cpython/Doc/library/operator.rst

:mod:`operator` --- Standard operators as functions
===================================================

.. module:: operator
   :synopsis: Functions corresponding to the standard operators.
.. sectionauthor:: Skip Montanaro <skip@automatrix.com>


.. testsetup::

   import operator
   from operator import itemgetter, iadd


The :mod:`operator` module exports a set of functions implemented in C
corresponding to the intrinsic operators of Python.  For example,
``operator.add(x, y)`` is equivalent to the expression ``x+y``.  The function
names are those used for special class methods; variants without leading and
trailing ``__`` are also provided for convenience.

The functions fall into categories that perform object comparisons, logical
operations, mathematical operations and sequence operations.

The object comparison functions are useful for all objects, and are named after
the rich comparison operators they support:


.. function:: lt(a, b)
              le(a, b)
              eq(a, b)
              ne(a, b)
              ge(a, b)
              gt(a, b)
              __lt__(a, b)
              __le__(a, b)
              __eq__(a, b)
              __ne__(a, b)
              __ge__(a, b)
              __gt__(a, b)

   Perform "rich comparisons" between *a* and *b*. Specifically, ``lt(a, b)`` is
   equivalent to ``a < b``, ``le(a, b)`` is equivalent to ``a <= b``, ``eq(a,
   b)`` is equivalent to ``a == b``, ``ne(a, b)`` is equivalent to ``a != b``,
   ``gt(a, b)`` is equivalent to ``a > b`` and ``ge(a, b)`` is equivalent to ``a
   >= b``.  Note that these functions can return any value, which may
   or may not be interpretable as a Boolean value.  See
   :ref:`comparisons` for more information about rich comparisons.


The logical operations are also generally applicable to all objects, and support
truth tests, identity tests, and boolean operations:


.. function:: not_(obj)
              __not__(obj)

   Return the outcome of :keyword:`not` *obj*.  (Note that there is no
   :meth:`__not__` method for object instances; only the interpreter core defines
   this operation.  The result is affected by the :meth:`__bool__` and
   :meth:`__len__` methods.)


.. function:: truth(obj)

   Return :const:`True` if *obj* is true, and :const:`False` otherwise.  This is
   equivalent to using the :class:`bool` constructor.


.. function:: is_(a, b)

   Return ``a is b``.  Tests object identity.


.. function:: is_not(a, b)

   Return ``a is not b``.  Tests object identity.


The mathematical and bitwise operations are the most numerous:


.. function:: abs(obj)
              __abs__(obj)

   Return the absolute value of *obj*.


.. function:: add(a, b)
              __add__(a, b)

   Return ``a + b``, for *a* and *b* numbers.


.. function:: and_(a, b)
              __and__(a, b)

   Return the bitwise and of *a* and *b*.


.. function:: floordiv(a, b)
              __floordiv__(a, b)

   Return ``a // b``.


.. function:: index(a)
              __index__(a)

   Return *a* converted to an integer.  Equivalent to ``a.__index__()``.


.. function:: inv(obj)
              invert(obj)
              __inv__(obj)
              __invert__(obj)

   Return the bitwise inverse of the number *obj*.  This is equivalent to ``~obj``.


.. function:: lshift(a, b)
              __lshift__(a, b)

   Return *a* shifted left by *b*.


.. function:: mod(a, b)
              __mod__(a, b)

   Return ``a % b``.


.. function:: mul(a, b)
              __mul__(a, b)

   Return ``a * b``, for *a* and *b* numbers.


.. function:: neg(obj)
              __neg__(obj)

   Return *obj* negated (``-obj``).


.. function:: or_(a, b)
              __or__(a, b)

   Return the bitwise or of *a* and *b*.


.. function:: pos(obj)
              __pos__(obj)

   Return *obj* positive (``+obj``).


.. function:: pow(a, b)
              __pow__(a, b)

   Return ``a ** b``, for *a* and *b* numbers.


.. function:: rshift(a, b)
              __rshift__(a, b)

   Return *a* shifted right by *b*.


.. function:: sub(a, b)
              __sub__(a, b)

   Return ``a - b``.


