| .. 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. |
| |
| .. note:: |
| |
| Historically (until release 2.2), Python's built-in types have differed from |
| user-defined types because it was not possible to use the built-in types as the |
| basis for object-oriented inheritance. This limitation no longer |
| exists. |
| |
| .. index:: pair: built-in; types |
| |
| The principal built-in types are numerics, sequences, mappings, files, classes, |
| instances and exceptions. |
| |
| .. index:: statement: print |
| |
| 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``, ``0L``, ``0.0``, ``0j``. |
| |
| * any empty sequence, for example, ``''``, ``()``, ``[]``. |
| |
| * any empty mapping, for example, ``{}``. |
| |
| * instances of user-defined classes, if the class defines a :meth:`__nonzero__` |
| 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 |
| |
| Comparison operations are supported by all objects. 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 | Notes | |
| +============+=========================+=======+ |
| | ``<`` | strictly less than | | |
| +------------+-------------------------+-------+ |
| | ``<=`` | less than or equal | | |
| +------------+-------------------------+-------+ |
| | ``>`` | strictly greater than | | |
| +------------+-------------------------+-------+ |
| | ``>=`` | greater than or equal | | |
| +------------+-------------------------+-------+ |
| | ``==`` | equal | | |
| +------------+-------------------------+-------+ |
| | ``!=`` | not equal | \(1) | |
| +------------+-------------------------+-------+ |
| | ``is`` | object identity | | |
| +------------+-------------------------+-------+ |
| | ``is not`` | negated object identity | | |
| +------------+-------------------------+-------+ |
| |
| Notes: |
| |
| (1) |
| ``!=`` can also be written ``<>``, but this is an obsolete usage |
| kept for backwards compatibility only. New code should always use |
| ``!=``. |
| |
| .. index:: |
| pair: object; numeric |
| pair: objects; comparing |
| |
| Objects of different types, except different numeric types and different string |
| types, never compare equal; such objects are ordered consistently but |
| arbitrarily (so that sorting a heterogeneous array yields a consistent result). |
| Furthermore, some types (for example, file objects) support only a degenerate |
| notion of comparison where any two objects of that type are unequal. Again, |
| such objects are ordered arbitrarily but consistently. The ``<``, ``<=``, ``>`` |
| and ``>=`` operators will raise a :exc:`TypeError` exception when any operand is |
| a complex number. |
| |
| .. index:: single: __cmp__() (instance method) |
| |
| Instances of a class normally compare as non-equal unless the class defines the |
| :meth:`__cmp__` method. Refer to :ref:`customization`) for information on the |
| use of this method to effect object comparisons. |
| |
| .. impl-detail:: |
| |
| Objects of different types except numbers are ordered by their type names; |
| objects of the same types that don't support proper comparison are ordered by |
| their address. |
| |
| .. index:: |
| operator: in |
| operator: not in |
| |
| Two more operations with the same syntactic priority, ``in`` and ``not in``, are |
| supported only by sequence types (below). |
| |
| |
| .. _typesnumeric: |
| |
| Numeric Types --- :class:`int`, :class:`float`, :class:`long`, :class:`complex` |
| =============================================================================== |
| |
| .. index:: |
| object: numeric |
| object: Boolean |
| object: integer |
| object: long integer |
| object: floating point |
| object: complex number |
| pair: C; language |
| |
| There are four distinct numeric types: :dfn:`plain integers`, :dfn:`long |
| integers`, :dfn:`floating point numbers`, and :dfn:`complex numbers`. In |
| addition, Booleans are a subtype of plain integers. Plain integers (also just |
| called :dfn:`integers`) are implemented using :c:type:`long` in C, which gives |
| them at least 32 bits of precision (``sys.maxint`` is always set to the maximum |
| plain integer value for the current platform, the minimum value is |
| ``-sys.maxint - 1``). Long 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 |
| triple: long; integer; literals |
| pair: floating point; literals |
| pair: complex number; literals |
| pair: hexadecimal; literals |
| pair: octal; literals |
| |
| Numbers are created by numeric literals or as the result of built-in functions |
| and operators. Unadorned integer literals (including binary, hex, and octal |
| numbers) yield plain integers unless the value they denote is too large to be |
| represented as a plain integer, in which case they yield a long integer. |
| Integer literals with an ``'L'`` or ``'l'`` suffix yield long integers (``'L'`` |
| is preferred because ``1l`` looks too much like eleven!). Numeric literals |
| containing a decimal point or an exponent sign yield floating point numbers. |
| Appending ``'j'`` or ``'J'`` to a numeric literal yields a complex number with a |
| zero real part. A complex numeric literal is the sum of a real and an imaginary |
| part. |
| |
| .. index:: |
| single: arithmetic |
| builtin: int |
| builtin: long |
| 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 plain integer is narrower than long integer |
| is narrower than floating point is narrower than complex. Comparisons between |
| numbers of mixed type use the same rule. [2]_ The constructors :func:`int`, |
| :func:`long`, :func:`float`, and :func:`complex` can be used to produce numbers |
| of a specific type. |
| |
| All built-in numeric types support the following operations. See |
| :ref:`power` and later sections for the operators' priorities. |
| |
| +--------------------+---------------------------------+--------+ |
| | Operation | Result | Notes | |
| +====================+=================================+========+ |
| | ``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* | \(1) | |
| +--------------------+---------------------------------+--------+ |
| | ``x // y`` | (floored) quotient of *x* and | (4)(5) | |
| | | *y* | | |
| +--------------------+---------------------------------+--------+ |
| | ``x % y`` | remainder of ``x / y`` | \(4) | |
| +--------------------+---------------------------------+--------+ |
| | ``-x`` | *x* negated | | |
| +--------------------+---------------------------------+--------+ |
| | ``+x`` | *x* unchanged | | |
| +--------------------+---------------------------------+--------+ |
| | ``abs(x)`` | absolute value or magnitude of | \(3) | |
| | | *x* | | |
| +--------------------+---------------------------------+--------+ |
| | ``int(x)`` | *x* converted to integer | \(2) | |
| +--------------------+---------------------------------+--------+ |
| | ``long(x)`` | *x* converted to long integer | \(2) | |
| +--------------------+---------------------------------+--------+ |
| | ``float(x)`` | *x* converted to floating point | \(6) | |
| +--------------------+---------------------------------+--------+ |
