| .. XXX: reference/datamodel and this have quite a few overlaps! |
| |
| |
| .. _bltin-types: |
| |
| ************** |
| Built-in Types |
| ************** |
| |
| The following sections describe the standard types that are built into the |
| interpreter. |
| |
| .. index:: pair: built-in; types |
| |
| The principal built-in types are numerics, sequences, mappings, files, classes, |
| instances and exceptions. |
| |
| Some operations are supported by several object types; in particular, |
| practically all objects can be compared, tested for truth value, and converted |
| to a string (with the :func:`repr` function or the slightly different |
| :func:`str` function). The latter function is implicitly used when an object is |
| written by the :func:`print` function. |
| |
| |
| .. _truth: |
| |
| Truth Value Testing |
| =================== |
| |
| .. index:: |
| statement: if |
| statement: while |
| pair: truth; value |
| pair: Boolean; operations |
| single: false |
| |
| Any object can be tested for truth value, for use in an :keyword:`if` or |
| :keyword:`while` condition or as operand of the Boolean operations below. The |
| following values are considered false: |
| |
| .. index:: single: None (Built-in object) |
| |
| * ``None`` |
| |
| .. index:: single: False (Built-in object) |
| |
| * ``False`` |
| |
| * zero of any numeric type, for example, ``0``, ``0.0``, ``0j``. |
| |
| * any empty sequence, for example, ``''``, ``()``, ``[]``. |
| |
| * any empty mapping, for example, ``{}``. |
| |
| * instances of user-defined classes, if the class defines a :meth:`__bool__` or |
| :meth:`__len__` method, when that method returns the integer zero or |
| :class:`bool` value ``False``. [#]_ |
| |
| .. 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 |
| |
| There are eight comparison operations in Python. They all have the same |
| priority (which is higher than that of the Boolean operations). Comparisons can |
| be chained arbitrarily; for example, ``x < y <= z`` is equivalent to ``x < y and |
| y <= z``, except that *y* is evaluated only once (but in both cases *z* is not |
| evaluated at all when ``x < y`` is found to be false). |
| |
| .. index:: |
| pair: operator; comparison |
| operator: == |
| operator: < |
| operator: > |
| operator: <= |
| operator: >= |
| operator: != |
| operator: is |
| operator: is not |
| |
| This table summarizes the comparison operations: |
| |
| +------------+-------------------------+ |
| | Operation | Meaning | |
| +============+=========================+ |
| | ``<`` | strictly less than | |
| +------------+-------------------------+ |
| | ``<=`` | less than or equal | |
| +------------+-------------------------+ |
| | ``>`` | strictly greater than | |
| +------------+-------------------------+ |
| | ``>=`` | greater than or equal | |
| +------------+-------------------------+ |
| | ``==`` | equal | |
| +------------+-------------------------+ |
| | ``!=`` | not equal | |
| +------------+-------------------------+ |
| | ``is`` | object identity | |
| +------------+-------------------------+ |
| | ``is not`` | negated object identity | |
| +------------+-------------------------+ |
| |
| .. index:: |
| pair: object; numeric |
| pair: objects; comparing |
| |
| Objects of different types, except different numeric types, never compare equal. |
| Furthermore, some types (for example, file objects) support only a degenerate |
| notion of comparison where any two objects of that type are unequal. The ``<``, |
| ``<=``, ``>`` and ``>=`` operators will raise a :exc:`TypeError` exception when |
| any operand is a complex number, the objects are of different types that cannot |
| be compared, or other cases where there is no defined ordering. |
| |
| .. index:: |
| single: __cmp__() (instance method) |
| single: __eq__() (instance method) |
| single: __ne__() (instance method) |
| single: __lt__() (instance method) |
| single: __le__() (instance method) |
| single: __gt__() (instance method) |
| single: __ge__() (instance method) |
| |
| Instances of a class normally compare as non-equal unless the class defines the |
| :meth:`__eq__` or :meth:`__cmp__` method. |
| |
| Instances of a class cannot be ordered with respect to other instances of the |
| same class, or other types of object, unless the class defines enough of the |
| methods :meth:`__cmp__`, :meth:`__lt__`, :meth:`__le__`, :meth:`__gt__`, and |
| :meth:`__ge__` (in general, either :meth:`__cmp__` or both :meth:`__lt__` and |
| :meth:`__eq__` are sufficient, if you want the conventional meanings of the |
| comparison operators). |
| |
| The behavior of the :keyword:`is` and :keyword:`is not` operators cannot be |
| customized; also they can be applied to any two objects and never raise an |
| exception. |
| |
| .. index:: |
| operator: in |
| operator: not in |
| |
| Two more operations with the same syntactic priority, ``in`` and ``not in``, are |
| supported only by sequence types (below). |
| |
| |
| .. _typesnumeric: |
| |
| Numeric Types --- :class:`int`, :class:`float`, :class:`complex` |
| ================================================================ |
| |
| .. index:: |
| object: numeric |
| object: Boolean |
| object: integer |
| object: floating point |
| object: complex number |
| pair: C; language |
| |
| There are three distinct numeric types: :dfn:`integers`, :dfn:`floating |
| point numbers`, and :dfn:`complex numbers`. In addition, Booleans are a |
| subtype of integers. Integers have unlimited precision. Floating point |
| numbers are implemented using :ctype:`double` in C---all bets on their |
| precision are off unless you happen to know the machine you are working |
| with. Complex numbers have a real and imaginary part, which are each |
| implemented using :ctype:`double` in C. To extract these parts from a |
| complex number *z*, use ``z.real`` and ``z.imag``. (The standard library |
| includes additional numeric types, :mod:`fractions` that hold rationals, |
| and :mod:`decimal` that hold floating-point numbers with user-definable |
| precision.) |
| |
| .. index:: |
| pair: numeric; literals |
| pair: integer; literals |
| pair: floating point; literals |
| pair: complex number; literals |
| pair: hexadecimal; literals |
| pair: octal; literals |
| pair: binary; literals |
| |
| Numbers are created by numeric literals or as the result of built-in functions |
| and operators. Unadorned integer literals (including hex, octal and binary |
| numbers) yield integers. Numeric literals containing a decimal point or an |
| exponent sign yield floating point numbers. Appending ``'j'`` or ``'J'`` to a |
| numeric literal yields an imaginary number (a complex number with a zero real |
| part) which you can add to an integer or float to get a complex number with real |
| and imaginary parts. |
| |
| .. index:: |
| single: arithmetic |
| builtin: int |
| builtin: float |
| builtin: complex |
| |
| Python fully supports mixed arithmetic: when a binary arithmetic operator has |
| operands of different numeric types, the operand with the "narrower" type is |
| widened to that of the other, where integer is narrower than floating point, |
| which is narrower than complex. Comparisons between numbers of mixed type use |
| the same rule. [#]_ The constructors :func:`int`, :func:`float`, and |
| :func:`complex` can be used to produce numbers of a specific type. |
| |
| All numeric types (except complex) support the following operations, sorted by |
| ascending priority (operations in the same box have the same priority; all |
| numeric operations have a higher priority than comparison operations): |
| |
| +---------------------+---------------------------------+-------+--------------------+ |