.. function:: truediv(a, b)
              __truediv__(a, b)

   Return ``a / b`` where 2/3 is .66 rather than 0.  This is also known as
   "true" division.


.. function:: xor(a, b)
              __xor__(a, b)

   Return the bitwise exclusive or of *a* and *b*.


Operations which work with sequences (some of them with mappings too) include:

.. function:: concat(a, b)
              __concat__(a, b)

   Return ``a + b`` for *a* and *b* sequences.


.. function:: contains(a, b)
              __contains__(a, b)

   Return the outcome of the test ``b in a``. Note the reversed operands.


.. function:: countOf(a, b)

   Return the number of occurrences of *b* in *a*.


.. function:: delitem(a, b)
              __delitem__(a, b)

   Remove the value of *a* at index *b*.


.. function:: getitem(a, b)
              __getitem__(a, b)

   Return the value of *a* at index *b*.


.. function:: indexOf(a, b)

   Return the index of the first of occurrence of *b* in *a*.


.. function:: setitem(a, b, c)
              __setitem__(a, b, c)

   Set the value of *a* at index *b* to *c*.

Example: Build a dictionary that maps the ordinals from ``0`` to ``255`` to
their character equivalents.

   >>> d = {}
   >>> keys = range(256)
   >>> vals = map(chr, keys)
   >>> map(operator.setitem, [d]*len(keys), keys, vals)   # doctest: +SKIP

.. XXX: find a better, readable, example

The :mod:`operator` module also defines tools for generalized attribute and item
lookups.  These are useful for making fast field extractors as arguments for
:func:`map`, :func:`sorted`, :meth:`itertools.groupby`, or other functions that
expect a function argument.


.. function:: attrgetter(attr[, args...])

   Return a callable object that fetches *attr* from its operand. If more than one
   attribute is requested, returns a tuple of attributes. After,
   ``f = attrgetter('name')``, the call ``f(b)`` returns ``b.name``.  After,
   ``f = attrgetter('name', 'date')``, the call ``f(b)`` returns ``(b.name,
   b.date)``.  Equivalent to::

      def attrgetter(*items):
          if any(not isinstance(item, str) for item in items):
              raise TypeError('attribute name must be a string')
          if len(items) == 1:
              attr = items[0]
              def g(obj):
                  return resolve_attr(obj, attr)
          else:
              def g(obj):
                  return tuple(resolve_att(obj, attr) for attr in items)
          return g

      def resolve_attr(obj, attr):
          for name in attr.split("."):
              obj = getattr(obj, name)
          return obj


   The attribute names can also contain dots; after ``f = attrgetter('date.month')``,
   the call ``f(b)`` returns ``b.date.month``.

.. function:: itemgetter(item[, args...])

   Return a callable object that fetches *item* from its operand using the
   operand's :meth:`__getitem__` method.  If multiple items are specified,
   returns a tuple of lookup values.  Equivalent to::

      def itemgetter(*items):
          if len(items) == 1:
              item = items[0]
              def g(obj):
                  return obj[item]
          else:
              def g(obj):
                  return tuple(obj[item] for item in items)
          return g

   The items can be any type accepted by the operand's :meth:`__getitem__`
   method.  Dictionaries accept any hashable value.  Lists, tuples, and
   strings accept an index or a slice:

      >>> itemgetter(1)('ABCDEFG')
      'B'
      >>> itemgetter(1,3,5)('ABCDEFG')
      ('B', 'D', 'F')
      >>> itemgetter(slice(2,None))('ABCDEFG')
      'CDEFG'


   Example of using :func:`itemgetter` to retrieve specific fields from a
   tuple record:

      >>> inventory = [('apple', 3), ('banana', 2), ('pear', 5), ('orange', 1)]
      >>> getcount = itemgetter(1)
      >>> list(map(getcount, inventory))
      [3, 2, 5, 1]
      >>> sorted(inventory, key=getcount)
      [('orange', 1), ('banana', 2), ('apple', 3), ('pear', 5)]


.. function:: methodcaller(name[, args...])