| | ``complex(re,im)`` | a complex number with real part | | |
| | | *re*, imaginary part *im*. | | |
| | | *im* defaults to zero. | | |
| +--------------------+---------------------------------+--------+ |
| | ``c.conjugate()`` | conjugate of the complex number | | |
| | | *c*. (Identity on real numbers) | | |
| +--------------------+---------------------------------+--------+ |
| | ``divmod(x, y)`` | the pair ``(x // y, x % y)`` | (3)(4) | |
| +--------------------+---------------------------------+--------+ |
| | ``pow(x, y)`` | *x* to the power *y* | (3)(7) | |
| +--------------------+---------------------------------+--------+ |
| | ``x ** y`` | *x* to the power *y* | \(7) | |
| +--------------------+---------------------------------+--------+ |
| |
| .. index:: |
| triple: operations on; numeric; types |
| single: conjugate() (complex number method) |
| |
| Notes: |
| |
| (1) |
| .. index:: |
| pair: integer; division |
| triple: long; integer; division |
| |
| For (plain or long) integer division, the result is an integer. 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. Note that the result is a long integer if either operand is a |
| long integer, regardless of the numeric value. |
| |
| (2) |
| .. index:: |
| module: math |
| single: floor() (in module math) |
| single: ceil() (in module math) |
| single: trunc() (in module math) |
| pair: numeric; conversions |
| |
| Conversion from floats using :func:`int` or :func:`long` truncates toward |
| zero like the related function, :func:`math.trunc`. Use the function |
| :func:`math.floor` to round downward and :func:`math.ceil` to round |
| upward. |
| |
| (3) |
| See :ref:`built-in-funcs` for a full description. |
| |
| (4) |
| .. deprecated:: 2.3 |
| The floor division operator, the modulo operator, and the :func:`divmod` |
| function are no longer defined for complex numbers. Instead, convert to |
| a floating point number using the :func:`abs` function if appropriate. |
| |
| (5) |
| Also referred to as integer division. The resultant value is a whole integer, |
| though the result's type is not necessarily int. |
| |
| (6) |
| float also accepts the strings "nan" and "inf" with an optional prefix "+" |
| or "-" for Not a Number (NaN) and positive or negative infinity. |
| |
| .. versionadded:: 2.6 |
| |
| (7) |
| Python defines ``pow(0, 0)`` and ``0 ** 0`` to be ``1``, as is common for |
| programming languages. |
| |
| All :class:`numbers.Real` types (:class:`int`, :class:`long`, 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* | | |
| +--------------------+------------------------------------+--------+ |
| |
| .. 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)``. A |
| long integer is returned if the result exceeds the range of plain integers. |
| |
| (3) |
| A right shift by *n* bits is equivalent to division by ``pow(2, n)``. |
| |
| |
| Additional Methods on Integer Types |
| ----------------------------------- |
| |
| The integer types implement the :class:`numbers.Integral` :term:`abstract base |
| class`. In addition, they provide one more method: |
| |
| .. method:: int.bit_length() |
| .. method:: long.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:: 2.7 |
| |
| |
| 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. |
| |
| .. versionadded:: 2.6 |
| |
| .. 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 |
| |
| .. versionadded:: 2.6 |
| |
| 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. |
| |
| .. versionadded:: 2.6 |
| |
| |
| .. method:: float.fromhex(s) |
| |
| Class method to return the float represented by a hexadecimal |
| string *s*. The string *s* may have leading and trailing |
| whitespace. |
| |
| .. versionadded:: 2.6 |
| |
| |
| 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' |
| |
| |
| .. _typeiter: |
| |
| Iterator Types |
| ============== |
| |
| .. versionadded:: 2.2 |
| |
| .. 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. |
| |
| The intention of the protocol is that once an iterator's :meth:`next` method |
| raises :exc:`StopIteration`, it will continue to do so on subsequent calls. |
| Implementations that do not obey this property are deemed broken. (This |
| constraint was added in Python 2.3; in Python 2.2, various iterators are broken |
| according to this rule.) |
| |
| |
| .. _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:`unicode`, :class:`list`, :class:`tuple`, :class:`bytearray`, :class:`buffer`, :class:`xrange` |
| ====================================================================================================================================== |
| |
| There are seven sequence types: strings, Unicode strings, lists, tuples, |
| bytearrays, buffers, and xrange 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: Unicode |
| object: tuple |
| object: list |
| object: bytearray |
| object: buffer |
| object: xrange |
| |
| String literals are written in single or double quotes: ``'xyzzy'``, |
| ``"frobozz"``. See :ref:`strings` for more about string literals. |
| Unicode strings are much like strings, but are specified in the syntax |
| using a preceding ``'u'`` character: ``u'abc'``, ``u"def"``. In addition |
| to the functionality described here, there are also string-specific |
| methods described in the :ref:`string-methods` section. 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,)``. |
| |
| Bytearray objects are created with the built-in function :func:`bytearray`. |
| |
| Buffer objects are not directly supported by Python syntax, but can be created |
| by calling the built-in function :func:`buffer`. They don't support |
| concatenation or repetition. |
| |
| Objects of type xrange are similar to buffers in that there is no specific syntax to |
| create them, but they are created using the :func:`xrange` function. They don't |
| support slicing, concatenation or repetition, and using ``in``, ``not in``, |
| :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* and *j* 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 |
| pair: extended slice; operation |
| operator: in |
| operator: not in |
| |
| Notes: |
| |
| (1) |
| When *s* is a string or Unicode string object the ``in`` and ``not in`` |
| operations act like a substring test. In Python versions before 2.3, *x* had to |
| be a string of length 1. In Python 2.3 and beyond, *x* may be a string of any |
| length. |
| |
| (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) |
| .. impl-detail:: |
| |
| If *s* and *t* are both strings, some Python implementations such as |
| CPython can usually perform an in-place optimization for assignments of |
| the form ``s = s + t`` or ``s += t``. When applicable, this optimization |
| makes quadratic run-time much less likely. This optimization is both |
| version and implementation dependent. For performance sensitive code, it |
| is preferable to use the :meth:`str.join` method which assures consistent |
| linear concatenation performance across versions and implementations. |
| |
