cpython/Doc/library/collections.rst

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:mod:`collections` --- High-performance container datatypes
===========================================================
.. module:: collections
:synopsis: High-performance datatypes
.. moduleauthor:: Raymond Hettinger <python@rcn.com>
.. sectionauthor:: Raymond Hettinger <python@rcn.com>
.. versionadded:: 2.4
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.
.. versionchanged:: 2.5
Added :class:`defaultdict`.
.. versionchanged:: 2.6
Added :class:`NamedTuple`.
.. _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.
.. versionadded:: 2.4
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`.
.. versionadded:: 2.5
.. 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.
.. versionadded:: 2.5
: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.
.. versionadded:: 2.6
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)