| | Operation | Result | Notes | Full documentation | |
| +=====================+=================================+=======+====================+ |
| | ``x + y`` | sum of *x* and *y* | | | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``x - y`` | difference of *x* and *y* | | | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``x * y`` | product of *x* and *y* | | | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``x / y`` | quotient of *x* and *y* | | | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``x // y`` | floored quotient of *x* and | \(1) | | |
| | | *y* | | | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``x % y`` | remainder of ``x / y`` | \(2) | | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``-x`` | *x* negated | | | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``+x`` | *x* unchanged | | | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``abs(x)`` | absolute value or magnitude of | | :func:`abs` | |
| | | *x* | | | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``int(x)`` | *x* converted to integer | \(3) | :func:`int` | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``float(x)`` | *x* converted to floating point | \(4) | :func:`float` | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``complex(re, im)`` | a complex number with real part | | :func:`complex` | |
| | | *re*, imaginary part *im*. | | | |
| | | *im* defaults to zero. | | | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``c.conjugate()`` | conjugate of the complex number | | | |
| | | *c* | | | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``divmod(x, y)`` | the pair ``(x // y, x % y)`` | \(2) | :func:`divmod` | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``pow(x, y)`` | *x* to the power *y* | \(5) | :func:`pow` | |
| +---------------------+---------------------------------+-------+--------------------+ |
| | ``x ** y`` | *x* to the power *y* | \(5) | | |
| +---------------------+---------------------------------+-------+--------------------+ |
| |
| .. index:: |
| triple: operations on; numeric; types |
| single: conjugate() (complex number method) |
| |
| Notes: |
| |
| (1) |
| Also referred to as integer division. The resultant value is a whole |
| integer, though the result's type is not necessarily int. The result is |
| always rounded towards minus infinity: ``1//2`` is ``0``, ``(-1)//2`` is |
| ``-1``, ``1//(-2)`` is ``-1``, and ``(-1)//(-2)`` is ``0``. |
| |
| (2) |
| Not for complex numbers. Instead convert to floats using :func:`abs` if |
| appropriate. |
| |
| (3) |
| .. index:: |
| module: math |
| single: floor() (in module math) |
| single: ceil() (in module math) |
| pair: numeric; conversions |
| pair: C; language |
| |
| Conversion from floating point to integer may round or truncate |
| as in C; see functions :func:`floor` and :func:`ceil` in the :mod:`math` module |
| for well-defined conversions. |
| |
| (4) |
| float also accepts the strings "nan" and "inf" with an optional prefix "+" |
| or "-" for Not a Number (NaN) and positive or negative infinity. |
| |
| (5) |
| Python defines ``pow(0, 0)`` and ``0 ** 0`` to be ``1``, as is common for |
| programming languages. |
| |
| |
| |
| All :class:`numbers.Real` types (:class:`int` and |
| :class:`float`) also include the following operations: |
| |
| +--------------------+--------------------------------+--------+ |
| | Operation | Result | Notes | |
| +====================+================================+========+ |
| | ``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 <= *x* | | |
| +--------------------+--------------------------------+--------+ |
| | ``math.ceil(x)`` | the least Integral >= *x* | | |
| +--------------------+--------------------------------+--------+ |
| |
| For additional numeric operations see the :mod:`math` and :mod:`cmath` |
| modules. |
| |
| .. XXXJH exceptions: overflow (when? what operations?) zerodivision |
| |
| |
| .. _bitstring-ops: |
| |
| Bit-string Operations on Integer Types |
| -------------------------------------- |
| |
| .. _bit-string-operations: |
| |
| Integers support additional operations that make sense only for bit-strings. |
| 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 bit-string 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 | | |
| +------------+--------------------------------+----------+ |
| |
| .. index:: |
| triple: operations on; integer; types |
| pair: bit-string; operations |
| pair: shifting; operations |
| pair: masking; operations |
| |
| Notes: |
| |
| (1) |
| Negative shift counts are illegal and cause a :exc:`ValueError` to be raised. |
| |
| (2) |
| A left shift by *n* bits is equivalent to multiplication by ``pow(2, n)`` |
| without overflow check. |
| |
| (3) |
| A right shift by *n* bits is equivalent to division by ``pow(2, n)`` without |
| overflow check. |
| |
| |
| .. _typeiter: |
| |
| Iterator Types |
| ============== |
| |
| .. index:: |
| single: iterator protocol |
| single: protocol; iterator |
| single: sequence; iteration |
| single: container; iteration over |
| |
| Python supports a concept of iteration over containers. This is implemented |
| using two distinct methods; these are used to allow user-defined classes to |
| support iteration. Sequences, described below in more detail, always support |
| the iteration methods. |
| |
| One method needs to be defined for container objects to provide iteration |
| support: |
| |
| .. XXX duplicated in reference/datamodel! |
| |
| .. method:: container.__iter__() |
| |
| Return an iterator object. The object is required to support the iterator |
| protocol described below. If a container supports different types of |
| iteration, additional methods can be provided to specifically request |
| iterators for those iteration types. (An example of an object supporting |
| multiple forms of iteration would be a tree structure which supports both |
| breadth-first and depth-first traversal.) This method corresponds to the |
| :attr:`tp_iter` slot of the type structure for Python objects in the Python/C |
| API. |
| |
| The iterator objects themselves are required to support the following two |
| methods, which together form the :dfn:`iterator protocol`: |
| |
| |
| .. method:: iterator.__iter__() |
| |
| Return the iterator object itself. This is required to allow both containers |
| and iterators to be used with the :keyword:`for` and :keyword:`in` statements. |
| This method corresponds to the :attr:`tp_iter` slot of the type structure for |
| Python objects in the Python/C API. |
| |
| |
| .. method:: iterator.__next__() |
| |
| Return the next item from the container. If there are no further items, raise |
| the :exc:`StopIteration` exception. This method corresponds to the |
| :attr:`tp_iternext` slot of the type structure for Python objects in the |
| Python/C API. |
| |
| Python defines several iterator objects to support iteration over general and |
| specific sequence types, dictionaries, and other more specialized forms. The |
| specific types are not important beyond their implementation of the iterator |
| protocol. |
| |
| Once an iterator's :meth:`__next__` method raises :exc:`StopIteration`, it must |
| continue to do so on subsequent calls. Implementations that do not obey this |
| property are deemed broken. |
| |
| 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. |
| |
| |
| .. _typesseq: |
| |
| Sequence Types --- :class:`str`, :class:`bytes`, :class:`bytearray`, :class:`list`, :class:`tuple`, :class:`range` |
| ================================================================================================================== |
| |
| There are five sequence types: strings, byte sequences, byte arrays, lists, |
