (Letter, uppercase), but e.g. "Lt" (Letter, |
| titlecase). |
| |
| The uppercasing algorithm used is described in section 3.13 of the Unicode |
| Standard. |
| |
| |
| .. method:: str.zfill(width) |
| |
| Return the numeric string left filled with zeros in a string of length |
| *width*. A sign prefix is handled correctly. The original string is |
| returned if *width* is less than or equal to ``len(s)``. |
| |
| |
| |
| .. _old-string-formatting: |
| |
| Old String Formatting Operations |
| -------------------------------- |
| |
| .. index:: |
| single: formatting, string (%) |
| single: interpolation, string (%) |
| single: string; formatting |
| single: string; interpolation |
| single: printf-style formatting |
| single: sprintf-style formatting |
| single: % formatting |
| single: % interpolation |
| |
| .. XXX is the note enough? |
| |
| .. note:: |
| |
| The formatting operations described here are modelled on C's printf() |
| syntax. They only support formatting of certain builtin types. The |
| use of a binary operator means that care may be needed in order to |
| format tuples and dictionaries correctly. As the new |
| :ref:`string-formatting` syntax is more flexible and handles tuples and |
| dictionaries naturally, it is recommended for new code. However, there |
| are no current plans to deprecate printf-style formatting. |
| |
| String objects have one unique built-in operation: the ``%`` operator (modulo). |
| This is also known as the string *formatting* or *interpolation* operator. |
| Given ``format % values`` (where *format* is a string), ``%`` conversion |
| specifications in *format* are replaced with zero or more elements of *values*. |
| The effect is similar to the using :c:func:`sprintf` in the C language. |
| |
| If *format* requires a single argument, *values* may be a single non-tuple |
| object. [5]_ Otherwise, *values* must be a tuple with exactly the number of |
| items specified by the format string, or a single mapping object (for example, a |
| dictionary). |
| |
| A conversion specifier contains two or more characters and has the following |
| components, which must occur in this order: |
| |
| #. The ``'%'`` character, which marks the start of the specifier. |
| |
| #. Mapping key (optional), consisting of a parenthesised sequence of characters |
| (for example, ``(somename)``). |
| |
| #. Conversion flags (optional), which affect the result of some conversion |
| types. |
| |
| #. Minimum field width (optional). If specified as an ``'*'`` (asterisk), the |
| actual width is read from the next element of the tuple in *values*, and the |
| object to convert comes after the minimum field width and optional precision. |
| |
| #. Precision (optional), given as a ``'.'`` (dot) followed by the precision. If |
| specified as ``'*'`` (an asterisk), the actual precision is read from the next |
| element of the tuple in *values*, and the value to convert comes after the |
| precision. |
| |
| #. Length modifier (optional). |
| |
| #. Conversion type. |
| |
| When the right argument is a dictionary (or other mapping type), then the |
| formats in the string *must* include a parenthesised mapping key into that |
| dictionary inserted immediately after the ``'%'`` character. The mapping key |
| selects the value to be formatted from the mapping. For example: |
| |
| >>> print('%(language)s has %(number)03d quote types.' % |
| ... {'language': "Python", "number": 2}) |
| Python has 002 quote types. |
| |
| In this case no ``*`` specifiers may occur in a format (since they require a |
| sequential parameter list). |
| |
| The conversion flag characters are: |
| |
| +---------+---------------------------------------------------------------------+ |
| | Flag | Meaning | |
| +=========+=====================================================================+ |
| | ``'#'`` | The value conversion will use the "alternate form" (where defined | |
| | | below). | |
| +---------+---------------------------------------------------------------------+ |
| | ``'0'`` | The conversion will be zero padded for numeric values. | |
| +---------+---------------------------------------------------------------------+ |
| | ``'-'`` | The converted value is left adjusted (overrides the ``'0'`` | |
| | | conversion if both are given). | |
| +---------+---------------------------------------------------------------------+ |
| | ``' '`` | (a space) A blank should be left before a positive number (or empty | |
| | | string) produced by a signed conversion. | |
| +---------+---------------------------------------------------------------------+ |
| | ``'+'`` | A sign character (``'+'`` or ``'-'``) will precede the conversion | |
| | | (overrides a "space" flag). | |
| +---------+---------------------------------------------------------------------+ |
| |
| A length modifier (``h``, ``l``, or ``L``) may be present, but is ignored as it |
| is not necessary for Python -- so e.g. ``%ld`` is identical to ``%d``. |
| |
| The conversion types are: |
| |
| +------------+-----------------------------------------------------+-------+ |
| | Conversion | Meaning | Notes | |
| +============+=====================================================+=======+ |
| | ``'d'`` | Signed integer decimal. | | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'i'`` | Signed integer decimal. | | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'o'`` | Signed octal value. | \(1) | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'u'`` | Obsolete type -- it is identical to ``'d'``. | \(7) | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'x'`` | Signed hexadecimal (lowercase). | \(2) | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'X'`` | Signed hexadecimal (uppercase). | \(2) | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'e'`` | Floating point exponential format (lowercase). | \(3) | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'E'`` | Floating point exponential format (uppercase). | \(3) | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'f'`` | Floating point decimal format. | \(3) | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'F'`` | Floating point decimal format. | \(3) | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'g'`` | Floating point format. Uses lowercase exponential | \(4) | |
| | | format if exponent is less than -4 or not less than | | |
| | | precision, decimal format otherwise. | | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'G'`` | Floating point format. Uses uppercase exponential | \(4) | |
| | | format if exponent is less than -4 or not less than | | |
| | | precision, decimal format otherwise. | | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'c'`` | Single character (accepts integer or single | | |
| | | character string). | | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'r'`` | String (converts any Python object using | \(5) | |
| | | :func:`repr`). | | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'s'`` | String (converts any Python object using | \(5) | |
| | | :func:`str`). | | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'a'`` | String (converts any Python object using | \(5) | |
| | | :func:`ascii`). | | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'%'`` | No argument is converted, results in a ``'%'`` | | |
| | | character in the result. | | |
| +------------+-----------------------------------------------------+-------+ |
| |
| Notes: |
| |
| (1) |
| The alternate form causes a leading zero (``'0'``) to be inserted between |
