Raymond Hettinger | e5820c6 | 2011-03-22 09:11:39 -0700 | [diff] [blame] | 1 | |
Raymond Hettinger | 53dbe39 | 2008-02-12 20:03:09 +0000 | [diff] [blame] | 2 | :mod:`collections` --- Container datatypes |
| 3 | ========================================== |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 4 | |
| 5 | .. module:: collections |
Raymond Hettinger | 53dbe39 | 2008-02-12 20:03:09 +0000 | [diff] [blame] | 6 | :synopsis: Container datatypes |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 7 | .. moduleauthor:: Raymond Hettinger <python@rcn.com> |
| 8 | .. sectionauthor:: Raymond Hettinger <python@rcn.com> |
| 9 | |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 10 | .. testsetup:: * |
| 11 | |
| 12 | from collections import * |
| 13 | import itertools |
| 14 | __name__ = '<doctest>' |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 15 | |
Raymond Hettinger | a199368 | 2011-01-27 01:20:32 +0000 | [diff] [blame] | 16 | **Source code:** :source:`Lib/collections.py` and :source:`Lib/_abcoll.py` |
Raymond Hettinger | 1048094 | 2011-01-10 03:26:08 +0000 | [diff] [blame] | 17 | |
Raymond Hettinger | 4f707fd | 2011-01-10 19:54:11 +0000 | [diff] [blame] | 18 | -------------- |
| 19 | |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 20 | This module implements specialized container datatypes providing alternatives to |
| 21 | Python's general purpose built-in containers, :class:`dict`, :class:`list`, |
| 22 | :class:`set`, and :class:`tuple`. |
Christian Heimes | 0bd4e11 | 2008-02-12 22:59:25 +0000 | [diff] [blame] | 23 | |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 24 | ===================== ==================================================================== |
| 25 | :func:`namedtuple` factory function for creating tuple subclasses with named fields |
| 26 | :class:`deque` list-like container with fast appends and pops on either end |
| 27 | :class:`Counter` dict subclass for counting hashable objects |
| 28 | :class:`OrderedDict` dict subclass that remembers the order entries were added |
| 29 | :class:`defaultdict` dict subclass that calls a factory function to supply missing values |
| 30 | :class:`UserDict` wrapper around dictionary objects for easier dict subclassing |
| 31 | :class:`UserList` wrapper around list objects for easier list subclassing |
| 32 | :class:`UserString` wrapper around string objects for easier string subclassing |
| 33 | ===================== ==================================================================== |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 34 | |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 35 | In addition to the concrete container classes, the collections module provides |
Éric Araujo | fa088db | 2011-06-04 18:42:38 +0200 | [diff] [blame] | 36 | :ref:`abstract base classes <collections-abstract-base-classes>` that can be |
| 37 | used to test whether a class provides a particular interface, for example, |
| 38 | whether it is hashable or a mapping. |
Mark Summerfield | 08898b4 | 2007-09-05 08:43:04 +0000 | [diff] [blame] | 39 | |
| 40 | |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 41 | :class:`Counter` objects |
| 42 | ------------------------ |
| 43 | |
| 44 | A counter tool is provided to support convenient and rapid tallies. |
| 45 | For example:: |
| 46 | |
Raymond Hettinger | 1c62dc9 | 2009-02-04 11:41:45 +0000 | [diff] [blame] | 47 | >>> # Tally occurrences of words in a list |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 48 | >>> cnt = Counter() |
Raymond Hettinger | 670eaec | 2009-01-21 23:14:07 +0000 | [diff] [blame] | 49 | >>> for word in ['red', 'blue', 'red', 'green', 'blue', 'blue']: |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 50 | ... cnt[word] += 1 |
| 51 | >>> cnt |
| 52 | Counter({'blue': 3, 'red': 2, 'green': 1}) |
| 53 | |
Raymond Hettinger | 1c62dc9 | 2009-02-04 11:41:45 +0000 | [diff] [blame] | 54 | >>> # Find the ten most common words in Hamlet |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 55 | >>> import re |
| 56 | >>> words = re.findall('\w+', open('hamlet.txt').read().lower()) |
Raymond Hettinger | 0bae662 | 2009-01-20 13:00:59 +0000 | [diff] [blame] | 57 | >>> Counter(words).most_common(10) |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 58 | [('the', 1143), ('and', 966), ('to', 762), ('of', 669), ('i', 631), |
| 59 | ('you', 554), ('a', 546), ('my', 514), ('hamlet', 471), ('in', 451)] |
| 60 | |
| 61 | .. class:: Counter([iterable-or-mapping]) |
| 62 | |
Raymond Hettinger | 670eaec | 2009-01-21 23:14:07 +0000 | [diff] [blame] | 63 | A :class:`Counter` is a :class:`dict` subclass for counting hashable objects. |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 64 | It is an unordered collection where elements are stored as dictionary keys |
| 65 | and their counts are stored as dictionary values. Counts are allowed to be |
| 66 | any integer value including zero or negative counts. The :class:`Counter` |
| 67 | class is similar to bags or multisets in other languages. |
| 68 | |
| 69 | Elements are counted from an *iterable* or initialized from another |
Benjamin Peterson | 25c95f1 | 2009-05-08 20:42:26 +0000 | [diff] [blame] | 70 | *mapping* (or counter): |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 71 | |
Raymond Hettinger | 73662a5 | 2009-01-27 02:38:22 +0000 | [diff] [blame] | 72 | >>> c = Counter() # a new, empty counter |
| 73 | >>> c = Counter('gallahad') # a new counter from an iterable |
| 74 | >>> c = Counter({'red': 4, 'blue': 2}) # a new counter from a mapping |
| 75 | >>> c = Counter(cats=4, dogs=8) # a new counter from keyword args |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 76 | |
Raymond Hettinger | 670eaec | 2009-01-21 23:14:07 +0000 | [diff] [blame] | 77 | Counter objects have a dictionary interface except that they return a zero |
Benjamin Peterson | 25c95f1 | 2009-05-08 20:42:26 +0000 | [diff] [blame] | 78 | count for missing items instead of raising a :exc:`KeyError`: |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 79 | |
Raymond Hettinger | 94adc8e | 2009-01-22 05:27:37 +0000 | [diff] [blame] | 80 | >>> c = Counter(['eggs', 'ham']) |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 81 | >>> c['bacon'] # count of a missing element is zero |
| 82 | 0 |
| 83 | |
Raymond Hettinger | 94adc8e | 2009-01-22 05:27:37 +0000 | [diff] [blame] | 84 | Setting a count to zero does not remove an element from a counter. |
| 85 | Use ``del`` to remove it entirely: |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 86 | |
Raymond Hettinger | 94adc8e | 2009-01-22 05:27:37 +0000 | [diff] [blame] | 87 | >>> c['sausage'] = 0 # counter entry with a zero count |
| 88 | >>> del c['sausage'] # del actually removes the entry |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 89 | |
Benjamin Peterson | d45bf58 | 2009-03-02 21:44:54 +0000 | [diff] [blame] | 90 | .. versionadded:: 3.1 |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 91 | |
| 92 | |
Ezio Melotti | 0be8b1c | 2010-04-04 06:53:44 +0000 | [diff] [blame] | 93 | Counter objects support three methods beyond those available for all |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 94 | dictionaries: |
| 95 | |
| 96 | .. method:: elements() |
| 97 | |
Raymond Hettinger | 670eaec | 2009-01-21 23:14:07 +0000 | [diff] [blame] | 98 | Return an iterator over elements repeating each as many times as its |
| 99 | count. Elements are returned in arbitrary order. If an element's count |
| 100 | is less than one, :meth:`elements` will ignore it. |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 101 | |
Raymond Hettinger | 0bae662 | 2009-01-20 13:00:59 +0000 | [diff] [blame] | 102 | >>> c = Counter(a=4, b=2, c=0, d=-2) |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 103 | >>> list(c.elements()) |
| 104 | ['a', 'a', 'a', 'a', 'b', 'b'] |
| 105 | |
| 106 | .. method:: most_common([n]) |
| 107 | |
Raymond Hettinger | 73662a5 | 2009-01-27 02:38:22 +0000 | [diff] [blame] | 108 | Return a list of the *n* most common elements and their counts from the |
Raymond Hettinger | d04fa31 | 2009-02-04 19:45:13 +0000 | [diff] [blame] | 109 | most common to the least. If *n* is not specified, :func:`most_common` |