| tuples, and range objects. (For other containers see the built-in |
| :class:`dict`, :class:`list`, :class:`set`, and :class:`tuple` classes, and the |
| :mod:`collections` module.) |
| |
| .. index:: |
| object: sequence |
| object: string |
| object: bytes |
| object: buffer |
| object: tuple |
| object: list |
| object: range |
| |
| Strings contain Unicode characters. Their literals are written in single or |
| double quotes: ``'xyzzy'``, ``"frobozz"``. See :ref:`strings` for more about |
| string literals. In addition to the functionality described here, there are |
| also string-specific methods described in the :ref:`string-methods` section. |
| |
| Bytes and bytearray objects contain single bytes -- the former is immutable |
| while the latter is a mutable sequence. Bytes objects can be constructed the |
| constructor, :func:`bytes`, and from literals; use a ``b`` prefix with normal |
| string syntax: ``b'xyzzy'``. To construct byte arrays, use the |
| :func:`bytearray` function. |
| |
| .. warning:: |
| |
| While string objects are sequences of characters (represented by strings of |
| length 1), bytes and bytearray objects are sequences of *integers* (between 0 |
| and 255), representing the ASCII value of single bytes. That means that for |
| a bytes or bytearray object *b*, ``b[0]`` will be an integer, while |
| ``b[0:1]`` will be a bytes or bytearray object of length 1. The |
| representation of bytes objects uses the literal format (``b'...'``) since it |
| is generally more useful than e.g. ``bytes([50, 19, 100])``. You can always |
| convert a bytes object into a list of integers using ``list(b)``. |
| |
| Also, while in previous Python versions, byte strings and Unicode strings |
| could be exchanged for each other rather freely (barring encoding issues), |
| strings and bytes are now completely separate concepts. There's no implicit |
| en-/decoding if you pass and object of the wrong type. A string always |
| compares unequal to a bytes or bytearray object. |
| |
| Lists are constructed with square brackets, separating items with commas: ``[a, |
| b, c]``. Tuples are constructed by the comma operator (not within square |
| brackets), with or without enclosing parentheses, but an empty tuple must have |
| the enclosing parentheses, such as ``a, b, c`` or ``()``. A single item tuple |
| must have a trailing comma, such as ``(d,)``. |
| |
| Objects of type range are created using the :func:`range` function. They don't |
| support 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. |
| [#]_ 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* | | |
| +------------------+--------------------------------+----------+ |
| |
| Sequence types also support comparisons. In particular, tuples and lists are |
| compared lexicographically by comparing corresponding elements. This means that |
| to compare equal, every element must compare equal and the two sequences must be |
| of the same type and have the same length. (For full details see |
| :ref:`comparisons` in the language reference.) |
| |
| .. index:: |
| triple: operations on; sequence; types |
| builtin: len |
| builtin: min |
| builtin: max |
| pair: concatenation; operation |
| pair: repetition; operation |
| pair: subscript; operation |
| pair: slice; operation |
| operator: in |
| operator: not in |
| |
| Notes: |
| |
| (1) |
| When *s* is a string object, the ``in`` and ``not in`` operations act like a |
| substring test. |
| |
| (2) |
| Values of *n* less than ``0`` are treated as ``0`` (which yields an empty |
| sequence of the same type as *s*). Note also that the copies are shallow; |
| nested structures are not copied. This often haunts new Python programmers; |
| consider: |
| |
| >>> lists = [[]] * 3 |
| >>> lists |
| [[], [], []] |
| >>> lists[0].append(3) |
| >>> lists |
| [[3], [3], [3]] |
| |
| What has happened is that ``[[]]`` is a one-element list containing an empty |
| list, so all three elements of ``[[]] * 3`` are (pointers to) this single empty |
| list. Modifying any of the elements of ``lists`` modifies this single list. |
| You can create a list of different lists this way: |
| |
| >>> lists = [[] for i in range(3)] |
| >>> lists[0].append(3) |
| >>> lists[1].append(5) |
| >>> lists[2].append(7) |
| >>> lists |
| [[3], [5], [7]] |
| |
| (3) |
| If *i* or *j* is negative, the index is relative to the end of the string: |
| ``len(s) + i`` or ``len(s) + j`` is substituted. But note that ``-0`` is |
| still ``0``. |
| |
| (4) |
| The slice of *s* from *i* to *j* is defined as the sequence of items with index |
| *k* such that ``i <= k < j``. If *i* or *j* is greater than ``len(s)``, use |
| ``len(s)``. If *i* is omitted or ``None``, use ``0``. If *j* is omitted or |
| ``None``, use ``len(s)``. If *i* is greater than or equal to *j*, the slice is |
| empty. |
| |
| (5) |
| The slice of *s* from *i* to *j* with step *k* is defined as the sequence of |
| items with index ``x = i + n*k`` such that ``0 <= n < (j-i)/k``. In other words, |
| the indices are ``i``, ``i+k``, ``i+2*k``, ``i+3*k`` and so on, stopping when |
| *j* is reached (but never including *j*). If *i* or *j* is greater than |
| ``len(s)``, use ``len(s)``. If *i* or *j* are omitted or ``None``, they become |
| "end" values (which end depends on the sign of *k*). Note, *k* cannot be zero. |
| If *k* is ``None``, it is treated like ``1``. |
| |
| (6) |
| 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. |
| |
| |
| .. _string-methods: |
| |
| String Methods |
| -------------- |
| |
| .. index:: pair: string; methods |
| |
| String objects support the methods listed below. Note that none of these |
| methods take keyword arguments. |
| |
| In addition, Python's strings support the sequence type methods described in |
| the :ref:`typesseq` section. To output formatted strings, see the |
| :ref:`string-formatting` section. Also, see the :mod:`re` module for string |
| functions based on regular expressions. |
| |
| .. method:: str.capitalize() |
| |
| Return a copy of the string with only its first character capitalized. |
| |
| |
| .. method:: str.center(width[, fillchar]) |
| |
| Return centered in a string of length *width*. Padding is done using the |
| specified *fillchar* (default is a space). |
| |
| |
| .. method:: str.count(sub[, start[, end]]) |
| |
| Return the number of occurrences of substring *sub* in the range [*start*, |
| *end*]. Optional arguments *start* and *end* are interpreted as in slice |
| notation. |
| |
| |
| .. 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`. |
| |
| |
| .. method:: str.endswith(suffix[, start[, end]]) |
| |
| Return ``True`` if the string ends with the specified *suffix*, otherwise return |
| ``False``. *suffix* can also be a tuple of suffixes to look for. With optional |
| *start*, test beginning at that position. With optional *end*, stop comparing |
| at that position. |
| |
| |
| .. method:: str.expandtabs([tabsize]) |
| |
| Return a copy of the string where all tab characters are replaced by 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 range [*start*, *end*]. Optional arguments *start* |
| and *end* are interpreted as in slice notation. Return ``-1`` if *sub* is not |
| found. |
| |
| |
| .. method:: str.format(format_string, *args, **kwargs) |
| |
| Perform a string formatting operation. The *format_string* argument 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 *format_string* where |
| each replacement field is replaced with the string value of the corresponding |
| argument. |
| |
| >>> "The sum of 1 + 2 is {0}".format(1+2) |
| 'The sum of 1 + 2 is 3' |
| |
| See :ref:`formatstrings` for a description of the various formatting options |
| that can be specified in format strings. |
| |
| |
