mirror of https://github.com/python/cpython
474 lines
16 KiB
ReStructuredText
474 lines
16 KiB
ReStructuredText
|
|
: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[, maxlen]])
|
|
|
|
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
|
|
|
|
If *maxlen* is not specified or is *-1*, deques may grow to an
|
|
arbitrary length. Otherwise, the deque is bounded to the specified maximum
|
|
length. Once a bounded length deque is full, when new items are added, a
|
|
corresponding number of items are discarded from the opposite end. Bounded
|
|
length deques provide functionality similar to the ``tail`` filter in
|
|
Unix. They are also useful for tracking transactions and other pools of data
|
|
where only the most recent activity is of interest.
|
|
|
|
.. versionchanged:: 2.6
|
|
Added *maxlen*
|
|
|
|
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:
|
|
|
|
:class:`deque` 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``.
|
|
|
|
Multi-pass data reduction algorithms can be succinctly expressed and efficiently
|
|
coded by extracting elements with multiple calls to :meth:`popleft`, applying
|
|
a reduction function, and calling :meth:`append` to add the result back to the
|
|
deque.
|
|
|
|
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']]]]
|
|
|
|
Bounded length deques provide functionality similar to the ``tail`` filter
|
|
in Unix::
|
|
|
|
def tail(filename, n=10):
|
|
'Return the last n lines of a file'
|
|
return deque(open(filename), n)
|
|
|
|
.. _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 :func:`itertools.repeat` which can supply any constant value (not just
|
|
zero)::
|
|
|
|
>>> def constant_factory(value):
|
|
... return itertools.repeat(value).next
|
|
>>> 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` Factory Function for Tuples with Named Fields
|
|
----------------------------------------------------------------
|
|
|
|
Named tuples assign meaning to each position in a tuple and allow for more readable,
|
|
self-documenting code. They can be used wherever regular tuples are used, and
|
|
they add the ability to access fields by name instead of position index.
|
|
|
|
.. function:: NamedTuple(typename, fieldnames, [verbose])
|
|
|
|
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 with each fieldname separated by
|
|
a space and/or comma. Any valid Python identifier may be used for a fieldname
|
|
except for names starting and ending with double underscores.
|
|
|
|
If *verbose* is true, will print the class definition.
|
|
|
|
*NamedTuple* instances do not have per-instance dictionaries, so they are
|
|
lightweight and require no more memory than regular tuples.
|
|
|
|
.. versionadded:: 2.6
|
|
|
|
Example::
|
|
|
|
>>> Point = NamedTuple('Point', 'x y', verbose=True)
|
|
class Point(tuple):
|
|
'Point(x, y)'
|
|
__slots__ = ()
|
|
__fields__ = ('x', 'y')
|
|
def __new__(cls, x, y):
|
|
return tuple.__new__(cls, (x, y))
|
|
def __repr__(self):
|
|
return 'Point(x=%r, y=%r)' % self
|
|
def __asdict__(self):
|
|
'Return a new dict mapping field names to their values'
|
|
return dict(zip(('x', 'y'), self))
|
|
def __replace__(self, field, value):
|
|
'Return a new Point object replacing one field with a new value'
|
|
return Point(**dict(zip(('x', 'y'), self) + [(field, value)]))
|
|
x = property(itemgetter(0))
|
|
y = property(itemgetter(1))
|
|
|
|
>>> p = Point(11, y=22) # instantiate with positional or keyword arguments
|
|
>>> p[0] + p[1] # indexable like the regular tuple (11, 22)
|
|
33
|
|
>>> x, y = p # unpack like a regular tuple
|
|
>>> x, y
|
|
(11, 22)
|
|
>>> p.x + p.y # fields also accessable by name
|
|
33
|
|
>>> p # readable __repr__ with a name=value style
|
|
Point(x=11, y=22)
|
|
|
|
Named tuples are especially useful for assigning field names to result tuples returned
|
|
by the :mod:`csv` or :mod:`sqlite3` modules::
|
|
|
|
from itertools import starmap
|
|
import csv
|
|
EmployeeRecord = NamedTuple('EmployeeRecord', 'name age title department paygrade')
|
|
for emp in starmap(EmployeeRecord, csv.reader(open("employees.csv", "rb"))):
|
|
print emp.name, emp.title
|
|
|
|
When casting a single record to a *NamedTuple*, use the star-operator [#]_ to unpack
|
|
the values::
|
|
|
|
>>> t = [11, 22]
|
|
>>> Point(*t) # the star-operator unpacks any iterable object
|
|
Point(x=11, y=22)
|
|
|
|
When casting a dictionary to a *NamedTuple*, use the double-star-operator::
|
|
|
|
>>> d = {'x': 11, 'y': 22}
|
|
>>> Point(**d)
|
|
Point(x=11, y=22)
|
|
|
|
In addition to the methods inherited from tuples, named tuples support
|
|
additonal methods and a read-only attribute.
|
|
|
|
.. method:: somenamedtuple.__asdict__()
|
|
|
|
Return a new dict which maps field names to their corresponding values:
|
|
|
|
::
|
|
|
|
>>> p.__asdict__()
|
|
{'x': 11, 'y': 22}
|
|
|
|
.. method:: somenamedtuple.__replace__(field, value)
|
|
|
|
Return a new instance of the named tuple replacing the named *field* with a new *value*:
|
|
|
|
::
|
|
|
|
>>> p = Point(x=11, y=22)
|
|
>>> p.__replace__('x', 33)
|
|
Point(x=33, y=22)
|
|
|
|
>>> for recordnum, record in inventory:
|
|
... inventory[recordnum] = record.replace('total', record.price * record.quantity)
|
|
|
|
.. attribute:: somenamedtuple.__fields__
|
|
|
|
Return a tuple of strings listing the field names. This is useful for introspection
|
|
and for creating new named tuple types from existing named tuples.
|
|
|
|
::
|
|
|
|
>>> p.__fields__ # view the field names
|
|
('x', 'y')
|
|
|
|
>>> Color = NamedTuple('Color', 'red green blue')
|
|
>>> Pixel = NamedTuple('Pixel', ' '.join(Point.__fields__ + Color.__fields__))
|
|
>>> Pixel(11, 22, 128, 255, 0)
|
|
Pixel(x=11, y=22, red=128, green=255, blue=0)'
|
|
|
|
.. rubric:: Footnotes
|
|
|
|
.. [#] For information on the star-operator see
|
|
:ref:`tut-unpacking-arguments` and :ref:`calls`.
|