| .. versionchanged:: 2.4 |
| Formerly, string concatenation never occurred in-place. |
| |
| |
| .. _string-methods: |
| |
| String Methods |
| -------------- |
| |
| .. index:: pair: string; methods |
| |
| Below are listed the string methods which both 8-bit strings and |
| Unicode objects support. Some of them are also available on :class:`bytearray` |
| objects. |
| |
| In addition, Python's strings support the sequence type methods |
| described in the :ref:`typesseq` section. To output formatted strings |
| use template strings or the ``%`` operator described in 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. |
| |
| For 8-bit strings, this method is locale-dependent. |
| |
| |
| .. method:: str.center(width[, fillchar]) |
| |
| Return centered in a string of length *width*. Padding is done using the |
| specified *fillchar* (default is a space). |
| |
| .. versionchanged:: 2.4 |
| Support for the *fillchar* argument. |
| |
| |
| .. 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.decode([encoding[, errors]]) |
| |
| Decodes the string using the codec registered for *encoding*. *encoding* |
| defaults to the default string encoding. *errors* may be given to set a |
| different error handling scheme. The default is ``'strict'``, meaning that |
| encoding errors raise :exc:`UnicodeError`. Other possible values are |
| ``'ignore'``, ``'replace'`` and any other name registered via |
| :func:`codecs.register_error`, see section :ref:`codec-base-classes`. |
| |
| .. versionadded:: 2.2 |
| |
| .. versionchanged:: 2.3 |
| Support for other error handling schemes added. |
| |
| .. versionchanged:: 2.7 |
| Support for keyword arguments added. |
| |
| .. method:: str.encode([encoding[,errors]]) |
| |
| Return an encoded version of the string. Default encoding is the current |
| default string encoding. *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`. |
| |
| .. versionadded:: 2.0 |
| |
| .. versionchanged:: 2.3 |
| Support for ``'xmlcharrefreplace'`` and ``'backslashreplace'`` and other error |
| handling schemes added. |
| |
| .. versionchanged:: 2.7 |
| 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. |
| |
| .. versionchanged:: 2.5 |
| Accept tuples as *suffix*. |
| |
| |
| .. method:: str.expandtabs([tabsize]) |
| |
| Return a copy of the string where all tab characters are replaced by one 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. |
| |
| This method of string formatting is the new standard in Python 3, and |
| should be preferred to the ``%`` formatting described in |
| :ref:`string-formatting` in new code. |
| |
| .. versionadded:: 2.6 |
| |
| |
| .. 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. |
| |
| For 8-bit strings, this method is locale-dependent. |
| |
| |
| .. method:: str.isalpha() |
| |
| Return true if all characters in the string are alphabetic and there is at least |
| one character, false otherwise. |
| |
| For 8-bit strings, this method is locale-dependent. |
| |
| |
| .. method:: str.isdigit() |
| |
| Return true if all characters in the string are digits and there is at least one |
| character, false otherwise. |
| |
| For 8-bit strings, this method is locale-dependent. |
| |
| |
| .. 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. |
| |
| For 8-bit strings, this method is locale-dependent. |
| |
| |
| .. method:: str.isspace() |
| |
| Return true if there are only whitespace characters in the string and there is |
| at least one character, false otherwise. |
| |
| For 8-bit strings, this method is locale-dependent. |
| |
| |
| .. 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. |
| |
| For 8-bit strings, this method is locale-dependent. |
| |
| |
| .. 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. |
| |
| For 8-bit strings, this method is locale-dependent. |
| |
| |
| .. method:: str.join(iterable) |
| |
| Return a string which is the concatenation of the strings in the |
| :term:`iterable` *iterable*. 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)``. |
| |
| .. versionchanged:: 2.4 |
| Support for the *fillchar* argument. |
| |
| |
| .. method:: str.lower() |
| |
| Return a copy of the string with all the cased characters [4]_ converted to |
| lowercase. |
| |
| For 8-bit strings, this method is locale-dependent. |
| |
| |
| .. 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' |
| |
| .. versionchanged:: 2.2.2 |
| Support for the *chars* argument. |
| |
| |
| .. 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. |
| |
| .. versionadded:: 2.5 |
| |
| |
| .. 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)``. |
| |
| .. versionchanged:: 2.4 |
| Support for the *fillchar* argument. |
| |
| |
| .. 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. |
| |
| .. versionadded:: 2.5 |
| |
| |
| .. method:: str.rsplit([sep [,maxsplit]]) |
| |
| 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. |
| |
| .. versionadded:: 2.4 |
| |
| |
| .. 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' |
| |
| .. versionchanged:: 2.2.2 |
| Support for the *chars* argument. |
| |
| |
| .. method:: str.split([sep[, maxsplit]]) |
| |
| 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 or ``-1``, 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. |
| This method uses the universal newlines approach to splitting lines. |
| Line breaks are not included in the resulting list unless *keepends* is |
| given and true. |
| |
| For example, ``'ab c\n\nde fg\rkl\r\n'.splitlines()`` returns |
| ``['ab c', '', 'de fg', 'kl']``, while the same call with ``splitlines(True)`` |
| returns ``['ab c\n', '\n, 'de fg\r', 'kl\r\n']``. |
| |
| Unlike :meth:`~str.split` when a delimiter string *sep* is given, this |
| method returns an empty list for the empty string, and a terminal line |
| break does not result in an extra line. |
| |
| |
| .. 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. |
| |
| .. versionchanged:: 2.5 |
| Accept tuples as *prefix*. |
| |
| |
| .. 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' |
| |
| .. versionchanged:: 2.2.2 |
| Support for the *chars* argument. |
| |
| |
| .. method:: str.swapcase() |
| |
| Return a copy of the string with uppercase characters converted to lowercase and |
| vice versa. |
| |
| For 8-bit strings, this method is locale-dependent. |
| |
| |
| .. 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." |
| |
| For 8-bit strings, this method is locale-dependent. |
| |
| |
| .. method:: str.translate(table[, deletechars]) |
| |
| Return a copy of the string where all characters occurring in the optional |
| argument *deletechars* are removed, and the remaining characters have been |
| mapped through the given translation table, which must be a string of length |
| 256. |
| |
| You can use the :func:`~string.maketrans` helper function in the :mod:`string` |
| module to create a translation table. For string objects, set the *table* |
| argument to ``None`` for translations that only delete characters: |
| |
| >>> 'read this short text'.translate(None, 'aeiou') |
