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516 lines
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ReStructuredText
516 lines
18 KiB
ReStructuredText
:mod:`operator` --- Standard operators as functions
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===================================================
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.. module:: operator
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:synopsis: Functions corresponding to the standard operators.
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.. sectionauthor:: Skip Montanaro <skip@automatrix.com>
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.. testsetup::
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import operator
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from operator import itemgetter, iadd
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The :mod:`operator` module exports a set of functions implemented in C
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corresponding to the intrinsic operators of Python. For example,
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``operator.add(x, y)`` is equivalent to the expression ``x+y``. The function
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names are those used for special class methods; variants without leading and
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trailing ``__`` are also provided for convenience.
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The functions fall into categories that perform object comparisons, logical
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operations, mathematical operations and sequence operations.
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The object comparison functions are useful for all objects, and are named after
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the rich comparison operators they support:
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.. function:: lt(a, b)
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le(a, b)
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eq(a, b)
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ne(a, b)
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ge(a, b)
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gt(a, b)
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__lt__(a, b)
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__le__(a, b)
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__eq__(a, b)
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__ne__(a, b)
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__ge__(a, b)
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__gt__(a, b)
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Perform "rich comparisons" between *a* and *b*. Specifically, ``lt(a, b)`` is
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equivalent to ``a < b``, ``le(a, b)`` is equivalent to ``a <= b``, ``eq(a,
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b)`` is equivalent to ``a == b``, ``ne(a, b)`` is equivalent to ``a != b``,
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``gt(a, b)`` is equivalent to ``a > b`` and ``ge(a, b)`` is equivalent to ``a
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>= b``. Note that these functions can return any value, which may
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or may not be interpretable as a Boolean value. See
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:ref:`comparisons` for more information about rich comparisons.
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The logical operations are also generally applicable to all objects, and support
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truth tests, identity tests, and boolean operations:
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.. function:: not_(obj)
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__not__(obj)
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Return the outcome of :keyword:`not` *obj*. (Note that there is no
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:meth:`__not__` method for object instances; only the interpreter core defines
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this operation. The result is affected by the :meth:`__bool__` and
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:meth:`__len__` methods.)
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.. function:: truth(obj)
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Return :const:`True` if *obj* is true, and :const:`False` otherwise. This is
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equivalent to using the :class:`bool` constructor.
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.. function:: is_(a, b)
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Return ``a is b``. Tests object identity.
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.. function:: is_not(a, b)
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Return ``a is not b``. Tests object identity.
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The mathematical and bitwise operations are the most numerous:
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.. function:: abs(obj)
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__abs__(obj)
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Return the absolute value of *obj*.
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.. function:: add(a, b)
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__add__(a, b)
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Return ``a + b``, for *a* and *b* numbers.
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.. function:: and_(a, b)
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__and__(a, b)
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Return the bitwise and of *a* and *b*.
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.. function:: floordiv(a, b)
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__floordiv__(a, b)
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Return ``a // b``.
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.. function:: index(a)
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__index__(a)
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Return *a* converted to an integer. Equivalent to ``a.__index__()``.
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.. function:: inv(obj)
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invert(obj)
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__inv__(obj)
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__invert__(obj)
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Return the bitwise inverse of the number *obj*. This is equivalent to ``~obj``.
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.. function:: lshift(a, b)
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__lshift__(a, b)
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Return *a* shifted left by *b*.
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.. function:: mod(a, b)
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__mod__(a, b)
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Return ``a % b``.
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.. function:: mul(a, b)
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__mul__(a, b)
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Return ``a * b``, for *a* and *b* numbers.
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.. function:: neg(obj)
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__neg__(obj)
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Return *obj* negated (``-obj``).
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.. function:: or_(a, b)
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__or__(a, b)
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Return the bitwise or of *a* and *b*.
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.. function:: pos(obj)
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__pos__(obj)
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Return *obj* positive (``+obj``).
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.. function:: pow(a, b)
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__pow__(a, b)
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Return ``a ** b``, for *a* and *b* numbers.
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.. function:: rshift(a, b)
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__rshift__(a, b)
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Return *a* shifted right by *b*.
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.. function:: sub(a, b)
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__sub__(a, b)
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Return ``a - b``.
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.. function:: truediv(a, b)
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__truediv__(a, b)
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Return ``a / b`` where 2/3 is .66 rather than 0. This is also known as
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"true" division.
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.. function:: xor(a, b)
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__xor__(a, b)
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Return the bitwise exclusive or of *a* and *b*.
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Operations which work with sequences (some of them with mappings too) include:
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.. function:: concat(a, b)
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__concat__(a, b)
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Return ``a + b`` for *a* and *b* sequences.
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.. function:: contains(a, b)
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__contains__(a, b)
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Return the outcome of the test ``b in a``. Note the reversed operands.