Raymond Hettinger | 73662a5 | 2009-01-27 02:38:22 +0000 | [diff] [blame] | 110 | returns *all* elements in the counter. Elements with equal counts are |
Benjamin Peterson | 25c95f1 | 2009-05-08 20:42:26 +0000 | [diff] [blame] | 111 | ordered arbitrarily: |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 112 | |
| 113 | >>> Counter('abracadabra').most_common(3) |
| 114 | [('a', 5), ('r', 2), ('b', 2)] |
| 115 | |
Raymond Hettinger | 9c01e44 | 2010-04-03 10:32:58 +0000 | [diff] [blame] | 116 | .. method:: subtract([iterable-or-mapping]) |
| 117 | |
| 118 | Elements are subtracted from an *iterable* or from another *mapping* |
| 119 | (or counter). Like :meth:`dict.update` but subtracts counts instead |
| 120 | of replacing them. Both inputs and outputs may be zero or negative. |
| 121 | |
| 122 | >>> c = Counter(a=4, b=2, c=0, d=-2) |
| 123 | >>> d = Counter(a=1, b=2, c=3, d=4) |
| 124 | >>> c.subtract(d) |
| 125 | Counter({'a': 3, 'b': 0, 'c': -3, 'd': -6}) |
| 126 | |
Ezio Melotti | 0be8b1c | 2010-04-04 06:53:44 +0000 | [diff] [blame] | 127 | .. versionadded:: 3.2 |
| 128 | |
Raymond Hettinger | 670eaec | 2009-01-21 23:14:07 +0000 | [diff] [blame] | 129 | The usual dictionary methods are available for :class:`Counter` objects |
| 130 | except for two which work differently for counters. |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 131 | |
| 132 | .. method:: fromkeys(iterable) |
| 133 | |
Raymond Hettinger | 73662a5 | 2009-01-27 02:38:22 +0000 | [diff] [blame] | 134 | This class method is not implemented for :class:`Counter` objects. |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 135 | |
| 136 | .. method:: update([iterable-or-mapping]) |
| 137 | |
Raymond Hettinger | 73662a5 | 2009-01-27 02:38:22 +0000 | [diff] [blame] | 138 | Elements are counted from an *iterable* or added-in from another |
| 139 | *mapping* (or counter). Like :meth:`dict.update` but adds counts |
| 140 | instead of replacing them. Also, the *iterable* is expected to be a |
| 141 | sequence of elements, not a sequence of ``(key, value)`` pairs. |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 142 | |
| 143 | Common patterns for working with :class:`Counter` objects:: |
| 144 | |
Raymond Hettinger | 73662a5 | 2009-01-27 02:38:22 +0000 | [diff] [blame] | 145 | sum(c.values()) # total of all counts |
| 146 | c.clear() # reset all counts |
| 147 | list(c) # list unique elements |
| 148 | set(c) # convert to a set |
| 149 | dict(c) # convert to a regular dictionary |
| 150 | c.items() # convert to a list of (elem, cnt) pairs |
| 151 | Counter(dict(list_of_pairs)) # convert from a list of (elem, cnt) pairs |
| 152 | c.most_common()[:-n:-1] # n least common elements |
| 153 | c += Counter() # remove zero and negative counts |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 154 | |
Raymond Hettinger | 72a95cc | 2009-02-25 22:51:40 +0000 | [diff] [blame] | 155 | Several mathematical operations are provided for combining :class:`Counter` |
| 156 | objects to produce multisets (counters that have counts greater than zero). |
| 157 | Addition and subtraction combine counters by adding or subtracting the counts |
| 158 | of corresponding elements. Intersection and union return the minimum and |
| 159 | maximum of corresponding counts. Each operation can accept inputs with signed |
| 160 | counts, but the output will exclude results with counts of zero or less. |
Raymond Hettinger | 4d2073a | 2009-01-20 03:41:22 +0000 | [diff] [blame] | 161 | |
Raymond Hettinger | e0d1b9f | 2009-01-21 20:36:27 +0000 | [diff] [blame] | 162 | >>> c = Counter(a=3, b=1) |
| 163 | >>> d = Counter(a=1, b=2) |
Raymond Hettinger | 73662a5 | 2009-01-27 02:38:22 +0000 | [diff] [blame] | 164 | >>> c + d # add two counters together: c[x] + d[x] |
Raymond Hettinger | 4d2073a | 2009-01-20 03:41:22 +0000 | [diff] [blame] | 165 | Counter({'a': 4, 'b': 3}) |
Raymond Hettinger | 73662a5 | 2009-01-27 02:38:22 +0000 | [diff] [blame] | 166 | >>> c - d # subtract (keeping only positive counts) |
Raymond Hettinger | 4d2073a | 2009-01-20 03:41:22 +0000 | [diff] [blame] | 167 | Counter({'a': 2}) |
Raymond Hettinger | 73662a5 | 2009-01-27 02:38:22 +0000 | [diff] [blame] | 168 | >>> c & d # intersection: min(c[x], d[x]) |
Raymond Hettinger | 4d2073a | 2009-01-20 03:41:22 +0000 | [diff] [blame] | 169 | Counter({'a': 1, 'b': 1}) |
Raymond Hettinger | 73662a5 | 2009-01-27 02:38:22 +0000 | [diff] [blame] | 170 | >>> c | d # union: max(c[x], d[x]) |
Raymond Hettinger | 4d2073a | 2009-01-20 03:41:22 +0000 | [diff] [blame] | 171 | Counter({'a': 3, 'b': 2}) |
| 172 | |
Raymond Hettinger | 22f1885 | 2010-04-12 21:45:14 +0000 | [diff] [blame] | 173 | .. note:: |
| 174 | |
| 175 | Counters were primarily designed to work with positive integers to represent |
| 176 | running counts; however, care was taken to not unnecessarily preclude use |
| 177 | cases needing other types or negative values. To help with those use cases, |
| 178 | this section documents the minimum range and type restrictions. |
| 179 | |
| 180 | * The :class:`Counter` class itself is a dictionary subclass with no |
| 181 | restrictions on its keys and values. The values are intended to be numbers |
| 182 | representing counts, but you *could* store anything in the value field. |
| 183 | |
| 184 | * The :meth:`most_common` method requires only that the values be orderable. |
| 185 | |
| 186 | * For in-place operations such as ``c[key] += 1``, the value type need only |
| 187 | support addition and subtraction. So fractions, floats, and decimals would |
| 188 | work and negative values are supported. The same is also true for |
| 189 | :meth:`update` and :meth:`subtract` which allow negative and zero values |
| 190 | for both inputs and outputs. |
| 191 | |
| 192 | * The multiset methods are designed only for use cases with positive values. |
| 193 | The inputs may be negative or zero, but only outputs with positive values |
| 194 | are created. There are no type restrictions, but the value type needs to |
| 195 | support support addition, subtraction, and comparison. |
| 196 | |
| 197 | * The :meth:`elements` method requires integer counts. It ignores zero and |
| 198 | negative counts. |
| 199 | |
Raymond Hettinger | b14043c | 2009-01-20 23:44:31 +0000 | [diff] [blame] | 200 | .. seealso:: |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 201 | |
Raymond Hettinger | 94adc8e | 2009-01-22 05:27:37 +0000 | [diff] [blame] | 202 | * `Counter class <http://code.activestate.com/recipes/576611/>`_ |
| 203 | adapted for Python 2.5 and an early `Bag recipe |
| 204 | <http://code.activestate.com/recipes/259174/>`_ for Python 2.4. |
| 205 | |
Raymond Hettinger | b14043c | 2009-01-20 23:44:31 +0000 | [diff] [blame] | 206 | * `Bag class <http://www.gnu.org/software/smalltalk/manual-base/html_node/Bag.html>`_ |
| 207 | in Smalltalk. |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 208 | |
Éric Araujo | 08c9bd5 | 2011-04-24 02:59:02 +0200 | [diff] [blame] | 209 | * Wikipedia entry for `Multisets <http://en.wikipedia.org/wiki/Multiset>`_. |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 210 | |
Raymond Hettinger | b14043c | 2009-01-20 23:44:31 +0000 | [diff] [blame] | 211 | * `C++ multisets <http://www.demo2s.com/Tutorial/Cpp/0380__set-multiset/Catalog0380__set-multiset.htm>`_ |
Raymond Hettinger | 94adc8e | 2009-01-22 05:27:37 +0000 | [diff] [blame] | 212 | tutorial with examples. |
Raymond Hettinger | b14043c | 2009-01-20 23:44:31 +0000 | [diff] [blame] | 213 | |
Raymond Hettinger | 94adc8e | 2009-01-22 05:27:37 +0000 | [diff] [blame] | 214 | * For mathematical operations on multisets and their use cases, see |
Raymond Hettinger | b14043c | 2009-01-20 23:44:31 +0000 | [diff] [blame] | 215 | *Knuth, Donald. The Art of Computer Programming Volume II, |
Éric Araujo | 08c9bd5 | 2011-04-24 02:59:02 +0200 | [diff] [blame] | 216 | Section 4.6.3, Exercise 19*. |
Raymond Hettinger | b14043c | 2009-01-20 23:44:31 +0000 | [diff] [blame] | 217 | |
Raymond Hettinger | 670eaec | 2009-01-21 23:14:07 +0000 | [diff] [blame] | 218 | * To enumerate all distinct multisets of a given size over a given set of |
