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