| 'rd ths shrt txt' |
| |
| .. versionadded:: 2.6 |
| Support for a ``None`` *table* argument. |
| |
| For Unicode objects, the :meth:`translate` method does not accept the optional |
| *deletechars* argument. Instead, it returns a copy of the *s* where all |
| characters have been mapped through the given translation table which must be a |
| mapping of Unicode ordinals to Unicode ordinals, Unicode strings or ``None``. |
| Unmapped characters are left untouched. Characters mapped to ``None`` are |
| deleted. Note, a 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). |
| |
| For 8-bit strings, this method is locale-dependent. |
| |
| |
| .. 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)``. |
| |
| |
| .. versionadded:: 2.2.2 |
| |
| The following methods are present only on unicode objects: |
| |
| .. method:: unicode.isnumeric() |
| |
| Return ``True`` if there are only numeric characters in S, ``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. |
| |
| .. method:: unicode.isdecimal() |
| |
| Return ``True`` if there are only decimal characters in S, ``False`` |
| otherwise. Decimal characters include digit characters, and all characters |
| that can be used to form decimal-radix numbers, e.g. U+0660, |
| ARABIC-INDIC DIGIT ZERO. |
| |
| |
| .. _string-formatting: |
| |
| 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 |
| |
| String and Unicode 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 |
| or Unicode object), ``%`` 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* is a Unicode object, or if any |
| of the objects being converted using the ``%s`` conversion are Unicode objects, |
| the result will also be a Unicode object. |
| |
| 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 width 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 | \(6) | |
| | | :func:`str`). | | |
| +------------+-----------------------------------------------------+-------+ |
| | ``'%'`` | 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) |
| The ``%r`` conversion was added in Python 2.0. |
| |
| The precision determines the maximal number of characters used. |
| |
| (6) |
| If the object or format provided is a :class:`unicode` string, the resulting |
| string will also be :class:`unicode`. |
| |
| The precision determines the maximal number of characters used. |
| |
| (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:: 2.7 |
| ``%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-xrange: |
| |
| XRange Type |
| ----------- |
| |
| .. index:: object: xrange |
| |
| The :class:`xrange` type is an immutable sequence which is commonly used for |
| looping. The advantage of the :class:`xrange` type is that an :class:`xrange` |
| object will always take the same amount of memory, no matter the size of the |
| range it represents. There are no consistent performance advantages. |
| |
| XRange objects have very little behavior: they only support indexing, iteration, |
| and the :func:`len` function. |
| |
| |
| .. _typesseq-mutable: |
| |
| Mutable Sequence Types |
| ---------------------- |
| |
| .. index:: |
| triple: mutable; sequence; types |
| object: list |
| |
| List and :class:`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): |
| |
| .. index:: |
| triple: operations on; sequence; types |
| triple: operations on; list; type |
| pair: subscript; assignment |
| pair: slice; assignment |
| pair: extended slice; assignment |
| statement: del |
| single: append() (list method) |
| single: extend() (list method) |
| single: count() (list method) |
| single: index() (list method) |
| single: insert() (list method) |
| single: pop() (list method) |
| single: remove() (list method) |
| single: reverse() (list method) |
| single: sort() (list 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)] = | \(2) | |
| | | [x]`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.extend(x)`` | same as ``s[len(s):len(s)] = | \(3) | |
| | | x`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.count(x)`` | return number of *i*'s for | | |
| | | which ``s[i] == x`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.index(x[, i[, j]])`` | return smallest *k* such that | \(4) | |
| | | ``s[k] == x`` and ``i <= k < | | |
| | | j`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.insert(i, x)`` | same as ``s[i:i] = [x]`` | \(5) | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.pop([i])`` | same as ``x = s[i]; del s[i]; | \(6) | |
| | | return x`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.remove(x)`` | same as ``del s[s.index(x)]`` | \(4) | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.reverse()`` | reverses the items of *s* in | \(7) | |
| | | place | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.sort([cmp[, key[, | sort the items of *s* in place | (7)(8)(9)(10) | |
| | reverse]]])`` | | | |
| +------------------------------+--------------------------------+---------------------+ |
| |
| Notes: |
| |
| (1) |
| *t* must have the same length as the slice it is replacing. |
| |
| (2) |
| The C implementation of Python has historically accepted multiple parameters and |
| implicitly joined them into a tuple; this no longer works in Python 2.0. Use of |
| this misfeature has been deprecated since Python 1.4. |
| |
| (3) |
| *x* can be any iterable object. |
| |
| (4) |
| 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 list |
| length is added, as for slice indices. If it is still negative, it is truncated |
| to zero, as for slice indices. |
| |
| .. versionchanged:: 2.3 |
| Previously, :meth:`index` didn't have arguments for specifying start and stop |
| positions. |
| |
| (5) |
| When a negative index is passed as the first parameter to the :meth:`insert` |
| method, the list length is added, as for slice indices. If it is still |
| negative, it is truncated to zero, as for slice indices. |
| |
| .. versionchanged:: 2.3 |
| Previously, all negative indices were truncated to zero. |
| |
| (6) |
| The :meth:`pop` method is only supported by the list and array types. The |
| optional argument *i* defaults to ``-1``, so that by default the last item is |
| removed and returned. |
| |
| (7) |
| The :meth:`sort` and :meth:`reverse` methods modify the list in place for |
| economy of space when sorting or reversing a large list. To remind you that |
| they operate by side effect, they don't return the sorted or reversed list. |
| |
| (8) |
| The :meth:`sort` method takes optional arguments for controlling the |
| comparisons. |
| |
| *cmp* specifies a custom comparison function of two arguments (list items) which |
| should return a negative, zero or positive number depending on whether the first |
| argument is considered smaller than, equal to, or larger than the second |
| argument: ``cmp=lambda x,y: cmp(x.lower(), y.lower())``. The default value |
| is ``None``. |
| |
| *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``. |
| |
| *reverse* is a boolean value. If set to ``True``, then the list elements are |
| sorted as if each comparison were reversed. |