Raymond Hettinger | d07d939 | 2009-01-27 04:20:44 +0000 | [diff] [blame] | 219 | elements, see :func:`itertools.combinations_with_replacement`. |
Raymond Hettinger | b14043c | 2009-01-20 23:44:31 +0000 | [diff] [blame] | 220 | |
Raymond Hettinger | 94adc8e | 2009-01-22 05:27:37 +0000 | [diff] [blame] | 221 | map(Counter, combinations_with_replacement('ABC', 2)) --> AA AB AC BB BC CC |
Raymond Hettinger | b8baf63 | 2009-01-14 02:20:07 +0000 | [diff] [blame] | 222 | |
| 223 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 224 | :class:`deque` objects |
| 225 | ---------------------- |
| 226 | |
Georg Brandl | c2a4f4f | 2009-04-10 09:03:43 +0000 | [diff] [blame] | 227 | .. class:: deque([iterable, [maxlen]]) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 228 | |
| 229 | Returns a new deque object initialized left-to-right (using :meth:`append`) with |
| 230 | data from *iterable*. If *iterable* is not specified, the new deque is empty. |
| 231 | |
| 232 | Deques are a generalization of stacks and queues (the name is pronounced "deck" |
| 233 | and is short for "double-ended queue"). Deques support thread-safe, memory |
| 234 | efficient appends and pops from either side of the deque with approximately the |
| 235 | same O(1) performance in either direction. |
| 236 | |
| 237 | Though :class:`list` objects support similar operations, they are optimized for |
| 238 | fast fixed-length operations and incur O(n) memory movement costs for |
| 239 | ``pop(0)`` and ``insert(0, v)`` operations which change both the size and |
| 240 | position of the underlying data representation. |
| 241 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 242 | |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 243 | If *maxlen* is not specified or is *None*, deques may grow to an |
| 244 | arbitrary length. Otherwise, the deque is bounded to the specified maximum |
| 245 | length. Once a bounded length deque is full, when new items are added, a |
| 246 | corresponding number of items are discarded from the opposite end. Bounded |
| 247 | length deques provide functionality similar to the ``tail`` filter in |
| 248 | Unix. They are also useful for tracking transactions and other pools of data |
| 249 | where only the most recent activity is of interest. |
| 250 | |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 251 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 252 | Deque objects support the following methods: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 253 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 254 | .. method:: append(x) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 255 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 256 | Add *x* to the right side of the deque. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 257 | |
| 258 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 259 | .. method:: appendleft(x) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 260 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 261 | Add *x* to the left side of the deque. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 262 | |
| 263 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 264 | .. method:: clear() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 265 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 266 | Remove all elements from the deque leaving it with length 0. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 267 | |
| 268 | |
Raymond Hettinger | 44459de | 2010-04-03 23:20:46 +0000 | [diff] [blame] | 269 | .. method:: count(x) |
| 270 | |
| 271 | Count the number of deque elements equal to *x*. |
| 272 | |
| 273 | .. versionadded:: 3.2 |
| 274 | |
Georg Brandl | 67b21b7 | 2010-08-17 15:07:14 +0000 | [diff] [blame] | 275 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 276 | .. method:: extend(iterable) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 277 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 278 | Extend the right side of the deque by appending elements from the iterable |
| 279 | argument. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 280 | |
| 281 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 282 | .. method:: extendleft(iterable) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 283 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 284 | Extend the left side of the deque by appending elements from *iterable*. |
| 285 | Note, the series of left appends results in reversing the order of |
| 286 | elements in the iterable argument. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 287 | |
| 288 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 289 | .. method:: pop() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 290 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 291 | Remove and return an element from the right side of the deque. If no |
| 292 | elements are present, raises an :exc:`IndexError`. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 293 | |
| 294 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 295 | .. method:: popleft() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 296 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 297 | Remove and return an element from the left side of the deque. If no |
| 298 | elements are present, raises an :exc:`IndexError`. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 299 | |
| 300 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 301 | .. method:: remove(value) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 302 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 303 | Removed the first occurrence of *value*. If not found, raises a |
| 304 | :exc:`ValueError`. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 305 | |
Georg Brandl | 67b21b7 | 2010-08-17 15:07:14 +0000 | [diff] [blame] | 306 | |
Raymond Hettinger | e5fdedb | 2009-12-10 00:47:21 +0000 | [diff] [blame] | 307 | .. method:: reverse() |
| 308 | |
| 309 | Reverse the elements of the deque in-place and then return ``None``. |
| 310 | |
| 311 | .. versionadded:: 3.2 |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 312 | |
Georg Brandl | 67b21b7 | 2010-08-17 15:07:14 +0000 | [diff] [blame] | 313 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 314 | .. method:: rotate(n) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 315 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 316 | Rotate the deque *n* steps to the right. If *n* is negative, rotate to |
| 317 | the left. Rotating one step to the right is equivalent to: |
| 318 | ``d.appendleft(d.pop())``. |
| 319 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 320 | |
Raymond Hettinger | 5bb0f0e | 2009-03-10 12:56:32 +0000 | [diff] [blame] | 321 | Deque objects also provide one read-only attribute: |
| 322 | |
| 323 | .. attribute:: maxlen |
| 324 | |
| 325 | Maximum size of a deque or *None* if unbounded. |
| 326 | |
Raymond Hettinger | 150fb9c | 2009-03-10 22:48:06 +0000 | [diff] [blame] | 327 | .. versionadded:: 3.1 |
Raymond Hettinger | 5bb0f0e | 2009-03-10 12:56:32 +0000 | [diff] [blame] | 328 | |
| 329 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 330 | In addition to the above, deques support iteration, pickling, ``len(d)``, |
| 331 | ``reversed(d)``, ``copy.copy(d)``, ``copy.deepcopy(d)``, membership testing with |
Benjamin Peterson | 206e307 | 2008-10-19 14:07:49 +0000 | [diff] [blame] | 332 | the :keyword:`in` operator, and subscript references such as ``d[-1]``. Indexed |
| 333 | access is O(1) at both ends but slows to O(n) in the middle. For fast random |
| 334 | access, use lists instead. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 335 | |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 336 | Example: |
| 337 | |
| 338 | .. doctest:: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 339 | |
| 340 | >>> from collections import deque |
| 341 | >>> d = deque('ghi') # make a new deque with three items |
| 342 | >>> for elem in d: # iterate over the deque's elements |
Neal Norwitz | 752abd0 | 2008-05-13 04:55:24 +0000 | [diff] [blame] | 343 | ... print(elem.upper()) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 344 | G |
| 345 | H |
| 346 | I |
| 347 | |
| 348 | >>> d.append('j') # add a new entry to the right side |
| 349 | >>> d.appendleft('f') # add a new entry to the left side |