| .. method:: str.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. |
| |
| |
| .. method:: str.isalpha() |
| |
| Return true if all characters in the string are alphabetic and there is at least |
| one character, false otherwise. |
| |
| |
| .. method:: str.isdecimal() |
| |
| Return true if all characters in the string are decimal |
| characters and there is at least one character, false |
| otherwise. Decimal characters include digit characters, and all characters |
| that that can be used to form decimal-radix numbers, e.g. U+0660, |
| ARABIC-INDIC DIGIT ZERO. |
| |
| |
| .. method:: str.isdigit() |
| |
| Return true if all characters in the string are digits and there is at least one |
| character, false otherwise. |
| |
| |
| .. method:: str.isidentifier() |
| |
| Return true if the string is a valid identifier according to the language |
| definition, section :ref:`identifiers`. |
| |
| |
| .. method:: str.islower() |
| |
| Return true if all cased characters in the string are lowercase and there is at |
| least one cased character, false otherwise. |
| |
| |
| .. method:: str.isnumeric() |
| |
| Return true if all characters in the string are numeric |
| characters, and there is at least one character, false |
| otherwise. Numeric characters include digit characters, and all characters |
| that have the Unicode numeric value property, e.g. U+2155, |
| VULGAR FRACTION ONE FIFTH. |
| |
| |
| .. method:: str.isprintable() |
| |
| Return true if all characters in the string are printable or the string is |
| empty, false otherwise. Nonprintable characters are those characters defined |
| in the Unicode character database as "Other" or "Separator", excepting the |
| ASCII space (0x20) which is considered printable. (Note that printable |
| characters in this context are those which should not be escaped when |
| :func:`repr` is invoked on a string. It has no bearing on the handling of |
| strings written to :data:`sys.stdout` or :data:`sys.stderr`.) |
| |
| |
| .. method:: str.isspace() |
| |
| Return true if there are only whitespace characters in the string and there is |
| at least one character, false otherwise. |
| |
| |
| .. method:: str.istitle() |
| |
| Return true if the string is a titlecased string and there is at least one |
| character, for example uppercase characters may only follow uncased characters |
| and lowercase characters only cased ones. Return false otherwise. |
| |
| |
| .. method:: str.isupper() |
| |
| Return true if all cased characters in the string are uppercase and there is at |
| least one cased character, false otherwise. |
| |
| |
| .. method:: str.join(seq) |
| |
| Return a string which is the concatenation of the values in the sequence |
| *seq*. Non-string values in *seq* will be converted to a string using their |
| respective ``str()`` value. If there are any :class:`bytes` objects in |
| *seq*, a :exc:`TypeError` will be raised. 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 ``len(s)``. |
| |
| |
| .. method:: str.lower() |
| |
| Return a copy of the string converted to lowercase. |
| |
| |
| .. 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' |
| |
| |
| .. method:: str.maketrans(x[, y[, z]]) |
| |
| This static method returns a translation table usable for :meth:`str.translate`. |
| |
| If there is only one argument, it must be a dictionary mapping Unicode |
| ordinals (integers) or characters (strings of length 1) to Unicode ordinals, |
| strings (of arbitrary lengths) or None. Character keys will then be |
| converted to ordinals. |
| |
| If there are two arguments, they must be strings of equal length, and in the |
| resulting dictionary, each character in x will be mapped to the character at |
| the same position in y. If there is a third argument, it must be a string, |
| whose characters will be mapped to None in the result. |
| |
| |
| .. method:: str.partition(sep) |
| |
| Split the string at the first occurrence of *sep*, and return a 3-tuple |
| containing the part before the separator, the separator itself, and the part |
| after the separator. If the separator is not found, return a 3-tuple containing |
| the string itself, followed by two empty strings. |
| |
| |
| .. method:: str.replace(old, new[, count]) |
| |
| Return a copy of the string with all occurrences of substring *old* replaced by |
| *new*. If the optional argument *count* is given, only the first *count* |
| occurrences are replaced. |
| |
| |
| .. method:: str.rfind(sub[, start[, end]]) |
| |
| Return the highest index in the string where substring *sub* is found, such that |
| *sub* is contained within s[start,end]. Optional arguments *start* and *end* |
| are interpreted as in slice notation. Return ``-1`` on failure. |
| |
| |
| .. method:: str.rindex(sub[, start[, end]]) |
| |
| Like :meth:`rfind` but raises :exc:`ValueError` when the substring *sub* is not |
| found. |
| |
| |
| .. method:: str.rjust(width[, fillchar]) |
| |
| Return the string right justified in a string of length *width*. Padding is done |
| using the specified *fillchar* (default is a space). The original string is |
| returned if *width* is less than ``len(s)``. |
| |
| |
| .. method:: str.rpartition(sep) |
| |
| Split the string at the last occurrence of *sep*, and return a 3-tuple |
| containing the part before the separator, the separator itself, and the part |
| after the separator. If the separator is not found, return a 3-tuple containing |
| two empty strings, followed by the string itself. |
| |
| |
| .. method:: str.rsplit([sep[, 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. |
| |
| |
| .. method:: str.rstrip([chars]) |
| |
| Return a copy of the string with trailing characters removed. The *chars* |
| argument is a string specifying the set of characters to be removed. If omitted |
| or ``None``, the *chars* argument defaults to removing whitespace. The *chars* |
| argument is not a suffix; rather, all combinations of its values are stripped: |
| |
| >>> ' spacious '.rstrip() |
| ' spacious' |
| >>> 'mississippi'.rstrip('ipz') |
| 'mississ' |
| |
| |
| .. method:: str.split([sep[, 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, then there is no limit on the number of splits (all possible |
| splits are made). |
| |
| If *sep* is given, consecutive delimiters are not grouped together and are |
| deemed to delimit empty strings (for example, ``'1,,2'.split(',')`` returns |
| ``['1', '', '2']``). The *sep* argument may consist of multiple characters |
| (for example, ``'1<>2<>3'.split('<>')`` returns ``['1', '2', '3']``). |
| Splitting an empty string with a specified separator returns ``['']``. |
| |
| If *sep* is not specified or is ``None``, a different splitting algorithm is |
| applied: runs of consecutive whitespace are regarded as a single separator, |
| and the result will contain no empty strings at the start or end if the |
| string has leading or trailing whitespace. Consequently, splitting an empty |
| string or a string consisting of just whitespace with a ``None`` separator |
| returns ``[]``. |
| |
| For example, ``' 1 2 3 '.split()`` returns ``['1', '2', '3']``, and |
| ``' 1 2 3 '.split(None, 1)`` returns ``['1', '2 3 ']``. |
| |
| |
| .. method:: str.splitlines([keepends]) |
| |
| Return a list of the lines in the string, breaking at line boundaries. Line |
| breaks are not included in the resulting list unless *keepends* is given and |
| true. |
| |
| |
| .. method:: str.startswith(prefix[, start[, end]]) |
| |
| Return ``True`` if string starts with the *prefix*, otherwise return ``False``. |
| *prefix* can also be a tuple of prefixes to look for. With optional *start*, |
| test string beginning at that position. With optional *end*, stop comparing |