| |
| In general, the *key* and *reverse* conversion processes are much faster than |
| specifying an equivalent *cmp* function. This is because *cmp* is called |
| multiple times for each list element while *key* and *reverse* touch each |
| element only once. Use :func:`functools.cmp_to_key` to convert an |
| old-style *cmp* function to a *key* function. |
| |
| .. versionchanged:: 2.3 |
| Support for ``None`` as an equivalent to omitting *cmp* was added. |
| |
| .. versionchanged:: 2.4 |
| Support for *key* and *reverse* was added. |
| |
| (9) |
| Starting with Python 2.3, 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). |
| |
| (10) |
| .. 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 2.3 and |
| newer 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. |
| |
| |
| .. _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.) |
| |
| |
| .. versionadded:: 2.4 |
| |
| 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. |
| |
| As of Python 2.7, 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. |
| |
| .. versionadded:: 2.6 |
| |
| .. 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. |
| |
| .. versionchanged:: 2.6 |
| Accepts multiple input iterables. |
| |
| .. method:: intersection(other, ...) |
| set & other & ... |
| |
| Return a new set with elements common to the set and all others. |
| |
| .. versionchanged:: 2.6 |
| Accepts multiple input iterables. |
| |
| .. method:: difference(other, ...) |
| set - other - ... |
| |
| Return a new set with elements in the set that are not in the others. |
| |
| .. versionchanged:: 2.6 |
| Accepts multiple input iterables. |
| |
| .. 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``. Accordingly, sets do not implement the :meth:`__cmp__` |
| method. |
| |
| 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. |
| |
| .. versionchanged:: 2.6 |
| Accepts multiple input iterables. |
| |
| .. method:: intersection_update(other, ...) |
| set &= other & ... |
| |
| Update the set, keeping only elements found in it and all others. |
| |
| .. versionchanged:: 2.6 |
| Accepts multiple input iterables. |
| |
| .. method:: difference_update(other, ...) |
| set -= other | ... |
| |
| Update the set, removing elements found in others. |
| |
| .. versionchanged:: 2.6 |
| Accepts multiple input iterables. |
| |
| .. 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. |
| |
| |
| .. seealso:: |
| |
| :ref:`comparison-to-builtin-set` |
| Differences between the :mod:`sets` module and the built-in set types. |
| |
| |
| .. _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. |
| |
| .. versionadded:: 2.2 |
| |
| .. versionchanged:: 2.3 |
| Support for building a dictionary from keyword arguments added. |
| |
| |
| 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. |
| |
| .. versionadded:: 2.5 |
| 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. For an example, see |
| :class:`collections.defaultdict`. |
| |
| .. 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``. |
| |
| .. versionadded:: 2.2 |
| |
| .. describe:: key not in d |
| |
| Equivalent to ``not key in d``. |
| |
| .. versionadded:: 2.2 |
| |
| .. describe:: iter(d) |
| |
| Return an iterator over the keys of the dictionary. This is a shortcut |
| for :meth:`iterkeys`. |
| |
| .. method:: clear() |
| |
| Remove all items from the dictionary. |
| |
| .. method:: copy() |
| |
| Return a shallow copy of the dictionary. |
| |
| .. method:: fromkeys(seq[, value]) |
| |
| Create a new dictionary with keys from *seq* and values set to *value*. |
| |
| :func:`fromkeys` is a class method that returns a new dictionary. *value* |
| defaults to ``None``. |
| |
| .. versionadded:: 2.3 |
| |
| .. 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:: has_key(key) |
| |
| Test for the presence of *key* in the dictionary. :meth:`has_key` is |
| deprecated in favor of ``key in d``. |
| |
| .. method:: items() |
| |
| Return a copy of the dictionary's list of ``(key, value)`` pairs. |
| |
| .. impl-detail:: |
| |
| Keys and values are listed in an arbitrary order which is non-random, |
| varies across Python implementations, and depends on the dictionary's |
| history of insertions and deletions. |
| |
| If :meth:`items`, :meth:`keys`, :meth:`values`, :meth:`iteritems`, |
| :meth:`iterkeys`, and :meth:`itervalues` are called with no intervening |
| modifications to the dictionary, the lists will directly correspond. This |
| allows the creation of ``(value, key)`` pairs using :func:`zip`: ``pairs = |
| zip(d.values(), d.keys())``. The same relationship holds for the |
| :meth:`iterkeys` and :meth:`itervalues` methods: ``pairs = |
| zip(d.itervalues(), d.iterkeys())`` provides the same value for |
| ``pairs``. Another way to create the same list is ``pairs = [(v, k) for |
| (k, v) in d.iteritems()]``. |
| |
| .. method:: iteritems() |
| |
| Return an iterator over the dictionary's ``(key, value)`` pairs. See the |
| note for :meth:`dict.items`. |
| |
| Using :meth:`iteritems` while adding or deleting entries in the dictionary |
| may raise a :exc:`RuntimeError` or fail to iterate over all entries. |
| |
| .. versionadded:: 2.2 |
| |
| .. method:: iterkeys() |
| |
| Return an iterator over the dictionary's keys. See the note for |
| :meth:`dict.items`. |
| |
| Using :meth:`iterkeys` while adding or deleting entries in the dictionary |
| may raise a :exc:`RuntimeError` or fail to iterate over all entries. |
| |
| .. versionadded:: 2.2 |
| |
| .. method:: itervalues() |
| |
| Return an iterator over the dictionary's values. See the note for |
| :meth:`dict.items`. |
| |
| Using :meth:`itervalues` while adding or deleting entries in the |
| dictionary may raise a :exc:`RuntimeError` or fail to iterate over all |
| entries. |
| |
| .. versionadded:: 2.2 |
| |
| .. method:: keys() |
| |
| Return a copy of the dictionary's list of keys. See the note for |
| :meth:`dict.items`. |
| |
| .. 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. |
| |
| .. versionadded:: 2.3 |
| |
| .. method:: popitem() |
| |
| Remove and return an arbitrary ``(key, value)`` pair from the dictionary. |
| |
| :func:`popitem` is useful to destructively iterate over a dictionary, as |
| often used in set algorithms. If the dictionary is empty, calling |
| :func:`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``. |
| |
| :func:`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)``. |
| |
| .. versionchanged:: 2.4 |
| Allowed the argument to be an iterable of key/value pairs and allowed |
| keyword arguments. |
| |
| .. method:: values() |
| |
| Return a copy of the dictionary's list of values. See the note for |
| :meth:`dict.items`. |
| |
| .. method:: viewitems() |
| |
| Return a new view of the dictionary's items (``(key, value)`` pairs). See |
| below for documentation of view objects. |
| |
| .. versionadded:: 2.7 |
| |
| .. method:: viewkeys() |
| |
| Return a new view of the dictionary's keys. See below for documentation of |
| view objects. |
| |
| .. versionadded:: 2.7 |
| |