| 350 | >>> d # show the representation of the deque |
| 351 | deque(['f', 'g', 'h', 'i', 'j']) |
| 352 | |
| 353 | >>> d.pop() # return and remove the rightmost item |
| 354 | 'j' |
| 355 | >>> d.popleft() # return and remove the leftmost item |
| 356 | 'f' |
| 357 | >>> list(d) # list the contents of the deque |
| 358 | ['g', 'h', 'i'] |
| 359 | >>> d[0] # peek at leftmost item |
| 360 | 'g' |
| 361 | >>> d[-1] # peek at rightmost item |
| 362 | 'i' |
| 363 | |
| 364 | >>> list(reversed(d)) # list the contents of a deque in reverse |
| 365 | ['i', 'h', 'g'] |
| 366 | >>> 'h' in d # search the deque |
| 367 | True |
| 368 | >>> d.extend('jkl') # add multiple elements at once |
| 369 | >>> d |
| 370 | deque(['g', 'h', 'i', 'j', 'k', 'l']) |
| 371 | >>> d.rotate(1) # right rotation |
| 372 | >>> d |
| 373 | deque(['l', 'g', 'h', 'i', 'j', 'k']) |
| 374 | >>> d.rotate(-1) # left rotation |
| 375 | >>> d |
| 376 | deque(['g', 'h', 'i', 'j', 'k', 'l']) |
| 377 | |
| 378 | >>> deque(reversed(d)) # make a new deque in reverse order |
| 379 | deque(['l', 'k', 'j', 'i', 'h', 'g']) |
| 380 | >>> d.clear() # empty the deque |
| 381 | >>> d.pop() # cannot pop from an empty deque |
| 382 | Traceback (most recent call last): |
| 383 | File "<pyshell#6>", line 1, in -toplevel- |
| 384 | d.pop() |
| 385 | IndexError: pop from an empty deque |
| 386 | |
| 387 | >>> d.extendleft('abc') # extendleft() reverses the input order |
| 388 | >>> d |
| 389 | deque(['c', 'b', 'a']) |
| 390 | |
| 391 | |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 392 | :class:`deque` Recipes |
| 393 | ^^^^^^^^^^^^^^^^^^^^^^ |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 394 | |
| 395 | This section shows various approaches to working with deques. |
| 396 | |
Raymond Hettinger | d2ee64d | 2009-03-31 22:52:48 +0000 | [diff] [blame] | 397 | Bounded length deques provide functionality similar to the ``tail`` filter |
| 398 | in Unix:: |
| 399 | |
| 400 | def tail(filename, n=10): |
| 401 | 'Return the last n lines of a file' |
| 402 | return deque(open(filename), n) |
| 403 | |
| 404 | Another approach to using deques is to maintain a sequence of recently |
| 405 | added elements by appending to the right and popping to the left:: |
| 406 | |
| 407 | def moving_average(iterable, n=3): |
| 408 | # moving_average([40, 30, 50, 46, 39, 44]) --> 40.0 42.0 45.0 43.0 |
| 409 | # http://en.wikipedia.org/wiki/Moving_average |
| 410 | it = iter(iterable) |
Raymond Hettinger | d40285a | 2009-05-22 01:11:26 +0000 | [diff] [blame] | 411 | d = deque(itertools.islice(it, n-1)) |
| 412 | d.appendleft(0) |
Raymond Hettinger | d2ee64d | 2009-03-31 22:52:48 +0000 | [diff] [blame] | 413 | s = sum(d) |
Raymond Hettinger | d2ee64d | 2009-03-31 22:52:48 +0000 | [diff] [blame] | 414 | for elem in it: |
| 415 | s += elem - d.popleft() |
| 416 | d.append(elem) |
| 417 | yield s / n |
| 418 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 419 | The :meth:`rotate` method provides a way to implement :class:`deque` slicing and |
Ezio Melotti | 0639d5a | 2009-12-19 23:26:38 +0000 | [diff] [blame] | 420 | deletion. For example, a pure Python implementation of ``del d[n]`` relies on |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 421 | the :meth:`rotate` method to position elements to be popped:: |
| 422 | |
| 423 | def delete_nth(d, n): |
| 424 | d.rotate(-n) |
| 425 | d.popleft() |
| 426 | d.rotate(n) |
| 427 | |
| 428 | To implement :class:`deque` slicing, use a similar approach applying |
| 429 | :meth:`rotate` to bring a target element to the left side of the deque. Remove |
| 430 | old entries with :meth:`popleft`, add new entries with :meth:`extend`, and then |
| 431 | reverse the rotation. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 432 | With minor variations on that approach, it is easy to implement Forth style |
| 433 | stack manipulations such as ``dup``, ``drop``, ``swap``, ``over``, ``pick``, |
| 434 | ``rot``, and ``roll``. |
| 435 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 436 | |
| 437 | :class:`defaultdict` objects |
| 438 | ---------------------------- |
| 439 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 440 | .. class:: defaultdict([default_factory[, ...]]) |
| 441 | |
| 442 | Returns a new dictionary-like object. :class:`defaultdict` is a subclass of the |
Georg Brandl | 22b3431 | 2009-07-26 14:54:51 +0000 | [diff] [blame] | 443 | built-in :class:`dict` class. It overrides one method and adds one writable |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 444 | instance variable. The remaining functionality is the same as for the |
| 445 | :class:`dict` class and is not documented here. |
| 446 | |
| 447 | The first argument provides the initial value for the :attr:`default_factory` |
| 448 | attribute; it defaults to ``None``. All remaining arguments are treated the same |
| 449 | as if they were passed to the :class:`dict` constructor, including keyword |
| 450 | arguments. |
| 451 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 452 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 453 | :class:`defaultdict` objects support the following method in addition to the |
| 454 | standard :class:`dict` operations: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 455 | |
Benjamin Peterson | d319ad5 | 2010-07-18 14:27:02 +0000 | [diff] [blame] | 456 | .. method:: __missing__(key) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 457 | |
Benjamin Peterson | 5478b47 | 2008-09-17 22:25:09 +0000 | [diff] [blame] | 458 | If the :attr:`default_factory` attribute is ``None``, this raises a |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 459 | :exc:`KeyError` exception with the *key* as argument. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 460 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 461 | If :attr:`default_factory` is not ``None``, it is called without arguments |
| 462 | to provide a default value for the given *key*, this value is inserted in |
| 463 | the dictionary for the *key*, and returned. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 464 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 465 | If calling :attr:`default_factory` raises an exception this exception is |
| 466 | propagated unchanged. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 467 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 468 | This method is called by the :meth:`__getitem__` method of the |
| 469 | :class:`dict` class when the requested key is not found; whatever it |
| 470 | returns or raises is then returned or raised by :meth:`__getitem__`. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 471 | |
| 472 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 473 | :class:`defaultdict` objects support the following instance variable: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 474 | |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 475 | |
Benjamin Peterson | d319ad5 | 2010-07-18 14:27:02 +0000 | [diff] [blame] | 476 | .. attribute:: default_factory |
Benjamin Peterson | e41251e | 2008-04-25 01:59:09 +0000 | [diff] [blame] | 477 | |
| 478 | This attribute is used by the :meth:`__missing__` method; it is |
| 479 | initialized from the first argument to the constructor, if present, or to |
| 480 | ``None``, if absent. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 481 | |
| 482 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 483 | :class:`defaultdict` Examples |
| 484 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| 485 | |
| 486 | Using :class:`list` as the :attr:`default_factory`, it is easy to group a |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 487 | sequence of key-value pairs into a dictionary of lists: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 488 | |
| 489 | >>> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)] |
| 490 | >>> d = defaultdict(list) |
| 491 | >>> for k, v in s: |
| 492 | ... d[k].append(v) |
| 493 | ... |
Ezio Melotti | c53a894 | 2009-09-12 01:52:05 +0000 | [diff] [blame] | 494 | >>> list(d.items()) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 495 | [('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])] |
| 496 | |