| left-hand padding and the formatting of the number if the leading character |
| of the result is not already a zero. |
| |
| (2) |
| The alternate form causes a leading ``'0x'`` or ``'0X'`` (depending on whether |
| the ``'x'`` or ``'X'`` format was used) to be inserted between left-hand padding |
| and the formatting of the number if the leading character of the result is not |
| already a zero. |
| |
| (3) |
| The alternate form causes the result to always contain a decimal point, even if |
| no digits follow it. |
| |
| The precision determines the number of digits after the decimal point and |
| defaults to 6. |
| |
| (4) |
| The alternate form causes the result to always contain a decimal point, and |
| trailing zeroes are not removed as they would otherwise be. |
| |
| The precision determines the number of significant digits before and after the |
| decimal point and defaults to 6. |
| |
| (5) |
| If precision is ``N``, the output is truncated to ``N`` characters. |
| |
| |
| (7) |
| See :pep:`237`. |
| |
| Since Python strings have an explicit length, ``%s`` conversions do not assume |
| that ``'\0'`` is the end of the string. |
| |
| .. XXX Examples? |
| |
| .. versionchanged:: 3.1 |
| ``%f`` conversions for numbers whose absolute value is over 1e50 are no |
| longer replaced by ``%g`` conversions. |
| |
| .. index:: |
| module: string |
| module: re |
| |
| Additional string operations are defined in standard modules :mod:`string` and |
| :mod:`re`. |
| |
| |
| .. _typesseq-range: |
| |
| Range Type |
| ---------- |
| |
| .. index:: object: range |
| |
| The :class:`range` type is an immutable sequence which is commonly used for |
| looping. The advantage of the :class:`range` type is that an :class:`range` |
| object will always take the same amount of memory, no matter the size of the |
| range it represents. |
| |
| Range objects have relatively little behavior: they support indexing, contains, |
| iteration, the :func:`len` function, and the following methods: |
| |
| .. method:: range.count(x) |
| |
| Return the number of *i*'s for which ``s[i] == x``. |
| |
| .. versionadded:: 3.2 |
| |
| .. method:: range.index(x) |
| |
| Return the smallest *i* such that ``s[i] == x``. Raises |
| :exc:`ValueError` when *x* is not in the range. |
| |
| .. versionadded:: 3.2 |
| |
| |
| .. _typesseq-mutable: |
| |
| Mutable Sequence Types |
| ---------------------- |
| |
| .. index:: |
| triple: mutable; sequence; types |
| object: list |
| object: bytearray |
| |
| List and bytearray objects support additional operations that allow in-place |
| modification of the object. Other mutable sequence types (when added to the |
| language) should also support these operations. Strings and tuples are |
| immutable sequence types: such objects cannot be modified once created. The |
| following operations are defined on mutable sequence types (where *x* is an |
| arbitrary object). |
| |
| Note that while lists allow their items to be of any type, bytearray object |
| "items" are all integers in the range 0 <= x < 256. |
| |
| .. index:: |
| triple: operations on; sequence; types |
| triple: operations on; list; type |
| pair: subscript; assignment |
| pair: slice; assignment |
| statement: del |
| single: append() (sequence method) |
| single: extend() (sequence method) |
| single: count() (sequence method) |
| single: clear() (sequence method) |
| single: copy() (sequence method) |
| single: index() (sequence method) |
| single: insert() (sequence method) |
| single: pop() (sequence method) |
| single: remove() (sequence method) |
| single: reverse() (sequence method) |
| single: sort() (sequence method) |
| |
| +------------------------------+--------------------------------+---------------------+ |
| | Operation | Result | Notes | |
| +==============================+================================+=====================+ |
| | ``s[i] = x`` | item *i* of *s* is replaced by | | |
| | | *x* | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s[i:j] = t`` | slice of *s* from *i* to *j* | | |
| | | is replaced by the contents of | | |
| | | the iterable *t* | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``del s[i:j]`` | same as ``s[i:j] = []`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s[i:j:k] = t`` | the elements of ``s[i:j:k]`` | \(1) | |
| | | are replaced by those of *t* | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``del s[i:j:k]`` | removes the elements of | | |
| | | ``s[i:j:k]`` from the list | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.append(x)`` | same as ``s[len(s):len(s)] = | | |
| | | [x]`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.extend(x)`` | same as ``s[len(s):len(s)] = | \(2) | |
| | | x`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.clear()`` | remove all items from ``s`` | | |
| | | | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.copy()`` | return a shallow copy of ``s`` | | |
| | | | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.count(x)`` | return number of *i*'s for | | |
| | | which ``s[i] == x`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.index(x[, i[, j]])`` | return smallest *k* such that | \(3) | |
| | | ``s[k] == x`` and ``i <= k < | | |
| | | j`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.insert(i, x)`` | same as ``s[i:i] = [x]`` | \(4) | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.pop([i])`` | same as ``x = s[i]; del s[i]; | \(5) | |
| | | return x`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.remove(x)`` | same as ``del s[s.index(x)]`` | \(3) | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.reverse()`` | reverses the items of *s* in | \(6) | |
| | | place | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.sort([key[, reverse]])`` | sort the items of *s* in place | (6), (7), (8) | |
| +------------------------------+--------------------------------+---------------------+ |
| |
| |
| Notes: |
| |
| (1) |
| *t* must have the same length as the slice it is replacing. |
| |
| (2) |
| *x* can be any iterable object. |
| |
| (3) |
| Raises :exc:`ValueError` when *x* is not found in *s*. When a negative index is |
| passed as the second or third parameter to the :meth:`index` method, the sequence |
| length is added, as for slice indices. If it is still negative, it is truncated |
| to zero, as for slice indices. |
| |
| (4) |
| When a negative index is passed as the first parameter to the :meth:`insert` |
| method, the sequence length is added, as for slice indices. If it is still |
| negative, it is truncated to zero, as for slice indices. |
| |
| (5) |
| The optional argument *i* defaults to ``-1``, so that by default the last |
| item is removed and returned. |
| |
| (6) |
| The :meth:`sort` and :meth:`reverse` methods modify the sequence in place for |
| economy of space when sorting or reversing a large sequence. To remind you |
| that they operate by side effect, they don't return the sorted or reversed |
| sequence. |
| |
| (7) |
| The :meth:`sort` method takes optional arguments for controlling the |
| comparisons. Each must be specified as a keyword argument. |
| |
| *key* specifies a function of one argument that is used to extract a comparison |
| key from each list element: ``key=str.lower``. The default value is ``None``. |
| Use :func:`functools.cmp_to_key` to convert an |
| old-style *cmp* function to a *key* function. |
| |
| |
| *reverse* is a boolean value. If set to ``True``, then the list elements are |
| sorted as if each comparison were reversed. |
| |
| The :meth:`sort` method is guaranteed to be stable. A |