| string at that position. |
| |
| |
| .. method:: str.strip([chars]) |
| |
| Return a copy of the string with the leading and trailing characters removed. |
| The *chars* argument is a string specifying the set of characters to be removed. |
| If omitted or ``None``, the *chars* argument defaults to removing whitespace. |
| The *chars* argument is not a prefix or suffix; rather, all combinations of its |
| values are stripped: |
| |
| >>> ' spacious '.strip() |
| 'spacious' |
| >>> 'www.example.com'.strip('cmowz.') |
| 'example' |
| |
| |
| .. method:: str.swapcase() |
| |
| Return a copy of the string with uppercase characters converted to lowercase and |
| vice versa. |
| |
| |
| .. method:: str.title() |
| |
| Return a titlecased version of the string: words start with uppercase |
| characters, all remaining cased characters are lowercase. |
| |
| |
| .. method:: str.translate(map) |
| |
| Return a copy of the *s* where all characters have been mapped through the |
| *map* which must be a dictionary of Unicode ordinals(integers) to Unicode |
| ordinals, strings or ``None``. Unmapped characters are left untouched. |
| Characters mapped to ``None`` are deleted. |
| |
| A *map* for :meth:`translate` is usually best created by |
| :meth:`str.maketrans`. |
| |
| You can use the :func:`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: |
| |
| .. note:: |
| |
| An even more flexible approach is to create a custom character mapping |
| codec using the :mod:`codecs` module (see :mod:`encodings.cp1251` for an |
| example). |
| |
| |
| .. method:: str.upper() |
| |
| Return a copy of the string converted to uppercase. |
| |
| |
| .. 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 ``len(s)``. |
| |
| |
| |
| .. _old-string-formatting: |
| |
| Old String Formatting Operations |
| -------------------------------- |
| |
| .. index:: |
| single: formatting, string (%) |
| single: interpolation, string (%) |
| single: string; formatting |
| single: string; interpolation |
| single: printf-style formatting |
| single: sprintf-style formatting |
| single: % formatting |
| single: % interpolation |
| |
| .. XXX is the note enough? |
| |
| .. note:: |
| |
| The formatting operations described here are obsolete and may go away in future |
| versions of Python. Use the new :ref:`string-formatting` in new code. |
| |
| String objects have one unique built-in operation: the ``%`` operator (modulo). |
| This is also known as the string *formatting* or *interpolation* operator. |
| Given ``format % values`` (where *format* is a string), ``%`` conversion |
| specifications in *format* are replaced with zero or more elements of *values*. |
| The effect is similar to the using :cfunc:`sprintf` in the C language. |
| |
| If *format* requires a single argument, *values* may be a single non-tuple |
| object. [#]_ 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 %(#)03d quote types.' % \ |
| ... {'language': "Python", "#": 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'`` | Obselete 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 | | |
| | | :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 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? |
| |
| For safety reasons, floating point precisions are clipped to 50; ``%f`` |
| conversions for numbers whose absolute value is over 1e25 are replaced by ``%g`` |
| conversions. [#]_ All other errors raise exceptions. |
| |
| .. index:: |
| module: string |
| module: re |
| |
| Additional string operations are defined in standard modules :mod:`string` and |
| :mod:`re`. |
| |
| |
| .. _typesseq-range: |
| |
| Range Type |
| ---------- |
| |
| .. index:: object: range |
| |
| The :class:`range` type is an immutable sequence which is commonly used for |
| looping. The advantage of the :class:`range` type is that an :class:`range` |
| object will always take the same amount of memory, no matter the size of the |
| range it represents. There are no consistent performance advantages. |
| |
| Range 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 |
| object: bytearray |
| |
| List and bytearray objects support additional operations that allow in-place |
| modification of the object. Other mutable sequence types (when added to the |
| language) should also support these operations. Strings and tuples are |
| immutable sequence types: such objects cannot be modified once created. The |
| following operations are defined on mutable sequence types (where *x* is an |
| arbitrary object). |
| |
| Note that while lists allow their items to be of any type, bytearray object |
| "items" are all integers in the range 0 <= x < 256. |
| |
| +------------------------------+--------------------------------+---------------------+ |
| | Operation | Result | Notes | |
| +==============================+================================+=====================+ |
| | ``s[i] = x`` | item *i* of *s* is replaced by | | |
| | | *x* | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s[i:j] = t`` | slice of *s* from *i* to *j* | | |
| | | is replaced by the contents of | | |
| | | the iterable *t* | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``del s[i:j]`` | same as ``s[i:j] = []`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s[i:j:k] = t`` | the elements of ``s[i:j:k]`` | \(1) | |
| | | are replaced by those of *t* | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``del s[i:j:k]`` | removes the elements of | | |
| | | ``s[i:j:k]`` from the list | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.append(x)`` | same as ``s[len(s):len(s)] = | | |
| | | [x]`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.extend(x)`` | same as ``s[len(s):len(s)] = | \(2) | |
| | | x`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.count(x)`` | return number of *i*'s for | | |
| | | which ``s[i] == x`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.index(x[, i[, j]])`` | return smallest *k* such that | \(3) | |
| | | ``s[k] == x`` and ``i <= k < | | |
| | | j`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.insert(i, x)`` | same as ``s[i:i] = [x]`` | \(4) | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.pop([i])`` | same as ``x = s[i]; del s[i]; | \(5) | |
| | | return x`` | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.remove(x)`` | same as ``del s[s.index(x)]`` | \(3) | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.reverse()`` | reverses the items of *s* in | \(6) | |
| | | place | | |
| +------------------------------+--------------------------------+---------------------+ |
| | ``s.sort([key[, reverse]])`` | sort the items of *s* in place | (6), (7), (8) | |
| +------------------------------+--------------------------------+---------------------+ |
| |
| .. index:: |
| triple: operations on; sequence; types |
| triple: operations on; list; type |
| pair: subscript; assignment |
| pair: slice; assignment |
| statement: del |
| single: append() (sequence method) |
| single: extend() (sequence method) |
| single: count() (sequence method) |
| single: index() (sequence method) |
| single: insert() (sequence method) |
| single: pop() (sequence method) |
| single: remove() (sequence method) |
| single: reverse() (sequence method) |
| single: sort() (sequence method) |
| |
| Notes: |
| |
| (1) |
| *t* must have the same length as the slice it is replacing. |
| |
| (2) |
| *x* can be any iterable object. |
| |
| (3) |
| Raises :exc:`ValueError` when *x* is not found in *s*. When a negative index is |
| passed as the second or third parameter to the :meth:`index` method, the sequence |
| length is added, as for slice indices. If it is still negative, it is truncated |
| to zero, as for slice indices. |
| |
| (4) |
| When a negative index is passed as the first parameter to the :meth:`insert` |