| 497 | When each key is encountered for the first time, it is not already in the |
| 498 | mapping; so an entry is automatically created using the :attr:`default_factory` |
| 499 | function which returns an empty :class:`list`. The :meth:`list.append` |
| 500 | operation then attaches the value to the new list. When keys are encountered |
| 501 | again, the look-up proceeds normally (returning the list for that key) and the |
| 502 | :meth:`list.append` operation adds another value to the list. This technique is |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 503 | simpler and faster than an equivalent technique using :meth:`dict.setdefault`: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 504 | |
| 505 | >>> d = {} |
| 506 | >>> for k, v in s: |
| 507 | ... d.setdefault(k, []).append(v) |
| 508 | ... |
Ezio Melotti | c53a894 | 2009-09-12 01:52:05 +0000 | [diff] [blame] | 509 | >>> list(d.items()) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 510 | [('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])] |
| 511 | |
| 512 | Setting the :attr:`default_factory` to :class:`int` makes the |
| 513 | :class:`defaultdict` useful for counting (like a bag or multiset in other |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 514 | languages): |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 515 | |
| 516 | >>> s = 'mississippi' |
| 517 | >>> d = defaultdict(int) |
| 518 | >>> for k in s: |
| 519 | ... d[k] += 1 |
| 520 | ... |
Ezio Melotti | c53a894 | 2009-09-12 01:52:05 +0000 | [diff] [blame] | 521 | >>> list(d.items()) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 522 | [('i', 4), ('p', 2), ('s', 4), ('m', 1)] |
| 523 | |
| 524 | When a letter is first encountered, it is missing from the mapping, so the |
| 525 | :attr:`default_factory` function calls :func:`int` to supply a default count of |
| 526 | zero. The increment operation then builds up the count for each letter. |
| 527 | |
| 528 | The function :func:`int` which always returns zero is just a special case of |
| 529 | constant functions. A faster and more flexible way to create constant functions |
| 530 | is to use a lambda function which can supply any constant value (not just |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 531 | zero): |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 532 | |
| 533 | >>> def constant_factory(value): |
| 534 | ... return lambda: value |
| 535 | >>> d = defaultdict(constant_factory('<missing>')) |
| 536 | >>> d.update(name='John', action='ran') |
| 537 | >>> '%(name)s %(action)s to %(object)s' % d |
| 538 | 'John ran to <missing>' |
| 539 | |
| 540 | Setting the :attr:`default_factory` to :class:`set` makes the |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 541 | :class:`defaultdict` useful for building a dictionary of sets: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 542 | |
| 543 | >>> s = [('red', 1), ('blue', 2), ('red', 3), ('blue', 4), ('red', 1), ('blue', 4)] |
| 544 | >>> d = defaultdict(set) |
| 545 | >>> for k, v in s: |
| 546 | ... d[k].add(v) |
| 547 | ... |
Ezio Melotti | c53a894 | 2009-09-12 01:52:05 +0000 | [diff] [blame] | 548 | >>> list(d.items()) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 549 | [('blue', set([2, 4])), ('red', set([1, 3]))] |
| 550 | |
| 551 | |
Guido van Rossum | 3d392eb | 2007-11-16 00:35:22 +0000 | [diff] [blame] | 552 | :func:`namedtuple` Factory Function for Tuples with Named Fields |
Christian Heimes | 790c823 | 2008-01-07 21:14:23 +0000 | [diff] [blame] | 553 | ---------------------------------------------------------------- |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 554 | |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 555 | Named tuples assign meaning to each position in a tuple and allow for more readable, |
| 556 | self-documenting code. They can be used wherever regular tuples are used, and |
| 557 | they add the ability to access fields by name instead of position index. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 558 | |
Georg Brandl | c2a4f4f | 2009-04-10 09:03:43 +0000 | [diff] [blame] | 559 | .. function:: namedtuple(typename, field_names, verbose=False, rename=False) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 560 | |
| 561 | Returns a new tuple subclass named *typename*. The new subclass is used to |
Christian Heimes | c3f30c4 | 2008-02-22 16:37:40 +0000 | [diff] [blame] | 562 | create tuple-like objects that have fields accessible by attribute lookup as |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 563 | well as being indexable and iterable. Instances of the subclass also have a |
Benjamin Peterson | 4469d0c | 2008-11-30 22:46:23 +0000 | [diff] [blame] | 564 | helpful docstring (with typename and field_names) and a helpful :meth:`__repr__` |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 565 | method which lists the tuple contents in a ``name=value`` format. |
| 566 | |
Benjamin Peterson | 4469d0c | 2008-11-30 22:46:23 +0000 | [diff] [blame] | 567 | The *field_names* are a single string with each fieldname separated by whitespace |
| 568 | and/or commas, for example ``'x y'`` or ``'x, y'``. Alternatively, *field_names* |
Christian Heimes | 25bb783 | 2008-01-11 16:17:00 +0000 | [diff] [blame] | 569 | can be a sequence of strings such as ``['x', 'y']``. |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 570 | |
| 571 | Any valid Python identifier may be used for a fieldname except for names |
Christian Heimes | 0449f63 | 2007-12-15 01:27:15 +0000 | [diff] [blame] | 572 | starting with an underscore. Valid identifiers consist of letters, digits, |
| 573 | and underscores but do not start with a digit or underscore and cannot be |
Georg Brandl | f694518 | 2008-02-01 11:56:49 +0000 | [diff] [blame] | 574 | a :mod:`keyword` such as *class*, *for*, *return*, *global*, *pass*, |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 575 | or *raise*. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 576 | |
Benjamin Peterson | a86f2c0 | 2009-02-10 02:41:10 +0000 | [diff] [blame] | 577 | If *rename* is true, invalid fieldnames are automatically replaced |
| 578 | with positional names. For example, ``['abc', 'def', 'ghi', 'abc']`` is |
Raymond Hettinger | 85737b8 | 2009-04-02 22:37:59 +0000 | [diff] [blame] | 579 | converted to ``['abc', '_1', 'ghi', '_3']``, eliminating the keyword |
Benjamin Peterson | a86f2c0 | 2009-02-10 02:41:10 +0000 | [diff] [blame] | 580 | ``def`` and the duplicate fieldname ``abc``. |
| 581 | |
Christian Heimes | 25bb783 | 2008-01-11 16:17:00 +0000 | [diff] [blame] | 582 | If *verbose* is true, the class definition is printed just before being built. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 583 | |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 584 | Named tuple instances do not have per-instance dictionaries, so they are |
Thomas Wouters | 8ce81f7 | 2007-09-20 18:22:40 +0000 | [diff] [blame] | 585 | lightweight and require no more memory than regular tuples. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 586 | |
Raymond Hettinger | b62ad24 | 2009-03-02 22:16:43 +0000 | [diff] [blame] | 587 | .. versionchanged:: 3.1 |
Georg Brandl | 67b21b7 | 2010-08-17 15:07:14 +0000 | [diff] [blame] | 588 | Added support for *rename*. |
Benjamin Peterson | a86f2c0 | 2009-02-10 02:41:10 +0000 | [diff] [blame] | 589 | |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 590 | |
| 591 | .. doctest:: |
| 592 | :options: +NORMALIZE_WHITESPACE |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 593 | |
Raymond Hettinger | 0ef956f | 2010-11-21 23:23:29 +0000 | [diff] [blame] | 594 | >>> # Basic example |
Raymond Hettinger | 15aded8 | 2011-03-15 17:25:51 -0700 | [diff] [blame] | 595 | >>> Point = namedtuple('Point', ['x', 'y']) |
Raymond Hettinger | 0ef956f | 2010-11-21 23:23:29 +0000 | [diff] [blame] | 596 | >>> p = Point(x=10, y=11) |
| 597 | |
| 598 | >>> # Example using the verbose option to print the class definition |
Guido van Rossum | 3d392eb | 2007-11-16 00:35:22 +0000 | [diff] [blame] | 599 | >>> Point = namedtuple('Point', 'x y', verbose=True) |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 600 | class Point(tuple): |