| sort is stable if it guarantees not to change the relative order of elements |
| that compare equal --- this is helpful for sorting in multiple passes (for |
| example, sort by department, then by salary grade). |
| |
| .. impl-detail:: |
| |
| While a list is being sorted, the effect of attempting to mutate, or even |
| inspect, the list is undefined. The C implementation of Python makes the |
| list appear empty for the duration, and raises :exc:`ValueError` if it can |
| detect that the list has been mutated during a sort. |
| |
| (8) |
| :meth:`sort` is not supported by :class:`bytearray` objects. |
| |
| .. versionadded:: 3.3 |
| :meth:`clear` and :meth:`!copy` methods. |
| |
| |
| .. _bytes-methods: |
| |
| Bytes and Byte Array Methods |
| ---------------------------- |
| |
| .. index:: pair: bytes; methods |
| pair: bytearray; methods |
| |
| Bytes and bytearray objects, being "strings of bytes", have all methods found on |
| strings, with the exception of :func:`encode`, :func:`format` and |
| :func:`isidentifier`, which do not make sense with these types. For converting |
| the objects to strings, they have a :func:`decode` method. |
| |
| Wherever one of these methods needs to interpret the bytes as characters |
| (e.g. the :func:`is...` methods), the ASCII character set is assumed. |
| |
| .. versionadded:: 3.3 |
| The functions :func:`count`, :func:`find`, :func:`index`, |
| :func:`rfind` and :func:`rindex` have additional semantics compared to |
| the corresponding string functions: They also accept an integer in |
| range 0 to 255 (a byte) as their first argument. |
| |
| .. note:: |
| |
| The methods on bytes and bytearray objects don't accept strings as their |
| arguments, just as the methods on strings don't accept bytes as their |
| arguments. For example, you have to write :: |
| |
| a = "abc" |
| b = a.replace("a", "f") |
| |
| and :: |
| |
| a = b"abc" |
| b = a.replace(b"a", b"f") |
| |
| |
| .. method:: bytes.decode(encoding="utf-8", errors="strict") |
| bytearray.decode(encoding="utf-8", errors="strict") |
| |
| Return a string decoded from the given bytes. Default encoding is |
| ``'utf-8'``. *errors* may be given to set a different |
| error handling scheme. The default for *errors* is ``'strict'``, meaning |
| that encoding errors raise a :exc:`UnicodeError`. Other possible values are |
| ``'ignore'``, ``'replace'`` and any other name registered via |
| :func:`codecs.register_error`, see section :ref:`codec-base-classes`. For a |
| list of possible encodings, see section :ref:`standard-encodings`. |
| |
| .. versionchanged:: 3.1 |
| Added support for keyword arguments. |
| |
| |
| The bytes and bytearray types have an additional class method: |
| |
| .. classmethod:: bytes.fromhex(string) |
| bytearray.fromhex(string) |
| |
| This :class:`bytes` class method returns a bytes or bytearray object, |
| decoding the given string object. The string must contain two hexadecimal |
| digits per byte, spaces are ignored. |
| |
| >>> bytes.fromhex('f0 f1f2 ') |
| b'\xf0\xf1\xf2' |
| |
| |
| The maketrans and translate methods differ in semantics from the versions |
| available on strings: |
| |
| .. method:: bytes.translate(table[, delete]) |
| bytearray.translate(table[, delete]) |
| |
| Return a copy of the bytes or bytearray object where all bytes occurring in |
| the optional argument *delete* are removed, and the remaining bytes have been |
| mapped through the given translation table, which must be a bytes object of |
| length 256. |
| |
| You can use the :func:`bytes.maketrans` method to create a translation table. |
| |
| Set the *table* argument to ``None`` for translations that only delete |
| characters:: |
| |
| >>> b'read this short text'.translate(None, b'aeiou') |
| b'rd ths shrt txt' |
| |
| |
| .. staticmethod:: bytes.maketrans(from, to) |
| bytearray.maketrans(from, to) |
| |
| This static method returns a translation table usable for |
| :meth:`bytes.translate` that will map each character in *from* into the |
| character at the same position in *to*; *from* and *to* must be bytes objects |
| and have the same length. |
| |
| .. versionadded:: 3.1 |
| |
| |
| .. _types-set: |
| |
| Set Types --- :class:`set`, :class:`frozenset` |
| ============================================== |
| |
| .. index:: object: set |
| |
| A :dfn:`set` object is an unordered collection of distinct :term:`hashable` objects. |
| Common uses include membership testing, removing duplicates from a sequence, and |
| computing mathematical operations such as intersection, union, difference, and |
| symmetric difference. |
| (For other containers see the built in :class:`dict`, :class:`list`, |
| and :class:`tuple` classes, and the :mod:`collections` module.) |
| |
| Like other collections, sets support ``x in set``, ``len(set)``, and ``for x in |
| set``. Being an unordered collection, sets do not record element position or |
| order of insertion. Accordingly, sets do not support indexing, slicing, or |
| other sequence-like behavior. |
| |
| There are currently two built-in set types, :class:`set` and :class:`frozenset`. |
| The :class:`set` type is mutable --- the contents can be changed using methods |
| like :meth:`add` and :meth:`remove`. Since it is mutable, it has no hash value |
| and cannot be used as either a dictionary key or as an element of another set. |
| The :class:`frozenset` type is immutable and :term:`hashable` --- its contents cannot be |
| altered after it is created; it can therefore be used as a dictionary key or as |
| an element of another set. |
| |
| Non-empty sets (not frozensets) can be created by placing a comma-separated list |
| of elements within braces, for example: ``{'jack', 'sjoerd'}``, in addition to the |
| :class:`set` constructor. |
| |
| The constructors for both classes work the same: |
| |
| .. class:: set([iterable]) |
| frozenset([iterable]) |
| |
| Return a new set or frozenset object whose elements are taken from |
| *iterable*. The elements of a set must be hashable. To represent sets of |
| sets, the inner sets must be :class:`frozenset` objects. If *iterable* is |
| not specified, a new empty set is returned. |
| |
| Instances of :class:`set` and :class:`frozenset` provide the following |
| operations: |
| |
| .. describe:: len(s) |
| |
| Return the cardinality of set *s*. |
| |
| .. describe:: x in s |
| |
| Test *x* for membership in *s*. |
| |
| .. describe:: x not in s |
| |
| Test *x* for non-membership in *s*. |
| |
| .. method:: isdisjoint(other) |
| |
| Return True if the set has no elements in common with *other*. Sets are |
| disjoint if and only if their intersection is the empty set. |
| |
| .. method:: issubset(other) |
| set <= other |
| |
| Test whether every element in the set is in *other*. |
| |
| .. method:: set < other |
| |
| Test whether the set is a true subset of *other*, that is, |
| ``set <= other and set != other``. |
| |
| .. method:: issuperset(other) |
| set >= other |
| |
| Test whether every element in *other* is in the set. |
| |
| .. method:: set > other |
| |
| Test whether the set is a true superset of *other*, that is, ``set >= |
| other and set != other``. |
| |
| .. method:: union(other, ...) |
| set | other | ... |
| |
| Return a new set with elements from the set and all others. |
| |
| .. method:: intersection(other, ...) |
| set & other & ... |
| |
| Return a new set with elements common to the set and all others. |
| |
| .. method:: difference(other, ...) |
| set - other - ... |
| |