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.. function:: countOf(a, b)
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Return the number of occurrences of *b* in *a*.
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.. function:: delitem(a, b)
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__delitem__(a, b)
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Remove the value of *a* at index *b*.
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.. function:: getitem(a, b)
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__getitem__(a, b)
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Return the value of *a* at index *b*.
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.. function:: indexOf(a, b)
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Return the index of the first of occurrence of *b* in *a*.
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.. function:: setitem(a, b, c)
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__setitem__(a, b, c)
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Set the value of *a* at index *b* to *c*.
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Example: Build a dictionary that maps the ordinals from ``0`` to ``255`` to
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their character equivalents.
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>>> d = {}
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>>> keys = range(256)
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>>> vals = map(chr, keys)
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>>> map(operator.setitem, [d]*len(keys), keys, vals) # doctest: +SKIP
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.. XXX: find a better, readable, example
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The :mod:`operator` module also defines tools for generalized attribute and item
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lookups. These are useful for making fast field extractors as arguments for
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:func:`map`, :func:`sorted`, :meth:`itertools.groupby`, or other functions that
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expect a function argument.
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.. function:: attrgetter(attr[, args...])
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Return a callable object that fetches *attr* from its operand. If more than one
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attribute is requested, returns a tuple of attributes. After,
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``f = attrgetter('name')``, the call ``f(b)`` returns ``b.name``. After,
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``f = attrgetter('name', 'date')``, the call ``f(b)`` returns ``(b.name,
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b.date)``. Equivalent to::
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def attrgetter(*items):
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if any(not isinstance(item, str) for item in items):
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raise TypeError('attribute name must be a string')
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if len(items) == 1:
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attr = items[0]
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def g(obj):
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return resolve_attr(obj, attr)
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else:
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def g(obj):
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return tuple(resolve_att(obj, attr) for attr in items)
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return g
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def resolve_attr(obj, attr):
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for name in attr.split("."):
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obj = getattr(obj, name)
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return obj
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The attribute names can also contain dots; after ``f = attrgetter('date.month')``,
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the call ``f(b)`` returns ``b.date.month``.
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.. function:: itemgetter(item[, args...])
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Return a callable object that fetches *item* from its operand using the
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operand's :meth:`__getitem__` method. If multiple items are specified,
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returns a tuple of lookup values. Equivalent to::
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def itemgetter(*items):
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if len(items) == 1:
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item = items[0]
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def g(obj):
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return obj[item]
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else:
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def g(obj):
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return tuple(obj[item] for item in items)
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return g
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The items can be any type accepted by the operand's :meth:`__getitem__`
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method. Dictionaries accept any hashable value. Lists, tuples, and
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strings accept an index or a slice:
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>>> itemgetter(1)('ABCDEFG')
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'B'
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>>> itemgetter(1,3,5)('ABCDEFG')
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('B', 'D', 'F')
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>>> itemgetter(slice(2,None))('ABCDEFG')
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'CDEFG'
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Example of using :func:`itemgetter` to retrieve specific fields from a
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tuple record:
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>>> inventory = [('apple', 3), ('banana', 2), ('pear', 5), ('orange', 1)]
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>>> getcount = itemgetter(1)
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>>> list(map(getcount, inventory))
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[3, 2, 5, 1]
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>>> sorted(inventory, key=getcount)
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[('orange', 1), ('banana', 2), ('apple', 3), ('pear', 5)]
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.. function:: methodcaller(name[, args...])
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Return a callable object that calls the method *name* on its operand. If
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additional arguments and/or keyword arguments are given, they will be given
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to the method as well. After ``f = methodcaller('name')``, the call ``f(b)``
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returns ``b.name()``. After ``f = methodcaller('name', 'foo', bar=1)``, the
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call ``f(b)`` returns ``b.name('foo', bar=1)``. Equivalent to::
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def methodcaller(name, *args, **kwargs):
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def caller(obj):
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return getattr(obj, name)(*args, **kwargs)
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return caller
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.. _operator-map:
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Mapping Operators to Functions
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------------------------------
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This table shows how abstract operations correspond to operator symbols in the
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Python syntax and the functions in the :mod:`operator` module.