| 601 | 'Point(x, y)' |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 602 | <BLANKLINE> |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 603 | __slots__ = () |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 604 | <BLANKLINE> |
Christian Heimes | faf2f63 | 2008-01-06 16:59:19 +0000 | [diff] [blame] | 605 | _fields = ('x', 'y') |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 606 | <BLANKLINE> |
Raymond Hettinger | 089ba7f | 2009-05-27 00:38:24 +0000 | [diff] [blame] | 607 | def __new__(_cls, x, y): |
Raymond Hettinger | 7b0d3c6 | 2010-04-02 18:54:02 +0000 | [diff] [blame] | 608 | 'Create a new instance of Point(x, y)' |
Raymond Hettinger | 089ba7f | 2009-05-27 00:38:24 +0000 | [diff] [blame] | 609 | return _tuple.__new__(_cls, (x, y)) |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 610 | <BLANKLINE> |
Christian Heimes | faf2f63 | 2008-01-06 16:59:19 +0000 | [diff] [blame] | 611 | @classmethod |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 612 | def _make(cls, iterable, new=tuple.__new__, len=len): |
Christian Heimes | faf2f63 | 2008-01-06 16:59:19 +0000 | [diff] [blame] | 613 | 'Make a new Point object from a sequence or iterable' |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 614 | result = new(cls, iterable) |
Christian Heimes | faf2f63 | 2008-01-06 16:59:19 +0000 | [diff] [blame] | 615 | if len(result) != 2: |
| 616 | raise TypeError('Expected 2 arguments, got %d' % len(result)) |
| 617 | return result |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 618 | <BLANKLINE> |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 619 | def __repr__(self): |
Raymond Hettinger | 7b0d3c6 | 2010-04-02 18:54:02 +0000 | [diff] [blame] | 620 | 'Return a nicely formatted representation string' |
Raymond Hettinger | d331ce9 | 2010-08-08 01:13:42 +0000 | [diff] [blame] | 621 | return self.__class__.__name__ + '(x=%r, y=%r)' % self |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 622 | <BLANKLINE> |
Raymond Hettinger | a4f52b1 | 2009-03-02 22:28:31 +0000 | [diff] [blame] | 623 | def _asdict(self): |
| 624 | 'Return a new OrderedDict which maps field names to their values' |
| 625 | return OrderedDict(zip(self._fields, self)) |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 626 | <BLANKLINE> |
Raymond Hettinger | 3d89057 | 2011-06-02 23:40:24 -0700 | [diff] [blame] | 627 | __dict__ = property(_asdict) |
| 628 | <BLANKLINE> |
Raymond Hettinger | 089ba7f | 2009-05-27 00:38:24 +0000 | [diff] [blame] | 629 | def _replace(_self, **kwds): |
Guido van Rossum | 3d392eb | 2007-11-16 00:35:22 +0000 | [diff] [blame] | 630 | 'Return a new Point object replacing specified fields with new values' |
Raymond Hettinger | 089ba7f | 2009-05-27 00:38:24 +0000 | [diff] [blame] | 631 | result = _self._make(map(kwds.pop, ('x', 'y'), _self)) |
Christian Heimes | faf2f63 | 2008-01-06 16:59:19 +0000 | [diff] [blame] | 632 | if kwds: |
Ezio Melotti | 8f7649e | 2009-09-13 04:48:45 +0000 | [diff] [blame] | 633 | raise ValueError('Got unexpected field names: %r' % list(kwds.keys())) |
Christian Heimes | faf2f63 | 2008-01-06 16:59:19 +0000 | [diff] [blame] | 634 | return result |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 635 | <BLANKLINE> |
| 636 | def __getnewargs__(self): |
Raymond Hettinger | 7b0d3c6 | 2010-04-02 18:54:02 +0000 | [diff] [blame] | 637 | 'Return self as a plain tuple. Used by copy and pickle.' |
Benjamin Peterson | 4118174 | 2008-07-02 20:22:54 +0000 | [diff] [blame] | 638 | return tuple(self) |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 639 | <BLANKLINE> |
Raymond Hettinger | 7b0d3c6 | 2010-04-02 18:54:02 +0000 | [diff] [blame] | 640 | x = _property(_itemgetter(0), doc='Alias for field number 0') |
| 641 | y = _property(_itemgetter(1), doc='Alias for field number 1') |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 642 | |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 643 | >>> p = Point(11, y=22) # instantiate with positional or keyword arguments |
Christian Heimes | 99170a5 | 2007-12-19 02:07:34 +0000 | [diff] [blame] | 644 | >>> p[0] + p[1] # indexable like the plain tuple (11, 22) |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 645 | 33 |
| 646 | >>> x, y = p # unpack like a regular tuple |
| 647 | >>> x, y |
| 648 | (11, 22) |
Christian Heimes | c3f30c4 | 2008-02-22 16:37:40 +0000 | [diff] [blame] | 649 | >>> p.x + p.y # fields also accessible by name |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 650 | 33 |
| 651 | >>> p # readable __repr__ with a name=value style |
| 652 | Point(x=11, y=22) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 653 | |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 654 | Named tuples are especially useful for assigning field names to result tuples returned |
| 655 | by the :mod:`csv` or :mod:`sqlite3` modules:: |
| 656 | |
Guido van Rossum | 3d392eb | 2007-11-16 00:35:22 +0000 | [diff] [blame] | 657 | EmployeeRecord = namedtuple('EmployeeRecord', 'name, age, title, department, paygrade') |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 658 | |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 659 | import csv |
Christian Heimes | faf2f63 | 2008-01-06 16:59:19 +0000 | [diff] [blame] | 660 | for emp in map(EmployeeRecord._make, csv.reader(open("employees.csv", "rb"))): |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 661 | print(emp.name, emp.title) |
| 662 | |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 663 | import sqlite3 |
| 664 | conn = sqlite3.connect('/companydata') |
| 665 | cursor = conn.cursor() |
| 666 | cursor.execute('SELECT name, age, title, department, paygrade FROM employees') |
Christian Heimes | faf2f63 | 2008-01-06 16:59:19 +0000 | [diff] [blame] | 667 | for emp in map(EmployeeRecord._make, cursor.fetchall()): |
Christian Heimes | 0041223 | 2008-01-10 16:02:19 +0000 | [diff] [blame] | 668 | print(emp.name, emp.title) |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 669 | |
Christian Heimes | 99170a5 | 2007-12-19 02:07:34 +0000 | [diff] [blame] | 670 | In addition to the methods inherited from tuples, named tuples support |
Christian Heimes | 2380ac7 | 2008-01-09 00:17:24 +0000 | [diff] [blame] | 671 | three additional methods and one attribute. To prevent conflicts with |
| 672 | field names, the method and attribute names start with an underscore. |
Christian Heimes | 99170a5 | 2007-12-19 02:07:34 +0000 | [diff] [blame] | 673 | |
Benjamin Peterson | 0b9fb80 | 2010-07-18 14:23:36 +0000 | [diff] [blame] | 674 | .. classmethod:: somenamedtuple._make(iterable) |
Christian Heimes | 99170a5 | 2007-12-19 02:07:34 +0000 | [diff] [blame] | 675 | |
Christian Heimes | faf2f63 | 2008-01-06 16:59:19 +0000 | [diff] [blame] | 676 | Class method that makes a new instance from an existing sequence or iterable. |
Christian Heimes | 99170a5 | 2007-12-19 02:07:34 +0000 | [diff] [blame] | 677 | |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 678 | .. doctest:: |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 679 | |
Christian Heimes | faf2f63 | 2008-01-06 16:59:19 +0000 | [diff] [blame] | 680 | >>> t = [11, 22] |
| 681 | >>> Point._make(t) |
| 682 | Point(x=11, y=22) |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 683 | |
Christian Heimes | 790c823 | 2008-01-07 21:14:23 +0000 | [diff] [blame] | 684 | .. method:: somenamedtuple._asdict() |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 685 | |
Raymond Hettinger | a4f52b1 | 2009-03-02 22:28:31 +0000 | [diff] [blame] | 686 | Return a new :class:`OrderedDict` which maps field names to their corresponding |
| 687 | values:: |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 688 | |
Christian Heimes | 0449f63 | 2007-12-15 01:27:15 +0000 | [diff] [blame] | 689 | >>> p._asdict() |
Raymond Hettinger | a4f52b1 | 2009-03-02 22:28:31 +0000 | [diff] [blame] | 690 | OrderedDict([('x', 11), ('y', 22)]) |
| 691 | |
Raymond Hettinger | a88e4da | 2009-03-03 05:12:27 +0000 | [diff] [blame] | 692 | .. versionchanged:: 3.1 |
Raymond Hettinger | a4f52b1 | 2009-03-02 22:28:31 +0000 | [diff] [blame] | 693 | Returns an :class:`OrderedDict` instead of a regular :class:`dict`. |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 694 | |
