
``operator`` --- Standard operators as functions
************************************************

The ``operator`` module exports a set of efficient functions
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:

operator.lt(a, b)
operator.le(a, b)
operator.eq(a, b)
operator.ne(a, b)
operator.ge(a, b)
operator.gt(a, b)
operator.__lt__(a, b)
operator.__le__(a, b)
operator.__eq__(a, b)
operator.__ne__(a, b)
operator.__ge__(a, b)
operator.__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 *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:

operator.not_(obj)
operator.__not__(obj)

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

operator.truth(obj)

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

operator.is_(a, b)

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

operator.is_not(a, b)

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

The mathematical and bitwise operations are the most numerous:

operator.abs(obj)
operator.__abs__(obj)

   Return the absolute value of *obj*.

operator.add(a, b)
operator.__add__(a, b)

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

operator.and_(a, b)
operator.__and__(a, b)

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

operator.floordiv(a, b)
operator.__floordiv__(a, b)

   Return ``a // b``.

operator.index(a)
operator.__index__(a)

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

operator.inv(obj)
operator.invert(obj)
operator.__inv__(obj)
operator.__invert__(obj)

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

operator.lshift(a, b)
operator.__lshift__(a, b)

   Return *a* shifted left by *b*.

operator.mod(a, b)
operator.__mod__(a, b)

   Return ``a % b``.

operator.mul(a, b)
operator.__mul__(a, b)

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

operator.neg(obj)
operator.__neg__(obj)

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

operator.or_(a, b)
operator.__or__(a, b)

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

operator.pos(obj)
operator.__pos__(obj)

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

operator.pow(a, b)
operator.__pow__(a, b)

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

operator.rshift(a, b)
operator.__rshift__(a, b)

   Return *a* shifted right by *b*.

operator.sub(a, b)
operator.__sub__(a, b)

   Return ``a - b``.

operator.truediv(a, b)
operator.__truediv__(a, b)

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

operator.xor(a, b)
operator.__xor__(a, b)

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

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

operator.concat(a, b)
operator.__concat__(a, b)

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

operator.contains(a, b)
operator.__contains__(a, b)

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

operator.countOf(a, b)

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

operator.delitem(a, b)
operator.__delitem__(a, b)

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

operator.getitem(a, b)
operator.__getitem__(a, b)

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

operator.indexOf(a, b)

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

operator.setitem(a, b, c)
operator.__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

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

operator.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``.

operator.itemgetter(item[, args...])

   Return a callable object that fetches *item* from its operand using
   the operand's ``__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
   ``__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 ``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)]

operator.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


Mapping Operators to Functions
==============================

This table shows how abstract operations correspond to operator
symbols in the Python syntax and the functions in the ``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``                 | ``truediv(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 *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']

operator.iadd(a, b)
operator.__iadd__(a, b)

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

operator.iand(a, b)
operator.__iand__(a, b)

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

operator.iconcat(a, b)
operator.__iconcat__(a, b)

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

operator.ifloordiv(a, b)
operator.__ifloordiv__(a, b)

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

operator.ilshift(a, b)
operator.__ilshift__(a, b)

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

operator.imod(a, b)
operator.__imod__(a, b)

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

operator.imul(a, b)
operator.__imul__(a, b)

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

operator.ior(a, b)
operator.__ior__(a, b)

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

operator.ipow(a, b)
operator.__ipow__(a, b)

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

operator.irshift(a, b)
operator.__irshift__(a, b)

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

operator.isub(a, b)
operator.__isub__(a, b)

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

operator.itruediv(a, b)
operator.__itruediv__(a, b)

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

operator.ixor(a, b)
operator.__ixor__(a, b)

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