| Return a new set with elements in the set that are not in the others. |
| |
| .. method:: symmetric_difference(other) |
| set ^ other |
| |
| Return a new set with elements in either the set or *other* but not both. |
| |
| .. method:: copy() |
| |
| Return a new set with a shallow copy of *s*. |
| |
| |
| Note, the non-operator versions of :meth:`union`, :meth:`intersection`, |
| :meth:`difference`, and :meth:`symmetric_difference`, :meth:`issubset`, and |
| :meth:`issuperset` methods will accept any iterable as an argument. In |
| contrast, their operator based counterparts require their arguments to be |
| sets. This precludes error-prone constructions like ``set('abc') & 'cbs'`` |
| in favor of the more readable ``set('abc').intersection('cbs')``. |
| |
| Both :class:`set` and :class:`frozenset` support set to set comparisons. Two |
| sets are equal if and only if every element of each set is contained in the |
| other (each is a subset of the other). A set is less than another set if and |
| only if the first set is a proper subset of the second set (is a subset, but |
| is not equal). A set is greater than another set if and only if the first set |
| is a proper superset of the second set (is a superset, but is not equal). |
| |
| Instances of :class:`set` are compared to instances of :class:`frozenset` |
| based on their members. For example, ``set('abc') == frozenset('abc')`` |
| returns ``True`` and so does ``set('abc') in set([frozenset('abc')])``. |
| |
| The subset and equality comparisons do not generalize to a complete ordering |
| function. For example, any two disjoint sets are not equal and are not |
| subsets of each other, so *all* of the following return ``False``: ``a<b``, |
| ``a==b``, or ``a>b``. |
| |
| Since sets only define partial ordering (subset relationships), the output of |
| the :meth:`list.sort` method is undefined for lists of sets. |
| |
| Set elements, like dictionary keys, must be :term:`hashable`. |
| |
| Binary operations that mix :class:`set` instances with :class:`frozenset` |
| return the type of the first operand. For example: ``frozenset('ab') | |
| set('bc')`` returns an instance of :class:`frozenset`. |
| |
| The following table lists operations available for :class:`set` that do not |
| apply to immutable instances of :class:`frozenset`: |
| |
| .. method:: update(other, ...) |
| set |= other | ... |
| |
| Update the set, adding elements from all others. |
| |
| .. method:: intersection_update(other, ...) |
| set &= other & ... |
| |
| Update the set, keeping only elements found in it and all others. |
| |
| .. method:: difference_update(other, ...) |
| set -= other | ... |
| |
| Update the set, removing elements found in others. |
| |
| .. method:: symmetric_difference_update(other) |
| set ^= other |
| |
| Update the set, keeping only elements found in either set, but not in both. |
| |
| .. method:: add(elem) |
| |
| Add element *elem* to the set. |
| |
| .. method:: remove(elem) |
| |
| Remove element *elem* from the set. Raises :exc:`KeyError` if *elem* is |
| not contained in the set. |
| |
| .. method:: discard(elem) |
| |
| Remove element *elem* from the set if it is present. |
| |
| .. method:: pop() |
| |
| Remove and return an arbitrary element from the set. Raises |
| :exc:`KeyError` if the set is empty. |
| |
| .. method:: clear() |
| |
| Remove all elements from the set. |
| |
| |
| Note, the non-operator versions of the :meth:`update`, |
| :meth:`intersection_update`, :meth:`difference_update`, and |
| :meth:`symmetric_difference_update` methods will accept any iterable as an |
| argument. |
| |
| Note, the *elem* argument to the :meth:`__contains__`, :meth:`remove`, and |
| :meth:`discard` methods may be a set. To support searching for an equivalent |
| frozenset, the *elem* set is temporarily mutated during the search and then |
| restored. During the search, the *elem* set should not be read or mutated |
| since it does not have a meaningful value. |
| |
| |
| .. _typesmapping: |
| |
| Mapping Types --- :class:`dict` |
| =============================== |
| |
| .. index:: |
| object: mapping |
| object: dictionary |
| triple: operations on; mapping; types |
| triple: operations on; dictionary; type |
| statement: del |
| builtin: len |
| |
| A :dfn:`mapping` object maps :term:`hashable` values to arbitrary objects. |
| Mappings are mutable objects. There is currently only one standard mapping |
| type, the :dfn:`dictionary`. (For other containers see the built in |
| :class:`list`, :class:`set`, and :class:`tuple` classes, and the |
| :mod:`collections` module.) |
| |
| A dictionary's keys are *almost* arbitrary values. Values that are not |
| :term:`hashable`, that is, values containing lists, dictionaries or other |
| mutable types (that are compared by value rather than by object identity) may |
| not be used as keys. Numeric types used for keys obey the normal rules for |
| numeric comparison: if two numbers compare equal (such as ``1`` and ``1.0``) |
| then they can be used interchangeably to index the same dictionary entry. (Note |
| however, that since computers store floating-point numbers as approximations it |
| is usually unwise to use them as dictionary keys.) |
| |
| Dictionaries can be created by placing a comma-separated list of ``key: value`` |
| pairs within braces, for example: ``{'jack': 4098, 'sjoerd': 4127}`` or ``{4098: |
| 'jack', 4127: 'sjoerd'}``, or by the :class:`dict` constructor. |
| |
| .. class:: dict([arg]) |
| |
| Return a new dictionary initialized from an optional positional argument or |
| from a set of keyword arguments. If no arguments are given, return a new |
| empty dictionary. If the positional argument *arg* is a mapping object, |
| return a dictionary mapping the same keys to the same values as does the |
| mapping object. Otherwise the positional argument must be a sequence, a |
| container that supports iteration, or an iterator object. The elements of |
| the argument must each also be of one of those kinds, and each must in turn |
| contain exactly two objects. The first is used as a key in the new |
| dictionary, and the second as the key's value. If a given key is seen more |
| than once, the last value associated with it is retained in the new |
| dictionary. |
| |
| If keyword arguments are given, the keywords themselves with their associated |
| values are added as items to the dictionary. If a key is specified both in |
| the positional argument and as a keyword argument, the value associated with |
| the keyword is retained in the dictionary. For example, these all return a |
| dictionary equal to ``{"one": 1, "two": 2}``: |
| |
| * ``dict(one=1, two=2)`` |
| * ``dict({'one': 1, 'two': 2})`` |
| * ``dict(zip(('one', 'two'), (1, 2)))`` |
| * ``dict([['two', 2], ['one', 1]])`` |
| |
| The first example only works for keys that are valid Python identifiers; the |
| others work with any valid keys. |
| |
| |
| These are the operations that dictionaries support (and therefore, custom |
| mapping types should support too): |
| |
| .. describe:: len(d) |
| |
| Return the number of items in the dictionary *d*. |
| |
| .. describe:: d[key] |
| |
| Return the item of *d* with key *key*. Raises a :exc:`KeyError` if *key* is |
| not in the map. |
| |
| If a subclass of dict defines a method :meth:`__missing__`, if the key *key* |
| is not present, the ``d[key]`` operation calls that method with the key *key* |
| as argument. The ``d[key]`` operation then returns or raises whatever is |
