cpython/Doc/library/collections.rst

:mod:`collections` --- High-performance container datatypes
===========================================================

.. module:: collections
   :synopsis: High-performance datatypes
.. moduleauthor:: Raymond Hettinger <python@rcn.com>
.. sectionauthor:: Raymond Hettinger <python@rcn.com>


This module implements high-performance container datatypes.  Currently,
there are two datatypes, :class:`deque` and :class:`defaultdict`, and
one datatype factory function, :func:`NamedTuple`. Python already
includes built-in containers, :class:`dict`, :class:`list`,
:class:`set`, and :class:`tuple`. In addition, the optional :mod:`bsddb`
module has a :meth:`bsddb.btopen` method that can be used to create in-memory
or file based ordered dictionaries with string keys.

Future editions of the standard library may include balanced trees and
ordered dictionaries.

In addition to containers, the collections module provides some ABCs
(abstract base classes) that can be used to test whether
a class provides a particular interface, for example, is it hashable or
a mapping. The ABCs provided include those in the following table:

=====================================  ========================================
ABC                                    Notes
=====================================  ========================================
:class:`collections.Container`         Defines ``__contains__()``
:class:`collections.Hashable`          Defines ``__hash__()``
:class:`collections.Iterable`          Defines ``__iter__()``
:class:`collections.Iterator`          Derived from :class:`Iterable` and in
                                       addition defines ``__next__()``
:class:`collections.Mapping`           Derived from :class:`Container`,
                                       :class:`Iterable`,
                                       and :class:`Sized`, and in addition
                                       defines ``__getitem__()``, ``get()``,
                                       ``__contains__()``, ``__len__()``,
                                       ``__iter__()``, ``keys()``,
                                       ``items()``, and ``values()``
:class:`collections.MutableMapping`    Derived from :class:`Mapping`
:class:`collections.MutableSequence`   Derived from :class:`Sequence`
:class:`collections.MutableSet`        Derived from :class:`Set` and in
                                       addition defines ``add()``,
                                       ``clear()``, ``discard()``, ``pop()``,
                                       and ``toggle()``
:class:`collections.Sequence`          Derived from :class:`Container`,
                                       :class:`Iterable`, and :class:`Sized`,
                                       and in addition defines
                                       ``__getitem__()``
:class:`collections.Set`               Derived from :class:`Container`, :class:`Iterable`, and :class:`Sized`
:class:`collections.Sized`             Defines ``__len__()``
=====================================  ========================================

.. XXX Have not included them all and the notes are imcomplete
.. Deliberately did one row wide to get a neater output

These ABCs allow us to ask classes or instances if they provide
particular functionality, for example::

    from collections import Sized

    size = None
    if isinstance(myvar, Sized):
	size = len(myvar)

(For more about ABCs, see the :mod:`abc` module and :pep:`3119`.)



.. _deque-objects:

:class:`deque` objects
----------------------


.. class:: deque([iterable])

   Returns a new deque object initialized left-to-right (using :meth:`append`) with
   data from *iterable*.  If *iterable* is not specified, the new deque is empty.

   Deques are a generalization of stacks and queues (the name is pronounced "deck"
   and is short for "double-ended queue").  Deques support thread-safe, memory
   efficient appends and pops from either side of the deque with approximately the
   same O(1) performance in either direction.

   Though :class:`list` objects support similar operations, they are optimized for
   fast fixed-length operations and incur O(n) memory movement costs for
   ``pop(0)`` and ``insert(0, v)`` operations which change both the size and
   position of the underlying data representation.


Deque objects support the following methods:

.. method:: deque.append(x)

   Add *x* to the right side of the deque.


.. method:: deque.appendleft(x)

   Add *x* to the left side of the deque.


.. method:: deque.clear()

   Remove all elements from the deque leaving it with length 0.


.. method:: deque.extend(iterable)

   Extend the right side of the deque by appending elements from the iterable
   argument.


.. method:: deque.extendleft(iterable)

   Extend the left side of the deque by appending elements from *iterable*.  Note,
   the series of left appends results in reversing the order of elements in the
   iterable argument.


.. method:: deque.pop()

   Remove and return an element from the right side of the deque. If no elements
   are present, raises an :exc:`IndexError`.


.. method:: deque.popleft()

   Remove and return an element from the left side of the deque. If no elements are
   present, raises an :exc:`IndexError`.


.. method:: deque.remove(value)

   Removed the first occurrence of *value*.  If not found, raises a
   :exc:`ValueError`.


.. method:: deque.rotate(n)

   Rotate the deque *n* steps to the right.  If *n* is negative, rotate to the
   left.  Rotating one step to the right is equivalent to:
   ``d.appendleft(d.pop())``.

In addition to the above, deques support iteration, pickling, ``len(d)``,
``reversed(d)``, ``copy.copy(d)``, ``copy.deepcopy(d)``, membership testing with
the :keyword:`in` operator, and subscript references such as ``d[-1]``.