| method, the sequence length is added, as for slice indices. If it is still |
| negative, it is truncated to zero, as for slice indices. |
| |
| (5) |
| The optional argument *i* defaults to ``-1``, so that by default the last |
| item is removed and returned. |
| |
| (6) |
| The :meth:`sort` and :meth:`reverse` methods modify the sequence in place for |
| economy of space when sorting or reversing a large sequence. To remind you |
| that they operate by side effect, they don't return the sorted or reversed |
| sequence. |
| |
| (7) |
| The :meth:`sort` method takes optional arguments for controlling the |
| comparisons. Each must be specified as a keyword argument. |
| |
| *key* specifies a function of one argument that is used to extract a comparison |
| key from each list element: ``key=str.lower``. The default value is ``None``. |
| |
| *reverse* is a boolean value. If set to ``True``, then the list elements are |
| sorted as if each comparison were reversed. |
| |
| The :meth:`sort` method is guaranteed to be stable. A |
| sort is stable if it guarantees not to change the relative order of elements |
| that compare equal --- this is helpful for sorting in multiple passes (for |
| example, sort by department, then by salary grade). |
| |
| While a list is being sorted, the effect of attempting to mutate, or even |
| inspect, the list is undefined. The C implementation |
| makes the list appear empty for the duration, and raises :exc:`ValueError` if it |
| can detect that the list has been mutated during a sort. |
| |
| (8) |
| :meth:`sort` is not supported by :class:`bytearray` objects. |
| |
| .. _bytes-methods: |
| |
| Bytes and Byte Array Methods |
| ---------------------------- |
| |
| .. index:: pair: bytes; methods |
| pair: bytearray; methods |
| |
| Bytes and bytearray objects, being "strings of bytes", have all methods found on |
| strings, with the exception of :func:`encode`, :func:`format` and |
| :func:`isidentifier`, which do not make sense with these types. For converting |
| the objects to strings, they have a :func:`decode` method. |
| |
| Wherever one of these methods needs to interpret the bytes as characters |
| (e.g. the :func:`is...` methods), the ASCII character set is assumed. |
| |
| .. note:: |
| |
| The methods on bytes and bytearray objects don't accept strings as their |
| arguments, just as the methods on strings don't accept bytes as their |
| arguments. For example, you have to write :: |
| |
| a = "abc" |
| b = a.replace("a", "f") |
| |
| and :: |
| |
| a = b"abc" |
| b = a.replace(b"a", b"f") |
| |
| |
| The bytes and bytearray types have an additional class method: |
| |
| .. method:: bytes.fromhex(string) |
| bytearray.fromhex(string) |
| |
| This :class:`bytes` class method returns a bytes or bytearray object, |
| decoding the given string object. The string must contain two hexadecimal |
| digits per byte, spaces are ignored. |
| |
| >>> bytes.fromhex('f0 f1f2 ') |
| b'\xf0\xf1\xf2' |
| |
| .. XXX verify/document translate() semantics! |
| |
| .. method:: bytes.translate(table[, delete]) |
| |
| Return a copy of the bytes object where all bytes occurring in the optional |
| argument *delete* are removed, and the remaining bytes have been mapped |
| through the given translation table, which must be a bytes object of length |
| 256. |
| |
| You can use the :func:`maketrans` helper function in the :mod:`string` module to |
| create a translation table. |
| |
| .. XXX a None table doesn't seem to be supported |
| Set the *table* argument to ``None`` for translations that only delete characters:: |
| |
| >>> 'read this short text'.translate(None, 'aeiou') |
| 'rd ths shrt txt' |
| |
| |
| .. _types-set: |
| |
| Set Types --- :class:`set`, :class:`frozenset` |
| ============================================== |
| |
| .. index:: object: set |
| |
| A :dfn:`set` object is an unordered collection of distinct :term:`hashable` objects. |
| Common uses include membership testing, removing duplicates from a sequence, and |
| computing mathematical operations such as intersection, union, difference, and |
| symmetric difference. |
| (For other containers see the built in :class:`dict`, :class:`list`, |
| and :class:`tuple` classes, and the :mod:`collections` module.) |
| |
| Like other collections, sets support ``x in set``, ``len(set)``, and ``for x in |
| set``. Being an unordered collection, sets do not record element position or |
| order of insertion. Accordingly, sets do not support indexing, slicing, or |
| other sequence-like behavior. |
| |
| There are currently two builtin 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. |
| |
| 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 interesection is the empty set. |
| |
| .. method:: issubset(other) |
| set <= other |
| |
| Test whether every element in the set is in *other*. |
| |
| .. method:: set < other |
| |
| Test whether the set is a true subset of *other*, that is, |
| ``set <= other and set != other``. |
| |
| .. method:: issuperset(other) |
| set >= other |
| |
| Test whether every element in *other* is in the set. |
| |
| .. method:: set > other |
| |
| Test whether the set is a true superset of *other*, that is, ``set >= |
| other and set != other``. |
| |
| .. method:: union(other, ...) |
| set | other | ... |
| |
| Return a new set with elements from both sets. |
| |
| .. method:: intersection(other, ...) |
| set & other & ... |
| |
| Return a new set with elements common to both sets. |
| |
| .. method:: difference(other, ...) |
| set - other - ... |
| |
| Return a new set with elements in the set that are not in the others. |
| |
| .. method:: symmetric_difference(other) |
| set ^ other |
| |
| Return a new set with elements in either the set or *other* but not both. |
| |
| .. method:: copy() |
| |
| Return a new set with a shallow copy of *s*. |
| |
| |
| Note, the non-operator versions of :meth:`union`, :meth:`intersection`, |
| :meth:`difference`, and :meth:`symmetric_difference`, :meth:`issubset`, and |
| :meth:`issuperset` methods will accept any iterable as an argument. In |
| contrast, their operator based counterparts require their arguments to be |
| sets. This precludes error-prone constructions like ``set('abc') & 'cbs'`` |
| in favor of the more readable ``set('abc').intersection('cbs')``. |
| |
| Both :class:`set` and :class:`frozenset` support set to set comparisons. Two |
| sets are equal if and only if every element of each set is contained in the |
| other (each is a subset of the other). A set is less than another set if and |
| only if the first set is a proper subset of the second set (is a subset, but |
| is not equal). A set is greater than another set if and only if the first set |
| is a proper superset of the second set (is a superset, but is not equal). |
| |
| Instances of :class:`set` are compared to instances of :class:`frozenset` |
| based on their members. For example, ``set('abc') == frozenset('abc')`` |
| returns ``True`` and so does ``set('abc') in set([frozenset('abc')])``. |
| |
| The subset and equality comparisons do not generalize to a complete ordering |
| function. For example, any two disjoint sets are not equal and are not |
| subsets of each other, so *all* of the following return ``False``: ``a<b``, |
| ``a==b``, or ``a>b``. 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 *other*. |
| |
| .. versionchanged:: 2.6 |
| Accepts multiple input iterables. |
| |
| .. method:: intersection_update(other, ...) |
| set &= other & ... |
| |
| Update the set, keeping only elements found in it and *other*. |
| |
| .. 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. |
| |
| |
| .. _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": 2, "two": 3}``: |