| returned or raised by the ``__missing__(key)`` call if the key is not |
| present. No other operations or methods invoke :meth:`__missing__`. If |
| :meth:`__missing__` is not defined, :exc:`KeyError` is raised. |
| :meth:`__missing__` must be a method; it cannot be an instance variable:: |
| |
| >>> class Counter(dict): |
| ... def __missing__(self, key): |
| ... return 0 |
| >>> c = Counter() |
| >>> c['red'] |
| 0 |
| >>> c['red'] += 1 |
| >>> c['red'] |
| 1 |
| |
| See :class:`collections.Counter` for a complete implementation including |
| other methods helpful for accumulating and managing tallies. |
| |
| .. versionchanged:: 3.3 |
| If the dict is modified during the lookup, a :exc:`RuntimeError` |
| exception is now raised. |
| |
| .. describe:: d[key] = value |
| |
| Set ``d[key]`` to *value*. |
| |
| .. describe:: del d[key] |
| |
| Remove ``d[key]`` from *d*. Raises a :exc:`KeyError` if *key* is not in the |
| map. |
| |
| .. describe:: key in d |
| |
| Return ``True`` if *d* has a key *key*, else ``False``. |
| |
| .. describe:: key not in d |
| |
| Equivalent to ``not key in d``. |
| |
| .. describe:: iter(d) |
| |
| Return an iterator over the keys of the dictionary. This is a shortcut |
| for ``iter(d.keys())``. |
| |
| .. method:: clear() |
| |
| Remove all items from the dictionary. |
| |
| .. method:: copy() |
| |
| Return a shallow copy of the dictionary. |
| |
| .. classmethod:: fromkeys(seq[, value]) |
| |
| Create a new dictionary with keys from *seq* and values set to *value*. |
| |
| :meth:`fromkeys` is a class method that returns a new dictionary. *value* |
| defaults to ``None``. |
| |
| .. method:: get(key[, default]) |
| |
| Return the value for *key* if *key* is in the dictionary, else *default*. |
| If *default* is not given, it defaults to ``None``, so that this method |
| never raises a :exc:`KeyError`. |
| |
| .. method:: items() |
| |
| Return a new view of the dictionary's items (``(key, value)`` pairs). See |
| below for documentation of view objects. |
| |
| .. method:: keys() |
| |
| Return a new view of the dictionary's keys. See below for documentation of |
| view objects. |
| |
| .. method:: pop(key[, default]) |
| |
| If *key* is in the dictionary, remove it and return its value, else return |
| *default*. If *default* is not given and *key* is not in the dictionary, |
| a :exc:`KeyError` is raised. |
| |
| .. method:: popitem() |
| |
| Remove and return an arbitrary ``(key, value)`` pair from the dictionary. |
| |
| :meth:`popitem` is useful to destructively iterate over a dictionary, as |
| often used in set algorithms. If the dictionary is empty, calling |
| :meth:`popitem` raises a :exc:`KeyError`. |
| |
| .. method:: setdefault(key[, default]) |
| |
| If *key* is in the dictionary, return its value. If not, insert *key* |
| with a value of *default* and return *default*. *default* defaults to |
| ``None``. |
| |
| .. method:: update([other]) |
| |
| Update the dictionary with the key/value pairs from *other*, overwriting |
| existing keys. Return ``None``. |
| |
| :meth:`update` accepts either another dictionary object or an iterable of |
| key/value pairs (as tuples or other iterables of length two). If keyword |
| arguments are specified, the dictionary is then updated with those |
| key/value pairs: ``d.update(red=1, blue=2)``. |
| |
| .. method:: values() |
| |
| Return a new view of the dictionary's values. See below for documentation of |
| view objects. |
| |
| |
| .. _dict-views: |
| |
| Dictionary view objects |
| ----------------------- |
| |
| The objects returned by :meth:`dict.keys`, :meth:`dict.values` and |
| :meth:`dict.items` are *view objects*. They provide a dynamic view on the |
| dictionary's entries, which means that when the dictionary changes, the view |
| reflects these changes. |
| |
| Dictionary views can be iterated over to yield their respective data, and |
| support membership tests: |
| |
| .. describe:: len(dictview) |
| |
| Return the number of entries in the dictionary. |
| |
| .. describe:: iter(dictview) |
| |
| Return an iterator over the keys, values or items (represented as tuples of |
| ``(key, value)``) in the dictionary. |
| |
| Keys and values are iterated over in an arbitrary order which is non-random, |
| varies across Python implementations, and depends on the dictionary's history |
| of insertions and deletions. If keys, values and items views are iterated |
| over with no intervening modifications to the dictionary, the order of items |
| will directly correspond. This allows the creation of ``(value, key)`` pairs |
| using :func:`zip`: ``pairs = zip(d.values(), d.keys())``. Another way to |
| create the same list is ``pairs = [(v, k) for (k, v) in d.items()]``. |
| |
| Iterating views while adding or deleting entries in the dictionary may raise |
| a :exc:`RuntimeError` or fail to iterate over all entries. |
| |
| .. describe:: x in dictview |
| |
| Return ``True`` if *x* is in the underlying dictionary's keys, values or |
| items (in the latter case, *x* should be a ``(key, value)`` tuple). |
| |
| |
| Keys views are set-like since their entries are unique and hashable. If all |
| values are hashable, so that ``(key, value)`` pairs are unique and hashable, |
| then the items view is also set-like. (Values views are not treated as set-like |
| since the entries are generally not unique.) For set-like views, all of the |
| operations defined for the abstract base class :class:`collections.Set` are |
| available (for example, ``==``, ``<``, or ``^``). |
| |
| An example of dictionary view usage:: |
| |
| >>> dishes = {'eggs': 2, 'sausage': 1, 'bacon': 1, 'spam': 500} |
| >>> keys = dishes.keys() |
| >>> values = dishes.values() |
| |
| >>> # iteration |
| >>> n = 0 |
| >>> for val in values: |
| ... n += val |
| >>> print(n) |
| 504 |
| |
| >>> # keys and values are iterated over in the same order |
| >>> list(keys) |
| ['eggs', 'bacon', 'sausage', 'spam'] |
| >>> list(values) |
| [2, 1, 1, 500] |
| |
| >>> # view objects are dynamic and reflect dict changes |
| >>> del dishes['eggs'] |
| >>> del dishes['sausage'] |
| >>> list(keys) |
| ['spam', 'bacon'] |
| |
| >>> # set operations |
| >>> keys & {'eggs', 'bacon', 'salad'} |
| {'bacon'} |
| >>> keys ^ {'sausage', 'juice'} |
| {'juice', 'sausage', 'bacon', 'spam'} |
| |
| |
| .. _typememoryview: |
| |
| memoryview type |
| =============== |
| |
| :class:`memoryview` objects allow Python code to access the internal data |
| of an object that supports the :ref:`buffer protocol <bufferobjects>` without |
| copying. |
| |
| .. class:: memoryview(obj) |
| |
| Create a :class:`memoryview` that references *obj*. *obj* must support the |
| buffer protocol. Built-in objects that support the buffer protocol include |
| :class:`bytes` and :class:`bytearray`. |
| |
| A :class:`memoryview` has the notion of an *element*, which is the |
| atomic memory unit handled by the originating object *obj*. For many |
| simple types such as :class:`bytes` and :class:`bytearray`, an element |
| is a single byte, but other types such as :class:`array.array` may have |
| bigger elements. |
| |
| ``len(view)`` is equal to the length of :class:`~memoryview.tolist`. |
| If ``view.ndim = 0``, the length is 1. If ``view.ndim = 1``, the length |
| is equal to the number of elements in the view. For higher dimensions, |
| the length is equal to the length of the nested list representation of |
| the view. The :class:`~memoryview.itemsize` attribute will give you the |
| number of bytes in a single element. |
| |