Example::

   >>> from collections import deque
   >>> d = deque('ghi')                 # make a new deque with three items
   >>> for elem in d:                   # iterate over the deque's elements
   ...     print(elem.upper())
   G
   H
   I

   >>> d.append('j')                    # add a new entry to the right side
   >>> d.appendleft('f')                # add a new entry to the left side
   >>> d                                # show the representation of the deque
   deque(['f', 'g', 'h', 'i', 'j'])

   >>> d.pop()                          # return and remove the rightmost item
   'j'
   >>> d.popleft()                      # return and remove the leftmost item
   'f'
   >>> list(d)                          # list the contents of the deque
   ['g', 'h', 'i']
   >>> d[0]                             # peek at leftmost item
   'g'
   >>> d[-1]                            # peek at rightmost item
   'i'

   >>> list(reversed(d))                # list the contents of a deque in reverse
   ['i', 'h', 'g']
   >>> 'h' in d                         # search the deque
   True
   >>> d.extend('jkl')                  # add multiple elements at once
   >>> d
   deque(['g', 'h', 'i', 'j', 'k', 'l'])
   >>> d.rotate(1)                      # right rotation
   >>> d
   deque(['l', 'g', 'h', 'i', 'j', 'k'])
   >>> d.rotate(-1)                     # left rotation
   >>> d
   deque(['g', 'h', 'i', 'j', 'k', 'l'])

   >>> deque(reversed(d))               # make a new deque in reverse order
   deque(['l', 'k', 'j', 'i', 'h', 'g'])
   >>> d.clear()                        # empty the deque
   >>> d.pop()                          # cannot pop from an empty deque
   Traceback (most recent call last):
     File "<pyshell#6>", line 1, in -toplevel-
       d.pop()
   IndexError: pop from an empty deque

   >>> d.extendleft('abc')              # extendleft() reverses the input order
   >>> d
   deque(['c', 'b', 'a'])


.. _deque-recipes:

Recipes
^^^^^^^

This section shows various approaches to working with deques.

The :meth:`rotate` method provides a way to implement :class:`deque` slicing and
deletion.  For example, a pure python implementation of ``del d[n]`` relies on
the :meth:`rotate` method to position elements to be popped::

   def delete_nth(d, n):
       d.rotate(-n)
       d.popleft()
       d.rotate(n)

To implement :class:`deque` slicing, use a similar approach applying
:meth:`rotate` to bring a target element to the left side of the deque. Remove
old entries with :meth:`popleft`, add new entries with :meth:`extend`, and then
reverse the rotation.

With minor variations on that approach, it is easy to implement Forth style
stack manipulations such as ``dup``, ``drop``, ``swap``, ``over``, ``pick``,
``rot``, and ``roll``.

A roundrobin task server can be built from a :class:`deque` using
:meth:`popleft` to select the current task and :meth:`append` to add it back to
the tasklist if the input stream is not exhausted::

   >>> def roundrobin(*iterables):
   ...     pending = deque(iter(i) for i in iterables)
   ...     while pending:
   ...         task = pending.popleft()
   ...         try:
   ...             yield next(task)
   ...         except StopIteration:
   ...             continue
   ...         pending.append(task)
   ...
   >>> for value in roundrobin('abc', 'd', 'efgh'):
   ...     print(value)

   a
   d
   e
   b
   f
   c
   g
   h


Multi-pass data reduction algorithms can be succinctly expressed and efficiently
coded by extracting elements with multiple calls to :meth:`popleft`, applying
the reduction function, and calling :meth:`append` to add the result back to the
queue.

For example, building a balanced binary tree of nested lists entails reducing
two adjacent nodes into one by grouping them in a list::

   >>> def maketree(iterable):
   ...     d = deque(iterable)
   ...     while len(d) > 1:
   ...         pair = [d.popleft(), d.popleft()]
   ...         d.append(pair)
   ...     return list(d)
   ...
   >>> print(maketree('abcdefgh'))
   [[[['a', 'b'], ['c', 'd']], [['e', 'f'], ['g', 'h']]]]



.. _defaultdict-objects:

:class:`defaultdict` objects
----------------------------


.. class:: defaultdict([default_factory[, ...]])

   Returns a new dictionary-like object.  :class:`defaultdict` is a subclass of the
   builtin :class:`dict` class.  It overrides one method and adds one writable
   instance variable.  The remaining functionality is the same as for the
   :class:`dict` class and is not documented here.

   The first argument provides the initial value for the :attr:`default_factory`
   attribute; it defaults to ``None``. All remaining arguments are treated the same
   as if they were passed to the :class:`dict` constructor, including keyword
   arguments.


:class:`defaultdict` objects support the following method in addition to the
standard :class:`dict` operations:

.. method:: defaultdict.__missing__(key)

   If the :attr:`default_factory` attribute is ``None``, this raises an
   :exc:`KeyError` exception with the *key* as argument.

   If :attr:`default_factory` is not ``None``, it is called without arguments to
   provide a default value for the given *key*, this value is inserted in the
   dictionary for the *key*, and returned.