| .. method:: viewvalues() |
| |
| Return a new view of the dictionary's values. See below for documentation of |
| view objects. |
| |
| .. versionadded:: 2.7 |
| |
| |
| .. _dict-views: |
| |
| Dictionary view objects |
| ----------------------- |
| |
| The objects returned by :meth:`dict.viewkeys`, :meth:`dict.viewvalues` and |
| :meth:`dict.viewitems` 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.) Then these set operations are |
| available ("other" refers either to another view or a set): |
| |
| .. describe:: dictview & other |
| |
| Return the intersection of the dictview and the other object as a new set. |
| |
| .. describe:: dictview | other |
| |
| Return the union of the dictview and the other object as a new set. |
| |
| .. describe:: dictview - other |
| |
| Return the difference between the dictview and the other object (all elements |
| in *dictview* that aren't in *other*) as a new set. |
| |
| .. describe:: dictview ^ other |
| |
| Return the symmetric difference (all elements either in *dictview* or |
| *other*, but not in both) of the dictview and the other object as a new set. |
| |
| |
| An example of dictionary view usage:: |
| |
| >>> dishes = {'eggs': 2, 'sausage': 1, 'bacon': 1, 'spam': 500} |
| >>> keys = dishes.viewkeys() |
| >>> values = dishes.viewvalues() |
| |
| >>> # 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'} |
| |
| |
| .. _bltin-file-objects: |
| |
| File Objects |
| ============ |
| |
| .. index:: |
| object: file |
| builtin: file |
| module: os |
| module: socket |
| |
| File objects are implemented using C's ``stdio`` package and can be |
| created with the built-in :func:`open` function. File |
| objects are also returned by some other built-in functions and methods, |
| such as :func:`os.popen` and :func:`os.fdopen` and the :meth:`makefile` |
| method of socket objects. Temporary files can be created using the |
| :mod:`tempfile` module, and high-level file operations such as copying, |
| moving, and deleting files and directories can be achieved with the |
| :mod:`shutil` module. |
| |
| When a file operation fails for an I/O-related reason, the exception |
| :exc:`IOError` is raised. This includes situations where the operation is not |
| defined for some reason, like :meth:`seek` on a tty device or writing a file |
| opened for reading. |
| |
| Files have the following methods: |
| |
| |
| .. method:: file.close() |
| |
| Close the file. A closed file cannot be read or written any more. Any operation |
| which requires that the file be open will raise a :exc:`ValueError` after the |
| file has been closed. Calling :meth:`close` more than once is allowed. |
| |
| As of Python 2.5, you can avoid having to call this method explicitly if you use |
| the :keyword:`with` statement. For example, the following code will |
| automatically close *f* when the :keyword:`with` block is exited:: |
| |
| from __future__ import with_statement # This isn't required in Python 2.6 |
| |
| with open("hello.txt") as f: |
| for line in f: |
| print line |
| |
| In older versions of Python, you would have needed to do this to get the same |
| effect:: |
| |
| f = open("hello.txt") |
| try: |
| for line in f: |
| print line |
| finally: |
| f.close() |
| |
| .. note:: |
| |
| Not all "file-like" types in Python support use as a context manager for the |
| :keyword:`with` statement. If your code is intended to work with any file-like |
| object, you can use the function :func:`contextlib.closing` instead of using |
| the object directly. |
| |
| |
| .. method:: file.flush() |
| |
| Flush the internal buffer, like ``stdio``'s :c:func:`fflush`. This may be a |
| no-op on some file-like objects. |
| |
| .. note:: |
| |
| :meth:`flush` does not necessarily write the file's data to disk. Use |
| :meth:`flush` followed by :func:`os.fsync` to ensure this behavior. |
| |
| |
| .. method:: file.fileno() |
| |
| .. index:: |
| pair: file; descriptor |
| module: fcntl |
| |
| Return the integer "file descriptor" that is used by the underlying |
| implementation to request I/O operations from the operating system. This can be |
| useful for other, lower level interfaces that use file descriptors, such as the |
| :mod:`fcntl` module or :func:`os.read` and friends. |
| |
| .. note:: |
| |
| File-like objects which do not have a real file descriptor should *not* provide |
| this method! |
| |
| |
| .. method:: file.isatty() |
| |
| Return ``True`` if the file is connected to a tty(-like) device, else ``False``. |
| |
| .. note:: |
| |
| If a file-like object is not associated with a real file, this method should |
| *not* be implemented. |
| |
| |
| .. method:: file.next() |
| |
| A file object is its own iterator, for example ``iter(f)`` returns *f* (unless |
| *f* is closed). When a file is used as an iterator, typically in a |
| :keyword:`for` loop (for example, ``for line in f: print line``), the |
| :meth:`~file.next` method is called repeatedly. This method returns the next input |
| line, or raises :exc:`StopIteration` when EOF is hit when the file is open for |
| reading (behavior is undefined when the file is open for writing). In order to |
| make a :keyword:`for` loop the most efficient way of looping over the lines of a |
| file (a very common operation), the :meth:`~file.next` method uses a hidden read-ahead |
| buffer. As a consequence of using a read-ahead buffer, combining :meth:`~file.next` |
| with other file methods (like :meth:`~file.readline`) does not work right. However, |
| using :meth:`seek` to reposition the file to an absolute position will flush the |
| read-ahead buffer. |
| |
| .. versionadded:: 2.3 |
| |
| |
| .. method:: file.read([size]) |
| |
| Read at most *size* bytes from the file (less if the read hits EOF before |
| obtaining *size* bytes). If the *size* argument is negative or omitted, read |
| all data until EOF is reached. The bytes are returned as a string object. An |
| empty string is returned when EOF is encountered immediately. (For certain |
| files, like ttys, it makes sense to continue reading after an EOF is hit.) Note |
| that this method may call the underlying C function :c:func:`fread` more than |
| once in an effort to acquire as close to *size* bytes as possible. Also note |
| that when in non-blocking mode, less data than was requested may be |
| returned, even if no *size* parameter was given. |
| |
| .. note:: |
| This function is simply a wrapper for the underlying |
| :c:func:`fread` C function, and will behave the same in corner cases, |
| such as whether the EOF value is cached. |
| |
| |
| .. method:: file.readline([size]) |
| |
| Read one entire line from the file. A trailing newline character is kept in |
| the string (but may be absent when a file ends with an incomplete line). [6]_ |
| If the *size* argument is present and non-negative, it is a maximum byte |
| count (including the trailing newline) and an incomplete line may be |
| returned. When *size* is not 0, an empty string is returned *only* when EOF |
| is encountered immediately. |