| A :class:`memoryview` supports slicing to expose its data. If |
| :class:`~memoryview.format` is one of the native format specifiers |
| from the :mod:`struct` module, indexing will return a single element |
| with the correct type. Full slicing will result in a subview:: |
| |
| >>> v = memoryview(b'abcefg') |
| >>> v[1] |
| 98 |
| >>> v[-1] |
| 103 |
| >>> v[1:4] |
| <memory at 0x7f3ddc9f4350> |
| >>> bytes(v[1:4]) |
| b'bce' |
| |
| Other native formats:: |
| |
| >>> import array |
| >>> a = array.array('l', [-11111111, 22222222, -33333333, 44444444]) |
| >>> a[0] |
| -11111111 |
| >>> a[-1] |
| 44444444 |
| >>> a[2:3].tolist() |
| [-33333333] |
| >>> a[::2].tolist() |
| [-11111111, -33333333] |
| >>> a[::-1].tolist() |
| [44444444, -33333333, 22222222, -11111111] |
| |
| .. versionadded:: 3.3 |
| |
| If the underlying object is writable, the memoryview supports slice |
| assignment. Resizing is not allowed:: |
| |
| >>> data = bytearray(b'abcefg') |
| >>> v = memoryview(data) |
| >>> v.readonly |
| False |
| >>> v[0] = ord(b'z') |
| >>> data |
| bytearray(b'zbcefg') |
| >>> v[1:4] = b'123' |
| >>> data |
| bytearray(b'z123fg') |
| >>> v[2:3] = b'spam' |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in <module> |
| ValueError: memoryview assignment: lvalue and rvalue have different structures |
| >>> v[2:6] = b'spam' |
| >>> data |
| bytearray(b'z1spam') |
| |
| Memoryviews of hashable (read-only) types are also hashable. The hash |
| is defined as ``hash(m) == hash(m.tobytes())``:: |
| |
| >>> v = memoryview(b'abcefg') |
| >>> hash(v) == hash(b'abcefg') |
| True |
| >>> hash(v[2:4]) == hash(b'ce') |
| True |
| >>> hash(v[::-2]) == hash(b'abcefg'[::-2]) |
| True |
| |
| Hashing of multi-dimensional objects is supported:: |
| |
| >>> buf = bytes(list(range(12))) |
| >>> x = memoryview(buf) |
| >>> y = x.cast('B', shape=[2,2,3]) |
| >>> x.tolist() |
| [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11] |
| >>> y.tolist() |
| [[[0, 1, 2], [3, 4, 5]], [[6, 7, 8], [9, 10, 11]]] |
| >>> hash(x) == hash(y) == hash(y.tobytes()) |
| True |
| |
| .. versionchanged:: 3.3 |
| Memoryview objects are now hashable. |
| |
| |
| :class:`memoryview` has several methods: |
| |
| .. method:: tobytes() |
| |
| Return the data in the buffer as a bytestring. This is equivalent to |
| calling the :class:`bytes` constructor on the memoryview. :: |
| |
| >>> m = memoryview(b"abc") |
| >>> m.tobytes() |
| b'abc' |
| >>> bytes(m) |
| b'abc' |
| |
| For non-contiguous arrays the result is equal to the flattened list |
| representation with all elements converted to bytes. |
| |
| .. method:: tolist() |
| |
| Return the data in the buffer as a list of elements. :: |
| |
| >>> memoryview(b'abc').tolist() |
| [97, 98, 99] |
| >>> import array |
| >>> a = array.array('d', [1.1, 2.2, 3.3]) |
| >>> m = memoryview(a) |
| >>> m.tolist() |
| [1.1, 2.2, 3.3] |
| |
| .. method:: release() |
| |
| Release the underlying buffer exposed by the memoryview object. Many |
| objects take special actions when a view is held on them (for example, |
| a :class:`bytearray` would temporarily forbid resizing); therefore, |
| calling release() is handy to remove these restrictions (and free any |
| dangling resources) as soon as possible. |
| |
| After this method has been called, any further operation on the view |
| raises a :class:`ValueError` (except :meth:`release()` itself which can |
| be called multiple times):: |
| |
| >>> m = memoryview(b'abc') |
| >>> m.release() |
| >>> m[0] |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in <module> |
| ValueError: operation forbidden on released memoryview object |
| |
| The context management protocol can be used for a similar effect, |
| using the ``with`` statement:: |
| |
| >>> with memoryview(b'abc') as m: |
| ... m[0] |
| ... |
| 97 |
| >>> m[0] |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in <module> |
| ValueError: operation forbidden on released memoryview object |
| |
| .. versionadded:: 3.2 |
| |
| .. method:: cast(format[, shape]) |
| |
| Cast a memoryview to a new format or shape. *shape* defaults to |
| ``[byte_length//new_itemsize]``, which means that the result view |
| will be one-dimensional. The return value is a new memoryview, but |
| the buffer itself is not copied. Supported casts are 1D -> C-contiguous |
| and C-contiguous -> 1D. One of the formats must be a byte format |
| ('B', 'b' or 'c'). The byte length of the result must be the same |
| as the original length. |
| |
| Cast 1D/long to 1D/unsigned bytes:: |
| |
| >>> import array |
| >>> a = array.array('l', [1,2,3]) |
| >>> x = memoryview(a) |
| >>> x.format |
| 'l' |
| >>> x.itemsize |
| 8 |
| >>> len(x) |
| 3 |
| >>> x.nbytes |
| 24 |
| >>> y = x.cast('B') |
| >>> y.format |
| 'B' |
| >>> y.itemsize |
| 1 |
| >>> len(y) |
| 24 |
| >>> y.nbytes |
| 24 |
| |
| Cast 1D/unsigned bytes to 1D/char:: |
| |
| >>> b = bytearray(b'zyz') |
| >>> x = memoryview(b) |
| >>> x[0] = b'a' |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in <module> |
| ValueError: memoryview: invalid value for format "B" |
| >>> y = x.cast('c') |
| >>> y[0] = b'a' |
| >>> b |
| bytearray(b'ayz') |
| |
| Cast 1D/bytes to 3D/ints to 1D/signed char:: |
| |
| >>> import struct |
| >>> buf = struct.pack("i"*12, *list(range(12))) |
| >>> x = memoryview(buf) |
| >>> y = x.cast('i', shape=[2,2,3]) |
| >>> y.tolist() |
| [[[0, 1, 2], [3, 4, 5]], [[6, 7, 8], [9, 10, 11]]] |
| >>> y.format |
| 'i' |
| >>> y.itemsize |
| 4 |
| >>> len(y) |
| 2 |
| >>> y.nbytes |
| 48 |
| >>> z = y.cast('b') |
| >>> z.format |
| 'b' |
| >>> z.itemsize |
| 1 |
| >>> len(z) |
| 48 |
| >>> z.nbytes |
| 48 |
| |
| Cast 1D/unsigned char to to 2D/unsigned long:: |
| |
| >>> buf = struct.pack("L"*6, *list(range(6))) |
| >>> x = memoryview(buf) |
| >>> y = x.cast('L', shape=[2,3]) |
| >>> len(y) |
| 2 |
| >>> y.nbytes |
| 48 |
| >>> y.tolist() |
| [[0, 1, 2], [3, 4, 5]] |
| |
| .. versionadded:: 3.3 |
| |
| There are also several readonly attributes available: |
| |
| .. attribute:: obj |
| |
| The underlying object of the memoryview:: |
| |
| >>> b = bytearray(b'xyz') |
| >>> m = memoryview(b) |
| >>> m.obj is b |
| True |
| |
| .. versionadded:: 3.3 |
| |
| .. attribute:: nbytes |
| |
| ``nbytes == product(shape) * itemsize == len(m.tobytes())``. This is |
| the amount of space in bytes that the array would use in a contiguous |
| representation. It is not necessarily equal to len(m):: |
| |
| >>> import array |
| >>> a = array.array('i', [1,2,3,4,5]) |
| >>> m = memoryview(a) |
| >>> len(m) |
| 5 |
| >>> m.nbytes |
| 20 |
| >>> y = m[::2] |
| >>> len(y) |
| 3 |
| >>> y.nbytes |
| 12 |
| >>> len(y.tobytes()) |
| 12 |
| |
| Multi-dimensional arrays:: |
| |
| >>> import struct |
| >>> buf = struct.pack("d"*12, *[1.5*x for x in range(12)]) |
| >>> x = memoryview(buf) |
| >>> y = x.cast('d', shape=[3,4]) |
| >>> y.tolist() |
| [[0.0, 1.5, 3.0, 4.5], [6.0, 7.5, 9.0, 10.5], [12.0, 13.5, 15.0, 16.5]] |
| >>> len(y) |
| 3 |
| >>> y.nbytes |
| 96 |
| |
| .. versionadded:: 3.3 |
| |
| .. attribute:: readonly |
| |
| A bool indicating whether the memory is read only. |
| |
| .. attribute:: format |
| |
| A string containing the format (in :mod:`struct` module style) for each |
| element in the view. A memoryview can be created from exporters with |
| arbitrary format strings, but some methods (e.g. :meth:`tolist`) are |
| restricted to native single element formats. Special care must be taken |
| when comparing memoryviews. Since comparisons are required to return a |
| value for ``==`` and ``!=``, two memoryviews referencing the same |