   If calling :attr:`default_factory` raises an exception this exception is
   propagated unchanged.

   This method is called by the :meth:`__getitem__` method of the :class:`dict`
   class when the requested key is not found; whatever it returns or raises is then
   returned or raised by :meth:`__getitem__`.

:class:`defaultdict` objects support the following instance variable:


.. attribute:: defaultdict.default_factory

   This attribute is used by the :meth:`__missing__` method; it is initialized from
   the first argument to the constructor, if present, or to ``None``,  if absent.


.. _defaultdict-examples:

:class:`defaultdict` Examples
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Using :class:`list` as the :attr:`default_factory`, it is easy to group a
sequence of key-value pairs into a dictionary of lists::

   >>> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
   >>> d = defaultdict(list)
   >>> for k, v in s:
   ...     d[k].append(v)
   ...
   >>> d.items()
   [('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

When each key is encountered for the first time, it is not already in the
mapping; so an entry is automatically created using the :attr:`default_factory`
function which returns an empty :class:`list`.  The :meth:`list.append`
operation then attaches the value to the new list.  When keys are encountered
again, the look-up proceeds normally (returning the list for that key) and the
:meth:`list.append` operation adds another value to the list. This technique is
simpler and faster than an equivalent technique using :meth:`dict.setdefault`::

   >>> d = {}
   >>> for k, v in s:
   ...     d.setdefault(k, []).append(v)
   ...
   >>> d.items()
   [('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

Setting the :attr:`default_factory` to :class:`int` makes the
:class:`defaultdict` useful for counting (like a bag or multiset in other
languages)::

   >>> s = 'mississippi'
   >>> d = defaultdict(int)
   >>> for k in s:
   ...     d[k] += 1
   ...
   >>> d.items()
   [('i', 4), ('p', 2), ('s', 4), ('m', 1)]

When a letter is first encountered, it is missing from the mapping, so the
:attr:`default_factory` function calls :func:`int` to supply a default count of
zero.  The increment operation then builds up the count for each letter.

The function :func:`int` which always returns zero is just a special case of
constant functions.  A faster and more flexible way to create constant functions
is to use a lambda function which can supply any constant value (not just
zero)::

   >>> def constant_factory(value):
   ...     return lambda: value
   >>> d = defaultdict(constant_factory('<missing>'))
   >>> d.update(name='John', action='ran')
   >>> '%(name)s %(action)s to %(object)s' % d
   'John ran to <missing>'

Setting the :attr:`default_factory` to :class:`set` makes the
:class:`defaultdict` useful for building a dictionary of sets::

   >>> s = [('red', 1), ('blue', 2), ('red', 3), ('blue', 4), ('red', 1), ('blue', 4)]
   >>> d = defaultdict(set)
   >>> for k, v in s:
   ...     d[k].add(v)
   ...
   >>> d.items()
   [('blue', set([2, 4])), ('red', set([1, 3]))]


.. _named-tuple-factory:

:func:`NamedTuple` datatype factory function
--------------------------------------------


.. function:: NamedTuple(typename, fieldnames)

   Returns a new tuple subclass named *typename*.  The new subclass is used to
   create tuple-like objects that have fields accessable by attribute lookup as
   well as being indexable and iterable.  Instances of the subclass also have a
   helpful docstring (with typename and fieldnames) and a helpful :meth:`__repr__`
   method which lists the tuple contents in a ``name=value`` format.

   The *fieldnames* are specified in a single string and are separated by spaces.
   Any valid Python identifier may be used for a field name.

   Example::

      >>> Point = NamedTuple('Point', 'x y')
      >>> Point.__doc__           # docstring for the new datatype
      'Point(x, y)'
      >>> p = Point(11, y=22)     # instantiate with positional or keyword arguments
      >>> p[0] + p[1]             # works just like the tuple (11, 22)
      33
      >>> x, y = p                # unpacks just like a tuple
      >>> x, y
      (11, 22)
      >>> p.x + p.y               # fields also accessable by name
      33
      >>> p                       # readable __repr__ with name=value style
      Point(x=11, y=22)  

   The use cases are the same as those for tuples.  The named factories assign
   meaning to each tuple position and allow for more readable, self-documenting
   code.  Named tuples can also be used to assign field names  to tuples returned
   by the :mod:`csv` or :mod:`sqlite3` modules. For example::

      from itertools import starmap
      import csv
      EmployeeRecord = NamedTuple('EmployeeRecord', 'name age title department paygrade')
      for record in starmap(EmployeeRecord, csv.reader(open("employees.csv", "rb"))):
          print(record)

   To cast an individual record stored as :class:`list`, :class:`tuple`, or some
   other iterable type, use the star-operator [#]_ to unpack the values::

      >>> Color = NamedTuple('Color', 'name code')
      >>> m = dict(red=1, green=2, blue=3)
      >>> print(Color(*m.popitem()))
      Color(name='blue', code=3)

.. rubric:: Footnotes

.. [#] For information on the star-operator see
   :ref:`tut-unpacking-arguments` and :ref:`calls`.