   Return a callable object that calls the method *name* on its operand.  If
   additional arguments and/or keyword arguments are given, they will be given
   to the method as well.  After ``f = methodcaller('name')``, the call ``f(b)``
   returns ``b.name()``.  After ``f = methodcaller('name', 'foo', bar=1)``, the
   call ``f(b)`` returns ``b.name('foo', bar=1)``.  Equivalent to::

      def methodcaller(name, *args, **kwargs):
          def caller(obj):
              return getattr(obj, name)(*args, **kwargs)
          return caller


.. _operator-map:

Mapping Operators to Functions
------------------------------

This table shows how abstract operations correspond to operator symbols in the
Python syntax and the functions in the :mod:`operator` module.

+-----------------------+-------------------------+---------------------------------------+
| Operation             | Syntax                  | Function                              |
+=======================+=========================+=======================================+
| Addition              | ``a + b``               | ``add(a, b)``                         |
+-----------------------+-------------------------+---------------------------------------+
| Concatenation         | ``seq1 + seq2``         | ``concat(seq1, seq2)``                |
+-----------------------+-------------------------+---------------------------------------+
| Containment Test      | ``obj in seq``          | ``contains(seq, obj)``                |
+-----------------------+-------------------------+---------------------------------------+
| Division              | ``a / b``               | ``div(a, b)``                         |
+-----------------------+-------------------------+---------------------------------------+
| Division              | ``a // b``              | ``floordiv(a, b)``                    |
+-----------------------+-------------------------+---------------------------------------+
| Bitwise And           | ``a & b``               | ``and_(a, b)``                        |
+-----------------------+-------------------------+---------------------------------------+
| Bitwise Exclusive Or  | ``a ^ b``               | ``xor(a, b)``                         |
+-----------------------+-------------------------+---------------------------------------+
| Bitwise Inversion     | ``~ a``                 | ``invert(a)``                         |
+-----------------------+-------------------------+---------------------------------------+
| Bitwise Or            | ``a | b``               | ``or_(a, b)``                         |
+-----------------------+-------------------------+---------------------------------------+
| Exponentiation        | ``a ** b``              | ``pow(a, b)``                         |
+-----------------------+-------------------------+---------------------------------------+
| Identity              | ``a is b``              | ``is_(a, b)``                         |
+-----------------------+-------------------------+---------------------------------------+
| Identity              | ``a is not b``          | ``is_not(a, b)``                      |
+-----------------------+-------------------------+---------------------------------------+
| Indexed Assignment    | ``obj[k] = v``          | ``setitem(obj, k, v)``                |
+-----------------------+-------------------------+---------------------------------------+
| Indexed Deletion      | ``del obj[k]``          | ``delitem(obj, k)``                   |
+-----------------------+-------------------------+---------------------------------------+
| Indexing              | ``obj[k]``              | ``getitem(obj, k)``                   |
+-----------------------+-------------------------+---------------------------------------+
| Left Shift            | ``a << b``              | ``lshift(a, b)``                      |
+-----------------------+-------------------------+---------------------------------------+
| Modulo                | ``a % b``               | ``mod(a, b)``                         |
+-----------------------+-------------------------+---------------------------------------+
| Multiplication        | ``a * b``               | ``mul(a, b)``                         |
+-----------------------+-------------------------+---------------------------------------+
| Negation (Arithmetic) | ``- a``                 | ``neg(a)``                            |
+-----------------------+-------------------------+---------------------------------------+
| Negation (Logical)    | ``not a``               | ``not_(a)``                           |
+-----------------------+-------------------------+---------------------------------------+
| Positive              | ``+ a``                 | ``pos(a)``                            |
+-----------------------+-------------------------+---------------------------------------+
| Right Shift           | ``a >> b``              | ``rshift(a, b)``                      |
+-----------------------+-------------------------+---------------------------------------+
| Slice Assignment      | ``seq[i:j] = values``   | ``setitem(seq, slice(i, j), values)`` |
+-----------------------+-------------------------+---------------------------------------+
| Slice Deletion        | ``del seq[i:j]``        | ``delitem(seq, slice(i, j))``         |
+-----------------------+-------------------------+---------------------------------------+
| Slicing               | ``seq[i:j]``            | ``getitem(seq, slice(i, j))``         |
+-----------------------+-------------------------+---------------------------------------+
| String Formatting     | ``s % obj``             | ``mod(s, obj)``                       |
+-----------------------+-------------------------+---------------------------------------+
| Subtraction           | ``a - b``               | ``sub(a, b)``                         |
+-----------------------+-------------------------+---------------------------------------+
| Truth Test            | ``obj``                 | ``truth(obj)``                        |
+-----------------------+-------------------------+---------------------------------------+
| Ordering              | ``a < b``               | ``lt(a, b)``                          |
+-----------------------+-------------------------+---------------------------------------+
| Ordering              | ``a <= b``              | ``le(a, b)``                          |
+-----------------------+-------------------------+---------------------------------------+
| Equality              | ``a == b``              | ``eq(a, b)``                          |
+-----------------------+-------------------------+---------------------------------------+
| Difference            | ``a != b``              | ``ne(a, b)``                          |
+-----------------------+-------------------------+---------------------------------------+
| Ordering              | ``a >= b``              | ``ge(a, b)``                          |
+-----------------------+-------------------------+---------------------------------------+
| Ordering              | ``a > b``               | ``gt(a, b)``                          |
+-----------------------+-------------------------+---------------------------------------+