| exporter can compare as not-equal if the exporter's format is not |
| understood:: |
| |
| >>> from ctypes import BigEndianStructure, c_long |
| >>> class BEPoint(BigEndianStructure): |
| ... _fields_ = [("x", c_long), ("y", c_long)] |
| ... |
| >>> point = BEPoint(100, 200) |
| >>> a = memoryview(point) |
| >>> b = memoryview(point) |
| >>> a == b |
| False |
| >>> a.tolist() |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in <module> |
| NotImplementedError: memoryview: unsupported format T{>l:x:>l:y:} |
| |
| .. attribute:: itemsize |
| |
| The size in bytes of each element of the memoryview:: |
| |
| >>> import array, struct |
| >>> m = memoryview(array.array('H', [32000, 32001, 32002])) |
| >>> m.itemsize |
| 2 |
| >>> m[0] |
| 32000 |
| >>> struct.calcsize('H') == m.itemsize |
| True |
| |
| .. attribute:: ndim |
| |
| An integer indicating how many dimensions of a multi-dimensional array the |
| memory represents. |
| |
| .. attribute:: shape |
| |
| A tuple of integers the length of :attr:`ndim` giving the shape of the |
| memory as a N-dimensional array. |
| |
| .. attribute:: strides |
| |
| A tuple of integers the length of :attr:`ndim` giving the size in bytes to |
| access each element for each dimension of the array. |
| |
| .. attribute:: suboffsets |
| |
| Used internally for PIL-style arrays. The value is informational only. |
| |
| .. attribute:: c_contiguous |
| |
| A bool indicating whether the memory is C-contiguous. |
| |
| .. versionadded:: 3.3 |
| |
| .. attribute:: f_contiguous |
| |
| A bool indicating whether the memory is Fortran contiguous. |
| |
| .. versionadded:: 3.3 |
| |
| .. attribute:: contiguous |
| |
| A bool indicating whether the memory is contiguous. |
| |
| .. versionadded:: 3.3 |
| |
| |
| .. _typecontextmanager: |
| |
| Context Manager Types |
| ===================== |
| |
| .. index:: |
| single: context manager |
| single: context management protocol |
| single: protocol; context management |
| |
| Python's :keyword:`with` statement supports the concept of a runtime context |
| defined by a context manager. This is implemented using a pair of methods |
| that allow user-defined classes to define a runtime context that is entered |
| before the statement body is executed and exited when the statement ends: |
| |
| |
| .. method:: contextmanager.__enter__() |
| |
| Enter the runtime context and return either this object or another object |
| related to the runtime context. The value returned by this method is bound to |
| the identifier in the :keyword:`as` clause of :keyword:`with` statements using |
| this context manager. |
| |
| An example of a context manager that returns itself is a :term:`file object`. |
| File objects return themselves from __enter__() to allow :func:`open` to be |
| used as the context expression in a :keyword:`with` statement. |
| |
| An example of a context manager that returns a related object is the one |
| returned by :func:`decimal.localcontext`. These managers set the active |
| decimal context to a copy of the original decimal context and then return the |
| copy. This allows changes to be made to the current decimal context in the body |
| of the :keyword:`with` statement without affecting code outside the |
| :keyword:`with` statement. |
| |
| |
| .. method:: contextmanager.__exit__(exc_type, exc_val, exc_tb) |
| |
| Exit the runtime context and return a Boolean flag indicating if any exception |
| that occurred should be suppressed. If an exception occurred while executing the |
| body of the :keyword:`with` statement, the arguments contain the exception type, |
| value and traceback information. Otherwise, all three arguments are ``None``. |
| |
| Returning a true value from this method will cause the :keyword:`with` statement |
| to suppress the exception and continue execution with the statement immediately |
| following the :keyword:`with` statement. Otherwise the exception continues |
| propagating after this method has finished executing. Exceptions that occur |
| during execution of this method will replace any exception that occurred in the |
| body of the :keyword:`with` statement. |
| |
| The exception passed in should never be reraised explicitly - instead, this |
| method should return a false value to indicate that the method completed |
| successfully and does not want to suppress the raised exception. This allows |
| context management code (such as ``contextlib.nested``) to easily detect whether |
| or not an :meth:`__exit__` method has actually failed. |
| |
| Python defines several context managers to support easy thread synchronisation, |
| prompt closure of files or other objects, and simpler manipulation of the active |
| decimal arithmetic context. The specific types are not treated specially beyond |
| their implementation of the context management protocol. See the |
| :mod:`contextlib` module for some examples. |
| |
| Python's :term:`generator`\s and the :class:`contextlib.contextmanager` decorator |
| provide a convenient way to implement these protocols. If a generator function is |
| decorated with the :class:`contextlib.contextmanager` decorator, it will return a |
| context manager implementing the necessary :meth:`__enter__` and |
| :meth:`__exit__` methods, rather than the iterator produced by an undecorated |
| generator function. |
| |
| Note that there is no specific slot for any of these methods in the type |
| structure for Python objects in the Python/C API. Extension types wanting to |
| define these methods must provide them as a normal Python accessible method. |
| Compared to the overhead of setting up the runtime context, the overhead of a |
| single class dictionary lookup is negligible. |
| |
| |
| .. _typesother: |
| |
| Other Built-in Types |
| ==================== |
| |
| The interpreter supports several other kinds of objects. Most of these support |
| only one or two operations. |
| |
| |
| .. _typesmodules: |
| |
| Modules |
| ------- |
| |
| The only special operation on a module is attribute access: ``m.name``, where |
| *m* is a module and *name* accesses a name defined in *m*'s symbol table. |
| Module attributes can be assigned to. (Note that the :keyword:`import` |
| statement is not, strictly speaking, an operation on a module object; ``import |
| foo`` does not require a module object named *foo* to exist, rather it requires |
| an (external) *definition* for a module named *foo* somewhere.) |
| |
| A special attribute of every module is :attr:`__dict__`. This is the dictionary |
| containing the module's symbol table. Modifying this dictionary will actually |
| change the module's symbol table, but direct assignment to the :attr:`__dict__` |
| attribute is not possible (you can write ``m.__dict__['a'] = 1``, which defines |
| ``m.a`` to be ``1``, but you can't write ``m.__dict__ = {}``). Modifying |
| :attr:`__dict__` directly is not recommended. |
| |
| Modules built into the interpreter are written like this: ``<module 'sys' |
| (built-in)>``. If loaded from a file, they are written as ``<module 'os' from |
| '/usr/local/lib/pythonX.Y/os.pyc'>``. |
| |
| |
| .. _typesobjects: |
| |
| Classes and Class Instances |
| --------------------------- |
| |
| See :ref:`objects` and :ref:`class` for these. |
| |
| |
| .. _typesfunctions: |
| |
| Functions |
| --------- |
| |
| Function objects are created by function definitions. The only operation on a |
| function object is to call it: ``func(argument-list)``. |
| |