| |
| .. note:: |
| |
| Unlike ``stdio``'s :c:func:`fgets`, the returned string contains null characters |
| (``'\0'``) if they occurred in the input. |
| |
| |
| .. method:: file.readlines([sizehint]) |
| |
| Read until EOF using :meth:`~file.readline` and return a list containing the lines |
| thus read. If the optional *sizehint* argument is present, instead of |
| reading up to EOF, whole lines totalling approximately *sizehint* bytes |
| (possibly after rounding up to an internal buffer size) are read. Objects |
| implementing a file-like interface may choose to ignore *sizehint* if it |
| cannot be implemented, or cannot be implemented efficiently. |
| |
| |
| .. method:: file.xreadlines() |
| |
| This method returns the same thing as ``iter(f)``. |
| |
| .. versionadded:: 2.1 |
| |
| .. deprecated:: 2.3 |
| Use ``for line in file`` instead. |
| |
| |
| .. method:: file.seek(offset[, whence]) |
| |
| Set the file's current position, like ``stdio``'s :c:func:`fseek`. The *whence* |
| argument is optional and defaults to ``os.SEEK_SET`` or ``0`` (absolute file |
| positioning); other values are ``os.SEEK_CUR`` or ``1`` (seek relative to the |
| current position) and ``os.SEEK_END`` or ``2`` (seek relative to the file's |
| end). There is no return value. |
| |
| For example, ``f.seek(2, os.SEEK_CUR)`` advances the position by two and |
| ``f.seek(-3, os.SEEK_END)`` sets the position to the third to last. |
| |
| Note that if the file is opened for appending |
| (mode ``'a'`` or ``'a+'``), any :meth:`seek` operations will be undone at the |
| next write. If the file is only opened for writing in append mode (mode |
| ``'a'``), this method is essentially a no-op, but it remains useful for files |
| opened in append mode with reading enabled (mode ``'a+'``). If the file is |
| opened in text mode (without ``'b'``), only offsets returned by :meth:`tell` are |
| legal. Use of other offsets causes undefined behavior. |
| |
| Note that not all file objects are seekable. |
| |
| .. versionchanged:: 2.6 |
| Passing float values as offset has been deprecated. |
| |
| |
| .. method:: file.tell() |
| |
| Return the file's current position, like ``stdio``'s :c:func:`ftell`. |
| |
| .. note:: |
| |
| On Windows, :meth:`tell` can return illegal values (after an :c:func:`fgets`) |
| when reading files with Unix-style line-endings. Use binary mode (``'rb'``) to |
| circumvent this problem. |
| |
| |
| .. method:: file.truncate([size]) |
| |
| Truncate the file's size. If the optional *size* argument is present, the file |
| is truncated to (at most) that size. The size defaults to the current position. |
| The current file position is not changed. Note that if a specified size exceeds |
| the file's current size, the result is platform-dependent: possibilities |
| include that the file may remain unchanged, increase to the specified size as if |
| zero-filled, or increase to the specified size with undefined new content. |
| Availability: Windows, many Unix variants. |
| |
| |
| .. method:: file.write(str) |
| |
| Write a string to the file. There is no return value. Due to buffering, the |
| string may not actually show up in the file until the :meth:`flush` or |
| :meth:`close` method is called. |
| |
| |
| .. method:: file.writelines(sequence) |
| |
| Write a sequence of strings to the file. The sequence can be any iterable |
| object producing strings, typically a list of strings. There is no return value. |
| (The name is intended to match :meth:`readlines`; :meth:`writelines` does not |
| add line separators.) |
| |
| Files support the iterator protocol. Each iteration returns the same result as |
| :meth:`~file.readline`, and iteration ends when the :meth:`~file.readline` method returns |
| an empty string. |
| |
| File objects also offer a number of other interesting attributes. These are not |
| required for file-like objects, but should be implemented if they make sense for |
| the particular object. |
| |
| |
| .. attribute:: file.closed |
| |
| bool indicating the current state of the file object. This is a read-only |
| attribute; the :meth:`close` method changes the value. It may not be available |
| on all file-like objects. |
| |
| |
| .. attribute:: file.encoding |
| |
| The encoding that this file uses. When Unicode strings are written to a file, |
| they will be converted to byte strings using this encoding. In addition, when |
| the file is connected to a terminal, the attribute gives the encoding that the |
| terminal is likely to use (that information might be incorrect if the user has |
| misconfigured the terminal). The attribute is read-only and may not be present |
| on all file-like objects. It may also be ``None``, in which case the file uses |
| the system default encoding for converting Unicode strings. |
| |
| .. versionadded:: 2.3 |
| |
| |
| .. attribute:: file.errors |
| |
| The Unicode error handler used along with the encoding. |
| |
| .. versionadded:: 2.6 |
| |
| |
| .. attribute:: file.mode |
| |
| The I/O mode for the file. If the file was created using the :func:`open` |
| built-in function, this will be the value of the *mode* parameter. This is a |
| read-only attribute and may not be present on all file-like objects. |
| |
| |
| .. attribute:: file.name |
| |
| If the file object was created using :func:`open`, the name of the file. |
| Otherwise, some string that indicates the source of the file object, of the |
| form ``<...>``. This is a read-only attribute and may not be present on all |
| file-like objects. |
| |
| |
| .. attribute:: file.newlines |
| |
| If Python was built with universal newlines enabled (the default) this |
| read-only attribute exists, and for files opened in universal newline read |
| mode it keeps track of the types of newlines encountered while reading the |
| file. The values it can take are ``'\r'``, ``'\n'``, ``'\r\n'``, ``None`` |
| (unknown, no newlines read yet) or a tuple containing all the newline types |
| seen, to indicate that multiple newline conventions were encountered. For |
| files not opened in universal newline read mode the value of this attribute |
| will be ``None``. |
| |
| |
| .. attribute:: file.softspace |
| |
| Boolean that indicates whether a space character needs to be printed before |
| another value when using the :keyword:`print` statement. Classes that are trying |
| to simulate a file object should also have a writable :attr:`softspace` |
| attribute, which should be initialized to zero. This will be automatic for most |
| classes implemented in Python (care may be needed for objects that override |
| attribute access); types implemented in C will have to provide a writable |
| :attr:`softspace` attribute. |
| |
| .. note:: |
| |
| This attribute is not used to control the :keyword:`print` statement, but to |
| allow the implementation of :keyword:`print` to keep track of its internal |
| state. |
| |
| |
| .. _typememoryview: |
| |
| memoryview type |
| =============== |
| |
| .. versionadded:: 2.7 |