Inplace Operators
=================

Many operations have an "in-place" version.  Listed below are functions
providing a more primitive access to in-place operators than the usual syntax
does; for example, the :term:`statement` ``x += y`` is equivalent to
``x = operator.iadd(x, y)``.  Another way to put it is to say that
``z = operator.iadd(x, y)`` is equivalent to the compound statement
``z = x; z += y``.

In those examples, note that when an in-place method is called, the computation
and assignment are performed in two separate steps.  The in-place functions
listed below only do the first step, calling the in-place method.  The second
step, assignment, is not handled.

For immutable targets such as strings, numbers, and tuples, the updated
value is computed, but not assigned back to the input variable:

>>> a = 'hello'
>>> iadd(a, ' world')
'hello world'
>>> a
'hello'

For mutable targets such as lists and dictionaries, the inplace method
will perform the update, so no subsequent assignment is necessary:

>>> s = ['h', 'e', 'l', 'l', 'o']
>>> iadd(s, [' ', 'w', 'o', 'r', 'l', 'd'])
['h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd']
>>> s
['h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd']

.. function:: iadd(a, b)
              __iadd__(a, b)

   ``a = iadd(a, b)`` is equivalent to ``a += b``.


.. function:: iand(a, b)
              __iand__(a, b)

   ``a = iand(a, b)`` is equivalent to ``a &= b``.


.. function:: iconcat(a, b)
              __iconcat__(a, b)

   ``a = iconcat(a, b)`` is equivalent to ``a += b`` for *a* and *b* sequences.


.. function:: ifloordiv(a, b)
              __ifloordiv__(a, b)

   ``a = ifloordiv(a, b)`` is equivalent to ``a //= b``.


.. function:: ilshift(a, b)
              __ilshift__(a, b)

   ``a = ilshift(a, b)`` is equivalent to ``a <<= b``.


.. function:: imod(a, b)
              __imod__(a, b)

   ``a = imod(a, b)`` is equivalent to ``a %= b``.


.. function:: imul(a, b)
              __imul__(a, b)

   ``a = imul(a, b)`` is equivalent to ``a *= b``.


.. function:: ior(a, b)
              __ior__(a, b)

   ``a = ior(a, b)`` is equivalent to ``a |= b``.


.. function:: ipow(a, b)
              __ipow__(a, b)

   ``a = ipow(a, b)`` is equivalent to ``a **= b``.


.. function:: irshift(a, b)
              __irshift__(a, b)

   ``a = irshift(a, b)`` is equivalent to ``a >>= b``.


.. function:: isub(a, b)
              __isub__(a, b)

   ``a = isub(a, b)`` is equivalent to ``a -= b``.


.. function:: itruediv(a, b)
              __itruediv__(a, b)

   ``a = itruediv(a, b)`` is equivalent to ``a /= b``.


.. function:: ixor(a, b)
              __ixor__(a, b)

   ``a = ixor(a, b)`` is equivalent to ``a ^= b``.