| There are really two flavors of function objects: built-in functions and |
| user-defined functions. Both support the same operation (to call the function), |
| but the implementation is different, hence the different object types. |
| |
| See :ref:`function` for more information. |
| |
| |
| .. _typesmethods: |
| |
| Methods |
| ------- |
| |
| .. index:: object: method |
| |
| Methods are functions that are called using the attribute notation. There are |
| two flavors: built-in methods (such as :meth:`append` on lists) and class |
| instance methods. Built-in methods are described with the types that support |
| them. |
| |
| If you access a method (a function defined in a class namespace) through an |
| instance, you get a special object: a :dfn:`bound method` (also called |
| :dfn:`instance method`) object. When called, it will add the ``self`` argument |
| to the argument list. Bound methods have two special read-only attributes: |
| ``m.__self__`` is the object on which the method operates, and ``m.__func__`` is |
| the function implementing the method. Calling ``m(arg-1, arg-2, ..., arg-n)`` |
| is completely equivalent to calling ``m.__func__(m.__self__, arg-1, arg-2, ..., |
| arg-n)``. |
| |
| Like function objects, bound method objects support getting arbitrary |
| attributes. However, since method attributes are actually stored on the |
| underlying function object (``meth.__func__``), setting method attributes on |
| bound methods is disallowed. Attempting to set a method attribute results in a |
| :exc:`TypeError` being raised. In order to set a method attribute, you need to |
| explicitly set it on the underlying function object:: |
| |
| class C: |
| def method(self): |
| pass |
| |
| c = C() |
| c.method.__func__.whoami = 'my name is c' |
| |
| See :ref:`types` for more information. |
| |
| |
| .. _bltin-code-objects: |
| |
| Code Objects |
| ------------ |
| |
| .. index:: object: code |
| |
| .. index:: |
| builtin: compile |
| single: __code__ (function object attribute) |
| |
| Code objects are used by the implementation to represent "pseudo-compiled" |
| executable Python code such as a function body. They differ from function |
| objects because they don't contain a reference to their global execution |
| environment. Code objects are returned by the built-in :func:`compile` function |
| and can be extracted from function objects through their :attr:`__code__` |
| attribute. See also the :mod:`code` module. |
| |
| .. index:: |
| builtin: exec |
| builtin: eval |
| |
| A code object can be executed or evaluated by passing it (instead of a source |
| string) to the :func:`exec` or :func:`eval` built-in functions. |
| |
| See :ref:`types` for more information. |
| |
| |
| .. _bltin-type-objects: |
| |
| Type Objects |
| ------------ |
| |
| .. index:: |
| builtin: type |
| module: types |
| |
| Type objects represent the various object types. An object's type is accessed |
| by the built-in function :func:`type`. There are no special operations on |
| types. The standard module :mod:`types` defines names for all standard built-in |
| types. |
| |
| Types are written like this: ``<class 'int'>``. |
| |
| |
| .. _bltin-null-object: |
| |
| The Null Object |
| --------------- |
| |
| This object is returned by functions that don't explicitly return a value. It |
| supports no special operations. There is exactly one null object, named |
| ``None`` (a built-in name). ``type(None)()`` produces the same singleton. |
| |
| It is written as ``None``. |
| |
| |
| .. _bltin-ellipsis-object: |
| |
| The Ellipsis Object |
| ------------------- |
| |
| This object is commonly used by slicing (see :ref:`slicings`), but may also |
| be used in other situations where a sentinel value other than :const:`None` |
| is needed. It supports no special operations. There is exactly one ellipsis |
| object, named :const:`Ellipsis` (a built-in name). ``type(Ellipsis)()`` |
| produces the :const:`Ellipsis` singleton. |
| |
| It is written as ``Ellipsis`` or ``...``. |
| |
| |
| .. _bltin-notimplemented-object: |
| |
| The NotImplemented Object |
| ------------------------- |
| |
| This object is returned from comparisons and binary operations when they are |
| asked to operate on types they don't support. See :ref:`comparisons` for more |
| information. There is exactly one ``NotImplemented`` object. |
| ``type(NotImplemented)()`` produces the singleton instance. |
| |
| It is written as ``NotImplemented``. |
| |
| |
| .. _bltin-boolean-values: |
| |
| Boolean Values |
| -------------- |
| |
| Boolean values are the two constant objects ``False`` and ``True``. They are |
| used to represent truth values (although other values can also be considered |
| false or true). In numeric contexts (for example when used as the argument to |
| an arithmetic operator), they behave like the integers 0 and 1, respectively. |
| The built-in function :func:`bool` can be used to convert any value to a |
| Boolean, if the value can be interpreted as a truth value (see section |
| :ref:`truth` above). |
| |
| .. index:: |
| single: False |
| single: True |
| pair: Boolean; values |
| |
| They are written as ``False`` and ``True``, respectively. |
| |
| |
| .. _typesinternal: |
| |
| Internal Objects |
| ---------------- |
| |
| See :ref:`types` for this information. It describes stack frame objects, |
| traceback objects, and slice objects. |
| |
| |
| .. _specialattrs: |
| |
| Special Attributes |
| ================== |
| |
| The implementation adds a few special read-only attributes to several object |
| types, where they are relevant. Some of these are not reported by the |
| :func:`dir` built-in function. |
| |
| |
| .. attribute:: object.__dict__ |
| |
| A dictionary or other mapping object used to store an object's (writable) |
| attributes. |
| |
| |
| .. attribute:: instance.__class__ |
| |
| The class to which a class instance belongs. |
| |
| |
| .. attribute:: class.__bases__ |
| |
| The tuple of base classes of a class object. |
| |
| |
| .. attribute:: class.__name__ |
| |
| The name of the class or type. |
| |
| |
| .. attribute:: class.__qualname__ |
| |
| The :term:`qualified name` of the class or type. |
| |
| .. versionadded:: 3.3 |
| |
| |
| .. attribute:: class.__mro__ |
| |
| This attribute is a tuple of classes that are considered when looking for |
| base classes during method resolution. |
| |
| |
| .. method:: class.mro() |
| |
| This method can be overridden by a metaclass to customize the method |
| resolution order for its instances. It is called at class instantiation, and |
| its result is stored in :attr:`__mro__`. |
| |
| |
| .. method:: class.__subclasses__ |
| |
| Each class keeps a list of weak references to its immediate subclasses. This |
| method returns a list of all those references still alive. |
| Example:: |
| |
| >>> int.__subclasses__() |
| [<class 'bool'>] |
| |
| |
| .. rubric:: Footnotes |
| |
| .. [1] Additional information on these special methods may be found in the Python |
| Reference Manual (:ref:`customization`). |
| |
| .. [2] As a consequence, the list ``[1, 2]`` is considered equal to ``[1.0, 2.0]``, and |
| similarly for tuples. |
| |
| .. [3] They must have since the parser can't tell the type of the operands. |
| |
| .. [4] Cased characters are those with general category property being one of |
| "Lu" (Letter, uppercase), "Ll" (Letter, lowercase), or "Lt" (Letter, titlecase). |
| |
| .. [5] To format only a tuple you should therefore provide a singleton tuple whose only |
| element is the tuple to be formatted. |