Christian Heimes | 790c823 | 2008-01-07 21:14:23 +0000 | [diff] [blame] | 695 | .. method:: somenamedtuple._replace(kwargs) |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 696 | |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 697 | Return a new instance of the named tuple replacing specified fields with new |
| 698 | values: |
Thomas Wouters | 8ce81f7 | 2007-09-20 18:22:40 +0000 | [diff] [blame] | 699 | |
| 700 | :: |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 701 | |
| 702 | >>> p = Point(x=11, y=22) |
Christian Heimes | 0449f63 | 2007-12-15 01:27:15 +0000 | [diff] [blame] | 703 | >>> p._replace(x=33) |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 704 | Point(x=33, y=22) |
| 705 | |
Guido van Rossum | 3d392eb | 2007-11-16 00:35:22 +0000 | [diff] [blame] | 706 | >>> for partnum, record in inventory.items(): |
Christian Heimes | 454f37b | 2008-01-10 00:10:02 +0000 | [diff] [blame] | 707 | ... inventory[partnum] = record._replace(price=newprices[partnum], timestamp=time.now()) |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 708 | |
Christian Heimes | 790c823 | 2008-01-07 21:14:23 +0000 | [diff] [blame] | 709 | .. attribute:: somenamedtuple._fields |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 710 | |
Christian Heimes | 2380ac7 | 2008-01-09 00:17:24 +0000 | [diff] [blame] | 711 | Tuple of strings listing the field names. Useful for introspection |
Georg Brandl | 9afde1c | 2007-11-01 20:32:30 +0000 | [diff] [blame] | 712 | and for creating new named tuple types from existing named tuples. |
Thomas Wouters | 8ce81f7 | 2007-09-20 18:22:40 +0000 | [diff] [blame] | 713 | |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 714 | .. doctest:: |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 715 | |
Christian Heimes | 0449f63 | 2007-12-15 01:27:15 +0000 | [diff] [blame] | 716 | >>> p._fields # view the field names |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 717 | ('x', 'y') |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 718 | |
Guido van Rossum | 3d392eb | 2007-11-16 00:35:22 +0000 | [diff] [blame] | 719 | >>> Color = namedtuple('Color', 'red green blue') |
Christian Heimes | 0449f63 | 2007-12-15 01:27:15 +0000 | [diff] [blame] | 720 | >>> Pixel = namedtuple('Pixel', Point._fields + Color._fields) |
Thomas Wouters | 1b7f891 | 2007-09-19 03:06:30 +0000 | [diff] [blame] | 721 | >>> Pixel(11, 22, 128, 255, 0) |
Christian Heimes | 454f37b | 2008-01-10 00:10:02 +0000 | [diff] [blame] | 722 | Pixel(x=11, y=22, red=128, green=255, blue=0) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 723 | |
Christian Heimes | 0449f63 | 2007-12-15 01:27:15 +0000 | [diff] [blame] | 724 | To retrieve a field whose name is stored in a string, use the :func:`getattr` |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 725 | function: |
Christian Heimes | 0449f63 | 2007-12-15 01:27:15 +0000 | [diff] [blame] | 726 | |
| 727 | >>> getattr(p, 'x') |
| 728 | 11 |
| 729 | |
Raymond Hettinger | 651453a | 2009-02-11 00:20:02 +0000 | [diff] [blame] | 730 | To convert a dictionary to a named tuple, use the double-star-operator |
| 731 | (as described in :ref:`tut-unpacking-arguments`): |
Christian Heimes | 99170a5 | 2007-12-19 02:07:34 +0000 | [diff] [blame] | 732 | |
| 733 | >>> d = {'x': 11, 'y': 22} |
| 734 | >>> Point(**d) |
| 735 | Point(x=11, y=22) |
| 736 | |
Guido van Rossum | 3d392eb | 2007-11-16 00:35:22 +0000 | [diff] [blame] | 737 | Since a named tuple is a regular Python class, it is easy to add or change |
Christian Heimes | 043d6f6 | 2008-01-07 17:19:16 +0000 | [diff] [blame] | 738 | functionality with a subclass. Here is how to add a calculated field and |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 739 | a fixed-width print format: |
Guido van Rossum | 3d392eb | 2007-11-16 00:35:22 +0000 | [diff] [blame] | 740 | |
Christian Heimes | 043d6f6 | 2008-01-07 17:19:16 +0000 | [diff] [blame] | 741 | >>> class Point(namedtuple('Point', 'x y')): |
Raymond Hettinger | 15aded8 | 2011-03-15 17:25:51 -0700 | [diff] [blame] | 742 | __slots__ = () |
| 743 | @property |
| 744 | def hypot(self): |
| 745 | return (self.x ** 2 + self.y ** 2) ** 0.5 |
| 746 | def __str__(self): |
| 747 | return 'Point: x=%6.3f y=%6.3f hypot=%6.3f' % (self.x, self.y, self.hypot) |
Guido van Rossum | 3d392eb | 2007-11-16 00:35:22 +0000 | [diff] [blame] | 748 | |
Georg Brandl | 0df7979 | 2008-10-04 18:33:26 +0000 | [diff] [blame] | 749 | >>> for p in Point(3, 4), Point(14, 5/7): |
Raymond Hettinger | 15aded8 | 2011-03-15 17:25:51 -0700 | [diff] [blame] | 750 | print(p) |
Christian Heimes | 25bb783 | 2008-01-11 16:17:00 +0000 | [diff] [blame] | 751 | Point: x= 3.000 y= 4.000 hypot= 5.000 |
| 752 | Point: x=14.000 y= 0.714 hypot=14.018 |
Christian Heimes | 043d6f6 | 2008-01-07 17:19:16 +0000 | [diff] [blame] | 753 | |
Georg Brandl | af5c238 | 2009-12-28 08:02:38 +0000 | [diff] [blame] | 754 | The subclass shown above sets ``__slots__`` to an empty tuple. This helps |
Christian Heimes | 679db4a | 2008-01-18 09:56:22 +0000 | [diff] [blame] | 755 | keep memory requirements low by preventing the creation of instance dictionaries. |
| 756 | |
Christian Heimes | 2380ac7 | 2008-01-09 00:17:24 +0000 | [diff] [blame] | 757 | |
| 758 | Subclassing is not useful for adding new, stored fields. Instead, simply |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 759 | create a new named tuple type from the :attr:`_fields` attribute: |
Christian Heimes | 2380ac7 | 2008-01-09 00:17:24 +0000 | [diff] [blame] | 760 | |
Christian Heimes | 25bb783 | 2008-01-11 16:17:00 +0000 | [diff] [blame] | 761 | >>> Point3D = namedtuple('Point3D', Point._fields + ('z',)) |
Christian Heimes | 2380ac7 | 2008-01-09 00:17:24 +0000 | [diff] [blame] | 762 | |
| 763 | Default values can be implemented by using :meth:`_replace` to |
Christian Heimes | fe337bf | 2008-03-23 21:54:12 +0000 | [diff] [blame] | 764 | customize a prototype instance: |
Guido van Rossum | 3d392eb | 2007-11-16 00:35:22 +0000 | [diff] [blame] | 765 | |
| 766 | >>> Account = namedtuple('Account', 'owner balance transaction_count') |
Christian Heimes | 587c2bf | 2008-01-19 16:21:02 +0000 | [diff] [blame] | 767 | >>> default_account = Account('<owner name>', 0.0, 0) |
| 768 | >>> johns_account = default_account._replace(owner='John') |
Guido van Rossum | 3d392eb | 2007-11-16 00:35:22 +0000 | [diff] [blame] | 769 | |
Christian Heimes | e4ca815 | 2008-05-08 17:18:53 +0000 | [diff] [blame] | 770 | Enumerated constants can be implemented with named tuples, but it is simpler |
| 771 | and more efficient to use a simple class declaration: |
| 772 | |
| 773 | >>> Status = namedtuple('Status', 'open pending closed')._make(range(3)) |
| 774 | >>> Status.open, Status.pending, Status.closed |
| 775 | (0, 1, 2) |
| 776 | >>> class Status: |
Raymond Hettinger | 15aded8 | 2011-03-15 17:25:51 -0700 | [diff] [blame] | 777 | open, pending, closed = range(3) |
Christian Heimes | e4ca815 | 2008-05-08 17:18:53 +0000 | [diff] [blame] | 778 | |
Raymond Hettinger | 651453a | 2009-02-11 00:20:02 +0000 | [diff] [blame] | 779 | .. seealso:: |
Thomas Wouters | 47b49bf | 2007-08-30 22:15:33 +0000 | [diff] [blame] | 780 | |
Raymond Hettinger | 6c94e6f | 2011-03-31 15:46:06 -0700 | [diff] [blame] | 781 | * `Named tuple recipe <http://code.activestate.com/recipes/500261/>`_ |
| 782 | adapted for Python 2.4. |
| 783 | |
| 784 | * `Recipe for named tuple abstract base class with a metaclass mix-in |
| 785 | <http://code.activestate.com/recipes/577629-namedtupleabc-abstract-base-class-mix-in-for-named/>`_ |
| 786 | by Jan Kaliszewski. Besides providing an :term:`abstract base class` for |
| 787 | named tuples, it also supports an alternate :term:`metaclass`-based |
| 788 | constructor that is convenient for use cases where named tuples are being |
| 789 | subclassed. |
Raymond Hettinger | e4c96ad | 2008-02-06 01:23:58 +0000 | [diff] [blame] | 790 | |
| 791 | |
Raymond Hettinger | 2d32f63 | 2009-03-02 21:24:57 +0000 | [diff] [blame] | 792 | :class:`OrderedDict` objects |
| 793 | ---------------------------- |
| 794 | |
| 795 | Ordered dictionaries are just like regular dictionaries but they remember the |
| 796 | order that items were inserted. When iterating over an ordered dictionary, |