| |
| * ``dict(one=2, two=3)`` |
| * ``dict({'one': 2, 'two': 3})`` |
| * ``dict(zip(('one', 'two'), (2, 3)))`` |
| * ``dict([['two', 3], ['one', 2]])`` |
| |
| The first example only works for keys that are valid Python identifiers; the |
| others work with any valid keys. |
| |
| |
| These are the operations that dictionaries support (and therefore, custom |
| mapping types should support too): |
| |
| .. describe:: len(d) |
| |
| Return the number of items in the dictionary *d*. |
| |
| .. describe:: d[key] |
| |
| Return the item of *d* with key *key*. Raises a :exc:`KeyError` if *key* is |
| not in the map. |
| |
| If a subclass of dict defines a method :meth:`__missing__`, if the key *key* |
| is not present, the ``d[key]`` operation calls that method with the key *key* |
| as argument. The ``d[key]`` operation then returns or raises whatever is |
| returned or raised by the ``__missing__(key)`` call if the key is not |
| present. No other operations or methods invoke :meth:`__missing__`. If |
| :meth:`__missing__` is not defined, :exc:`KeyError` is raised. |
| :meth:`__missing__` must be a method; it cannot be an instance variable. 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``. |
| |
| .. describe:: key not in d |
| |
| Equivalent to ``not key in d``. |
| |
| .. 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*. |
| |
| :meth:`fromkeys` is a class method that returns a new dictionary. *value* |
| defaults to ``None``. |
| |
| .. method:: get(key[, default]) |
| |
| Return the value for *key* if *key* is in the dictionary, else *default*. |
| If *default* is not given, it defaults to ``None``, so that this method |
| never raises a :exc:`KeyError`. |
| |
| .. method:: items() |
| |
| Return a new view of the dictionary's items (``(key, value)`` pairs). See |
| below for documentation of view objects. |
| |
| .. method:: keys() |
| |
| Return a new view of the dictionary's keys. See below for documentation of |
| view objects. |
| |
| .. method:: pop(key[, default]) |
| |
| If *key* is in the dictionary, remove it and return its value, else return |
| *default*. If *default* is not given and *key* is not in the dictionary, |
| a :exc:`KeyError` is raised. |
| |
| .. method:: popitem() |
| |
| Remove and return an arbitrary ``(key, value)`` pair from the dictionary. |
| |
| :meth:`popitem` is useful to destructively iterate over a dictionary, as |
| often used in set algorithms. If the dictionary is empty, calling |
| :meth:`popitem` raises a :exc:`KeyError`. |
| |
| .. method:: setdefault(key[, default]) |
| |
| If *key* is in the dictionary, return its value. If not, insert *key* |
| with a value of *default* and return *default*. *default* defaults to |
| ``None``. |
| |
| .. method:: update([other]) |
| |
| Update the dictionary with the key/value pairs from *other*, overwriting |
| existing keys. Return ``None``. |
| |
| :meth:`update` accepts either another dictionary object or an iterable of |
| key/value pairs (as a tuple or other iterable of length two). If keyword |
| arguments are specified, the dictionary is then is updated with those |
| key/value pairs: ``d.update(red=1, blue=2)``. |
| |
| .. method:: values() |
| |
| Return a new view of the dictionary's values. See below for documentation of |
| view objects. |
| |
| |
| Dictionary view objects |
| ----------------------- |
| |
| The objects returned by :meth:`dict.keys`, :meth:`dict.values` and |
| :meth:`dict.items` are *view objects*. They provide a dynamic view on the |
| dictionary's entries, which means that when the dictionary changes, the view |
| reflects these changes. The keys and items views have a set-like character |
| since their entries |
| |
| 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()]``. |
| |
| .. 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). |
| |
| |
| The keys and items views also provide set-like operations ("other" here refers |
| to another dictionary 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. |
| |
| .. warning:: |
| |
| Since a dictionary's values are not required to be hashable, any of these |
| four operations will fail if an involved dictionary contains such a value. |
| |
| |
| An example of dictionary view usage:: |
| |
| >>> dishes = {'eggs': 2, 'sausage': 1, 'bacon': 1, 'spam': 500} |
| >>> keys = dishes.keys() |
| >>> values = dishes.values() |
| |
| >>> # iteration |
| >>> n = 0 |
| >>> for val in values: |
| ... n += val |
| >>> print(n) |
| 504 |
| |
| >>> # keys and values are iterated over in the same order |
| >>> list(keys) |
| ['eggs', 'bacon', 'sausage', 'spam'] |
| >>> list(values) |
| [2, 1, 1, 500] |
| |
| >>> # view objects are dynamic and reflect dict changes |
| >>> del dishes['eggs'] |
| >>> del dishes['sausage'] |
| >>> list(keys) |
| ['spam', 'bacon'] |
| |
| >>> # set operations |
| >>> keys & {'eggs', 'bacon', 'salad'} |
| {'eggs', 'bacon'} |
| |
| |
| .. _bltin-file-objects: |
| |
| File Objects |
| ============ |
| |
| .. index:: |
| object: file |
| builtin: file |
| module: os |
| module: socket |
| |
| .. XXX this is quite out of date, must be updated with "io" module |
| |
| 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. |
| |
| 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 |
| |
| 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 :cfunc:`fflush`. This may be a |
| no-op on some file-like objects. |
| |
| |
| .. 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:`__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:`__next__` method uses a hidden |
| read-ahead buffer. As a consequence of using a read-ahead buffer, combining |
| :meth:`__next__` with other file methods (like :meth:`readline`) does not work |
| right. However, using :meth:`seek` to reposition the file to an absolute |
| position will flush the read-ahead buffer. |
| |
| |
| .. 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 :cfunc:`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 what was requested may be |
| returned, even if no *size* parameter was given. |
| |
| |
| .. 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). [#]_ 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. An |
| empty string is returned *only* when EOF is encountered immediately. |
| |
| .. note:: |
| |
| Unlike ``stdio``'s :cfunc:`fgets`, the returned string contains null characters |
| (``'\0'``) if they occurred in the input. |
| |
| |
| .. method:: file.readlines([sizehint]) |
| |
| Read until EOF using :meth:`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.seek(offset[, whence]) |
| |
| Set the file's current position, like ``stdio``'s :cfunc:`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. |
| |
| |
| .. method:: file.tell() |
| |
| Return the file's current position, like ``stdio``'s :cfunc:`ftell`. |
| |
| .. note:: |
| |
| On Windows, :meth:`tell` can return illegal values (after an :cfunc:`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. Due to buffering, the string may not actually |
| show up in the file until the :meth:`flush` or :meth:`close` method is |
| called. |
| |
| The meaning of the return value is not defined for every file-like object. |
| Some (mostly low-level) file-like objects may return the number of bytes |
| actually written, others return ``None``. |
| |
| |
| .. 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 |
| ``file.readline()``, and iteration ends when the :meth:`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. |
| |
| |
| .. XXX does this still apply? |
| .. attribute:: file.encoding |
| |
| The encoding that this file uses. When 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 strings. |
| |
| |
| .. attribute:: file.errors |
| |