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+-----------------------+-------------------------+---------------------------------------+
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| Operation | Syntax | Function |
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+=======================+=========================+=======================================+
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| Addition | ``a + b`` | ``add(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Concatenation | ``seq1 + seq2`` | ``concat(seq1, seq2)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Containment Test | ``obj in seq`` | ``contains(seq, obj)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Division | ``a / b`` | ``div(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Division | ``a // b`` | ``floordiv(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Bitwise And | ``a & b`` | ``and_(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Bitwise Exclusive Or | ``a ^ b`` | ``xor(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Bitwise Inversion | ``~ a`` | ``invert(a)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Bitwise Or | ``a | b`` | ``or_(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Exponentiation | ``a ** b`` | ``pow(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Identity | ``a is b`` | ``is_(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Identity | ``a is not b`` | ``is_not(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Indexed Assignment | ``obj[k] = v`` | ``setitem(obj, k, v)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Indexed Deletion | ``del obj[k]`` | ``delitem(obj, k)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Indexing | ``obj[k]`` | ``getitem(obj, k)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Left Shift | ``a << b`` | ``lshift(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Modulo | ``a % b`` | ``mod(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Multiplication | ``a * b`` | ``mul(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Negation (Arithmetic) | ``- a`` | ``neg(a)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Negation (Logical) | ``not a`` | ``not_(a)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Positive | ``+ a`` | ``pos(a)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Right Shift | ``a >> b`` | ``rshift(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Sequence Repetition | ``seq * i`` | ``repeat(seq, i)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Slice Assignment | ``seq[i:j] = values`` | ``setitem(seq, slice(i, j), values)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Slice Deletion | ``del seq[i:j]`` | ``delitem(seq, slice(i, j))`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Slicing | ``seq[i:j]`` | ``getitem(seq, slice(i, j))`` |
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+-----------------------+-------------------------+---------------------------------------+
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| String Formatting | ``s % obj`` | ``mod(s, obj)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Subtraction | ``a - b`` | ``sub(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Truth Test | ``obj`` | ``truth(obj)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Ordering | ``a < b`` | ``lt(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Ordering | ``a <= b`` | ``le(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Equality | ``a == b`` | ``eq(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Difference | ``a != b`` | ``ne(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Ordering | ``a >= b`` | ``ge(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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| Ordering | ``a > b`` | ``gt(a, b)`` |
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+-----------------------+-------------------------+---------------------------------------+
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Inplace Operators
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=================
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Many operations have an "in-place" version. Listed below are functions
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providing a more primitive access to in-place operators than the usual syntax
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does; for example, the :term:`statement` ``x += y`` is equivalent to
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``x = operator.iadd(x, y)``. Another way to put it is to say that
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``z = operator.iadd(x, y)`` is equivalent to the compound statement
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``z = x; z += y``.
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In those examples, note that when an in-place method is called, the computation
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and assignment are performed in two separate steps. The in-place functions
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listed below only do the first step, calling the in-place method. The second
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step, assignment, is not handled.
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For immutable targets such as strings, numbers, and tuples, the updated
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value is computed, but not assigned back to the input variable:
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>>> a = 'hello'
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>>> iadd(a, ' world')
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'hello world'
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>>> a
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'hello'
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For mutable targets such as lists and dictionaries, the inplace method
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will perform the update, so no subsequent assignment is necessary:
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>>> s = ['h', 'e', 'l', 'l', 'o']
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>>> iadd(s, [' ', 'w', 'o', 'r', 'l', 'd'])
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['h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd']
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>>> s
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['h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd']
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.. function:: iadd(a, b)
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__iadd__(a, b)
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``a = iadd(a, b)`` is equivalent to ``a += b``.
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.. function:: iand(a, b)
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__iand__(a, b)
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``a = iand(a, b)`` is equivalent to ``a &= b``.
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.. function:: iconcat(a, b)
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__iconcat__(a, b)
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``a = iconcat(a, b)`` is equivalent to ``a += b`` for *a* and *b* sequences.
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.. function:: ifloordiv(a, b)
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__ifloordiv__(a, b)
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``a = ifloordiv(a, b)`` is equivalent to ``a //= b``.
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.. function:: ilshift(a, b)
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__ilshift__(a, b)
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``a = ilshift(a, b)`` is equivalent to ``a <<= b``.
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.. function:: imod(a, b)
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__imod__(a, b)
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``a = imod(a, b)`` is equivalent to ``a %= b``.
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.. function:: imul(a, b)
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__imul__(a, b)
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``a = imul(a, b)`` is equivalent to ``a *= b``.
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.. function:: ior(a, b)
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__ior__(a, b)
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``a = ior(a, b)`` is equivalent to ``a |= b``.
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.. function:: ipow(a, b)
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__ipow__(a, b)
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``a = ipow(a, b)`` is equivalent to ``a **= b``.
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.. function:: irshift(a, b)
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__irshift__(a, b)
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``a = irshift(a, b)`` is equivalent to ``a >>= b``.
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.. function:: isub(a, b)
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__isub__(a, b)
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``a = isub(a, b)`` is equivalent to ``a -= b``.
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.. function:: itruediv(a, b)
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__itruediv__(a, b)
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``a = itruediv(a, b)`` is equivalent to ``a /= b``.
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.. function:: ixor(a, b)
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__ixor__(a, b)
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``a = ixor(a, b)`` is equivalent to ``a ^= b``.
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