| |
| :class:`memoryview` objects allow Python code to access the internal data |
| of an object that supports the buffer protocol without copying. Memory |
| is generally interpreted as simple bytes. |
| |
| .. 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:`str` and :class:`bytearray` (but not :class:`unicode`). |
| |
| 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:`str` and :class:`bytearray`, an element |
| is a single byte, but other third-party types may expose larger elements. |
| |
| ``len(view)`` returns the total number of elements in the memoryview, |
| *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. Taking a single |
| index will return a single element as a :class:`str` object. Full |
| slicing will result in a subview:: |
| |
| >>> v = memoryview('abcefg') |
| >>> v[1] |
| 'b' |
| >>> v[-1] |
| 'g' |
| >>> v[1:4] |
| <memory at 0x77ab28> |
| >>> v[1:4].tobytes() |
| 'bce' |
| |
| If the object the memoryview is over supports changing its data, the |
| memoryview supports slice assignment:: |
| |
| >>> data = bytearray('abcefg') |
| >>> v = memoryview(data) |
| >>> v.readonly |
| False |
| >>> v[0] = 'z' |
| >>> data |
| bytearray(b'zbcefg') |
| >>> v[1:4] = '123' |
| >>> data |
| bytearray(b'z123fg') |
| >>> v[2] = 'spam' |
| Traceback (most recent call last): |
| File "<stdin>", line 1, in <module> |
| ValueError: cannot modify size of memoryview object |
| |
| Notice how the size of the memoryview object cannot be changed. |
| |
| :class:`memoryview` has two methods: |
| |
| .. method:: tobytes() |
| |
| Return the data in the buffer as a bytestring (an object of class |
| :class:`str`). :: |
| |
| >>> m = memoryview("abc") |
| >>> m.tobytes() |
| 'abc' |
| |
| .. method:: tolist() |
| |
| Return the data in the buffer as a list of integers. :: |
| |
| >>> memoryview("abc").tolist() |
| [97, 98, 99] |
| |
| There are also several readonly attributes available: |
| |
| .. attribute:: format |
| |
| A string containing the format (in :mod:`struct` module style) for each |
| element in the view. This defaults to ``'B'``, a simple bytestring. |
| |
| .. attribute:: itemsize |
| |
| The size in bytes of each element of the memoryview. |
| |
| .. attribute:: shape |
| |
| A tuple of integers the length of :attr:`ndim` giving the shape of the |
| memory as a N-dimensional array. |
| |
| .. attribute:: ndim |
| |
| An integer indicating how many dimensions of a multi-dimensional array the |
| memory represents. |
| |
| .. 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:: readonly |
| |
| A bool indicating whether the memory is read only. |
| |
| .. memoryview.suboffsets isn't documented because it only seems useful for C |
| |
| |
| .. _typecontextmanager: |
| |
| Context Manager Types |
| ===================== |
| |
| .. versionadded:: 2.5 |
| |
| .. 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 two separate 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. |
| |
| The :dfn:`context management protocol` consists of a pair of methods that need |
| to be provided for a context manager object to define a runtime context: |
| |
| |
| .. 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 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 ``contextlib.contextmanager`` :term:`decorator` |
| provide a convenient way to implement these protocols. If a generator function is |
| decorated with the ``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. |
| |
| The implementation adds two special read-only attributes to class instance |
| methods: ``m.im_self`` is the object on which the method operates, and |
| ``m.im_func`` is the function implementing the method. Calling ``m(arg-1, |
| arg-2, ..., arg-n)`` is completely equivalent to calling ``m.im_func(m.im_self, |
| arg-1, arg-2, ..., arg-n)``. |
| |
| Class instance methods are either *bound* or *unbound*, referring to whether the |
| method was accessed through an instance or a class, respectively. When a method |
| is unbound, its ``im_self`` attribute will be ``None`` and if called, an |
| explicit ``self`` object must be passed as the first argument. In this case, |
| ``self`` must be an instance of the unbound method's class (or a subclass of |
| that class), otherwise a :exc:`TypeError` is raised. |
| |
| Like function objects, methods objects support getting arbitrary attributes. |
| However, since method attributes are actually stored on the underlying function |
| object (``meth.im_func``), setting method attributes on either bound or unbound |
| 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.im_func.whoami = 'my name is c' |
| |
| See :ref:`types` for more information. |
| |
| |
| .. _bltin-code-objects: |
| |
| Code Objects |
| ------------ |
| |
| .. index:: object: code |
| |
| .. index:: |
| builtin: compile |
| single: func_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:`func_code` |
| attribute. See also the :mod:`code` module. |
| |
| .. index:: |
| statement: exec |
| builtin: eval |
| |
| A code object can be executed or evaluated by passing it (instead of a source |
| string) to the :keyword:`exec` statement or the built-in :func:`eval` function. |
| |
| 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: ``<type '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). |
| |
| It is written as ``None``. |
| |
| |
| .. _bltin-ellipsis-object: |
| |
| The Ellipsis Object |
| ------------------- |
| |
| This object is used by extended slice notation (see :ref:`slicings`). It |
| supports no special operations. There is exactly one ellipsis object, named |
| :const:`Ellipsis` (a built-in name). |
| |
| It is written as ``Ellipsis``. When in a subscript, it can also be written as |
| ``...``, for example ``seq[...]``. |
| |
| |
| 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. |
| |
| It is written as ``NotImplemented``. |
| |
| |
| 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:: object.__methods__ |
| |
| .. deprecated:: 2.2 |
| Use the built-in function :func:`dir` to get a list of an object's attributes. |
| This attribute is no longer available. |
| |
| |
| .. attribute:: object.__members__ |
| |
| .. deprecated:: 2.2 |
| Use the built-in function :func:`dir` to get a list of an object's attributes. |
| This attribute is no longer available. |
| |
| |
| .. 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. |
| |
| |
| The following attributes are only supported by :term:`new-style class`\ es. |
| |
| .. 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 new-style 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__() |
| [<type '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. |
| |
| .. [6] The advantage of leaving the newline on is that returning an empty string is |
| then an unambiguous EOF indication. It is also possible (in cases where it |
| might matter, for example, if you want to make an exact copy of a file while |
| scanning its lines) to tell whether the last line of a file ended in a newline |
| or not (yes this happens!). |