| The Unicode error handler used along with the encoding. |
| |
| |
| .. 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 the :option:`--with-universal-newlines` option to |
| :program:`configure` (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``. |
| |
| |
| .. _typecontextmanager: |
| |
| Context Manager Types |
| ===================== |
| |
| .. index:: |
| single: context manager |
| single: context management protocol |
| single: protocol; context management |
| |
| Python's :keyword:`with` statement supports the concept of a runtime context |
| defined by a context manager. This is implemented using 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.contextfactory`` :term:`decorator` |
| provide a convenient way to implement these protocols. If a generator function is |
| decorated with the ``contextlib.contextfactory`` 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 member of every module is :attr:`__dict__`. This is the dictionary |
| containing the module's symbol table. Modifying this dictionary will actually |
| change the module's symbol table, but direct assignment to the :attr:`__dict__` |
| attribute is not possible (you can write ``m.__dict__['a'] = 1``, which defines |
| ``m.a`` to be ``1``, but you can't write ``m.__dict__ = {}``). Modifying |
| :attr:`__dict__` directly is not recommended. |
| |
| Modules built into the interpreter are written like this: ``<module 'sys' |
| (built-in)>``. If loaded from a file, they are written as ``<module 'os' from |
| '/usr/local/lib/pythonX.Y/os.pyc'>``. |
| |
| |
| .. _typesobjects: |
| |
| Classes and Class Instances |
| --------------------------- |
| |
| See :ref:`objects` and :ref:`class` for these. |
| |
| |
| .. _typesfunctions: |
| |
| Functions |
| --------- |
| |
| Function objects are created by function definitions. The only operation on a |
| function object is to call it: ``func(argument-list)``. |
| |
| There are really two flavors of function objects: built-in functions and |
| user-defined functions. Both support the same operation (to call the function), |
| but the implementation is different, hence the different object types. |
| |
| See :ref:`function` for more information. |
| |
| |
| .. _typesmethods: |
| |
| Methods |
| ------- |
| |
| .. index:: object: method |
| |
| Methods are functions that are called using the attribute notation. There are |
| two flavors: built-in methods (such as :meth:`append` on lists) and class |
| instance methods. Built-in methods are described with the types that support |
| them. |
| |
| If you access a method (a function defined in a class namespace) through an |
| instance, you get a special object: a :dfn:`bound method` (also called |
| :dfn:`instance method`) object. When called, it will add the ``self`` argument |
| to the argument list. Bound methods have two special read-only attributes: |
| ``m.__self__`` is the object on which the method operates, and ``m.__func__`` is |
| the function implementing the method. Calling ``m(arg-1, arg-2, ..., arg-n)`` |
| is completely equivalent to calling ``m.__func__(m.__self__, arg-1, arg-2, ..., |
| arg-n)``. |
| |
| Like function objects, bound method objects support getting arbitrary |
| attributes. However, since method attributes are actually stored on the |
| underlying function object (``meth.__func__``), setting method attributes on |
| bound methods is disallowed. Attempting to set a method attribute results in a |
| :exc:`TypeError` being raised. In order to set a method attribute, you need to |
| explicitly set it on the underlying function object:: |
| |
| class C: |
| def method(self): |
| pass |
| |
| c = C() |
| c.method.__func__.whoami = 'my name is c' |
| |
| See :ref:`types` for more information. |
| |
| |
| .. _bltin-code-objects: |
| |
| Code Objects |
| ------------ |
| |
| .. index:: object: code |
| |
| .. index:: |
| builtin: compile |
| single: __code__ (function object attribute) |
| |
| Code objects are used by the implementation to represent "pseudo-compiled" |
| executable Python code such as a function body. They differ from function |
| objects because they don't contain a reference to their global execution |
| environment. Code objects are returned by the built-in :func:`compile` function |
| and can be extracted from function objects through their :attr:`__code__` |
| attribute. See also the :mod:`code` module. |
| |
| .. index:: |
| builtin: exec |
| builtin: eval |
| |
| A code object can be executed or evaluated by passing it (instead of a source |
| string) to the :func:`exec` or :func:`eval` built-in functions. |
| |
| See :ref:`types` for more information. |
| |
| |
| .. _bltin-type-objects: |
| |
| Type Objects |
| ------------ |
| |
| .. index:: |
| builtin: type |
| module: types |
| |
| Type objects represent the various object types. An object's type is accessed |
| by the built-in function :func:`type`. There are no special operations on |
| types. The standard module :mod:`types` defines names for all standard built-in |
| types. |
| |
| Types are written like this: ``<class 'int'>``. |
| |
| |
| .. _bltin-null-object: |
| |
| The Null Object |
| --------------- |
| |
| This object is returned by functions that don't explicitly return a value. It |
| supports no special operations. There is exactly one null object, named |
| ``None`` (a built-in name). |
| |
| It is written as ``None``. |
| |
| |
| .. _bltin-ellipsis-object: |
| |
| The Ellipsis Object |
| ------------------- |
| |
| This object is commonly used by slicing (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`` or ``...``. |
| |
| |
| 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 cast any value to a Boolean, |
| if the value can be interpreted as a truth value (see section Truth Value |
| Testing above). |
| |
| .. index:: |
| single: False |
| single: True |
| pair: Boolean; values |
| |
| They are written as ``False`` and ``True``, respectively. |
| |
| |
| .. _typesinternal: |
| |
| Internal Objects |
| ---------------- |
| |
| See :ref:`types` for this information. It describes stack frame objects, |
| traceback objects, and slice objects. |
| |
| |
| .. _specialattrs: |
| |
| Special Attributes |
| ================== |
| |
| The implementation adds a few special read-only attributes to several object |
| types, where they are relevant. Some of these are not reported by the |
| :func:`dir` built-in function. |
| |
| |
| .. attribute:: object.__dict__ |
| |
| A dictionary or other mapping object used to store an object's (writable) |
| attributes. |
| |
| |
| .. attribute:: instance.__class__ |
| |
| The class to which a class instance belongs. |
| |
| |
| .. attribute:: class.__bases__ |
| |
| The tuple of base classes of a class object. If there are no base classes, this |
| will be an empty tuple. |
| |
| |
| .. attribute:: class.__name__ |
| |
| The name of the class or type. |
| |
| .. rubric:: Footnotes |
| |
| .. [#] Additional information on these special methods may be found in the Python |
| Reference Manual (:ref:`customization`). |
| |
| .. [#] As a consequence, the list ``[1, 2]`` is considered equal to ``[1.0, 2.0]``, and |
| similarly for tuples. |
| |
| .. [#] They must have since the parser can't tell the type of the operands. |
| |
| .. [#] To format only a tuple you should therefore provide a singleton tuple whose only |
| element is the tuple to be formatted. |
| |
| .. [#] These numbers are fairly arbitrary. They are intended to avoid printing endless |
| strings of meaningless digits without hampering correct use and without having |
| to know the exact precision of floating point values on a particular machine. |
| |
| .. [#] 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!). |