| 797 | the items are returned in the order their keys were first added. |
| 798 | |
| 799 | .. class:: OrderedDict([items]) |
| 800 | |
| 801 | Return an instance of a dict subclass, supporting the usual :class:`dict` |
| 802 | methods. An *OrderedDict* is a dict that remembers the order that keys |
| 803 | were first inserted. If a new entry overwrites an existing entry, the |
| 804 | original insertion position is left unchanged. Deleting an entry and |
| 805 | reinserting it will move it to the end. |
| 806 | |
Benjamin Peterson | d45bf58 | 2009-03-02 21:44:54 +0000 | [diff] [blame] | 807 | .. versionadded:: 3.1 |
Raymond Hettinger | 2d32f63 | 2009-03-02 21:24:57 +0000 | [diff] [blame] | 808 | |
Benjamin Peterson | d319ad5 | 2010-07-18 14:27:02 +0000 | [diff] [blame] | 809 | .. method:: popitem(last=True) |
Raymond Hettinger | dc879f0 | 2009-03-19 20:30:56 +0000 | [diff] [blame] | 810 | |
Benjamin Peterson | d319ad5 | 2010-07-18 14:27:02 +0000 | [diff] [blame] | 811 | The :meth:`popitem` method for ordered dictionaries returns and removes a |
| 812 | (key, value) pair. The pairs are returned in LIFO order if *last* is true |
| 813 | or FIFO order if false. |
Raymond Hettinger | 2d32f63 | 2009-03-02 21:24:57 +0000 | [diff] [blame] | 814 | |
Raymond Hettinger | f45abc9 | 2010-09-06 21:26:09 +0000 | [diff] [blame] | 815 | .. method:: move_to_end(key, last=True) |
| 816 | |
| 817 | Move an existing *key* to either end of an ordered dictionary. The item |
| 818 | is moved to the right end if *last* is true (the default) or to the |
| 819 | beginning if *last* is false. Raises :exc:`KeyError` if the *key* does |
| 820 | not exist:: |
| 821 | |
| 822 | >>> d = OrderedDict.fromkeys('abcde') |
| 823 | >>> d.move_to_end('b') |
Raymond Hettinger | 4d5208d | 2011-06-25 11:39:00 +0200 | [diff] [blame] | 824 | >>> ''.join(d.keys()) |
Raymond Hettinger | f45abc9 | 2010-09-06 21:26:09 +0000 | [diff] [blame] | 825 | 'acdeb' |
Éric Araujo | 1cb25aa | 2010-11-06 07:03:07 +0000 | [diff] [blame] | 826 | >>> d.move_to_end('b', last=False) |
Raymond Hettinger | 4d5208d | 2011-06-25 11:39:00 +0200 | [diff] [blame] | 827 | >>> ''.join(d.keys()) |
Raymond Hettinger | f45abc9 | 2010-09-06 21:26:09 +0000 | [diff] [blame] | 828 | 'bacde' |
| 829 | |
| 830 | .. versionadded:: 3.2 |
| 831 | |
Raymond Hettinger | e909150 | 2009-05-19 17:40:07 +0000 | [diff] [blame] | 832 | In addition to the usual mapping methods, ordered dictionaries also support |
| 833 | reverse iteration using :func:`reversed`. |
| 834 | |
Raymond Hettinger | 2d32f63 | 2009-03-02 21:24:57 +0000 | [diff] [blame] | 835 | Equality tests between :class:`OrderedDict` objects are order-sensitive |
| 836 | and are implemented as ``list(od1.items())==list(od2.items())``. |
| 837 | Equality tests between :class:`OrderedDict` objects and other |
| 838 | :class:`Mapping` objects are order-insensitive like regular dictionaries. |
| 839 | This allows :class:`OrderedDict` objects to be substituted anywhere a |
| 840 | regular dictionary is used. |
| 841 | |
Raymond Hettinger | 3618078 | 2009-04-09 22:34:23 +0000 | [diff] [blame] | 842 | The :class:`OrderedDict` constructor and :meth:`update` method both accept |
| 843 | keyword arguments, but their order is lost because Python's function call |
| 844 | semantics pass-in keyword arguments using a regular unordered dictionary. |
| 845 | |
Raymond Hettinger | dc879f0 | 2009-03-19 20:30:56 +0000 | [diff] [blame] | 846 | .. seealso:: |
| 847 | |
| 848 | `Equivalent OrderedDict recipe <http://code.activestate.com/recipes/576693/>`_ |
| 849 | that runs on Python 2.4 or later. |
| 850 | |
Raymond Hettinger | 7bba683 | 2011-04-15 17:43:19 -0700 | [diff] [blame] | 851 | :class:`OrderedDict` Examples and Recipes |
| 852 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| 853 | |
Raymond Hettinger | 0e31201 | 2009-11-10 18:35:46 +0000 | [diff] [blame] | 854 | Since an ordered dictionary remembers its insertion order, it can be used |
| 855 | in conjuction with sorting to make a sorted dictionary:: |
| 856 | |
| 857 | >>> # regular unsorted dictionary |
| 858 | >>> d = {'banana': 3, 'apple':4, 'pear': 1, 'orange': 2} |
| 859 | |
| 860 | >>> # dictionary sorted by key |
| 861 | >>> OrderedDict(sorted(d.items(), key=lambda t: t[0])) |
| 862 | OrderedDict([('apple', 4), ('banana', 3), ('orange', 2), ('pear', 1)]) |
| 863 | |
| 864 | >>> # dictionary sorted by value |
| 865 | >>> OrderedDict(sorted(d.items(), key=lambda t: t[1])) |
| 866 | OrderedDict([('pear', 1), ('orange', 2), ('banana', 3), ('apple', 4)]) |
| 867 | |
| 868 | >>> # dictionary sorted by length of the key string |
| 869 | >>> OrderedDict(sorted(d.items(), key=lambda t: len(t[0]))) |
| 870 | OrderedDict([('pear', 1), ('apple', 4), ('orange', 2), ('banana', 3)]) |
| 871 | |
| 872 | The new sorted dictionaries maintain their sort order when entries |
| 873 | are deleted. But when new keys are added, the keys are appended |
| 874 | to the end and the sort is not maintained. |
| 875 | |
Raymond Hettinger | 4821ef8 | 2010-07-31 10:14:41 +0000 | [diff] [blame] | 876 | It is also straight-forward to create an ordered dictionary variant |
| 877 | that the remembers the order the keys were *last* inserted. |
| 878 | If a new entry overwrites an existing entry, the |
| 879 | original insertion position is changed and moved to the end:: |
| 880 | |
| 881 | class LastUpdatedOrderedDict(OrderedDict): |
Georg Brandl | 77570e2 | 2010-12-18 16:21:58 +0000 | [diff] [blame] | 882 | 'Store items in the order the keys were last added' |
Raymond Hettinger | 7bba683 | 2011-04-15 17:43:19 -0700 | [diff] [blame] | 883 | |
Raymond Hettinger | 4821ef8 | 2010-07-31 10:14:41 +0000 | [diff] [blame] | 884 | def __setitem__(self, key, value): |
| 885 | if key in self: |
| 886 | del self[key] |
| 887 | OrderedDict.__setitem__(self, key, value) |
| 888 | |
Raymond Hettinger | 7bba683 | 2011-04-15 17:43:19 -0700 | [diff] [blame] | 889 | An ordered dictionary can combined with the :class:`Counter` class |
| 890 | so that the counter remembers the order elements are first encountered:: |
| 891 | |
| 892 | class OrderedCounter(Counter, OrderedDict): |
| 893 | 'Counter that remembers the order elements are first encountered' |
| 894 | |
Raymond Hettinger | 7bba683 | 2011-04-15 17:43:19 -0700 | [diff] [blame] | 895 | def __repr__(self): |
| 896 | return '%s(%r)' % (self.__class__.__name__, OrderedDict(self)) |
| 897 | |
| 898 | def __reduce__(self): |
| 899 | return self.__class__, (OrderedDict(self),) |
| 900 | |
Raymond Hettinger | e4c96ad | 2008-02-06 01:23:58 +0000 | [diff] [blame] | 901 | |
| 902 | :class:`UserDict` objects |
Mark Summerfield | 8f2d006 | 2008-02-06 13:30:44 +0000 | [diff] [blame] | 903 | ------------------------- |
Raymond Hettinger | e4c96ad | 2008-02-06 01:23:58 +0000 | [diff] [blame] | 904 | |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 905 | The class, :class:`UserDict` acts as a wrapper around dictionary objects. |
| 906 | The need for this class has been partially supplanted by the ability to |
Raymond Hettinger | e4c96ad | 2008-02-06 01:23:58 +0000 | [diff] [blame] | 907 | subclass directly from :class:`dict`; however, this class can be easier |
| 908 | to work with because the underlying dictionary is accessible as an |
| 909 | attribute. |
| 910 | |
| 911 | .. class:: UserDict([initialdata]) |
| 912 | |
| 913 | Class that simulates a dictionary. The instance's contents are kept in a |
| 914 | regular dictionary, which is accessible via the :attr:`data` attribute of |
| 915 | :class:`UserDict` instances. If *initialdata* is provided, :attr:`data` is |
| 916 | initialized with its contents; note that a reference to *initialdata* will not |
| 917 | be kept, allowing it be used for other purposes. |
| 918 | |
Benjamin Peterson | d319ad5 | 2010-07-18 14:27:02 +0000 | [diff] [blame] | 919 | In addition to supporting the methods and operations of mappings, |
| 920 | :class:`UserDict` instances provide the following attribute: |
Raymond Hettinger | e4c96ad | 2008-02-06 01:23:58 +0000 | [diff] [blame] | 921 | |
Benjamin Peterson | d319ad5 | 2010-07-18 14:27:02 +0000 | [diff] [blame] | 922 | .. attribute:: data |
Raymond Hettinger | e4c96ad | 2008-02-06 01:23:58 +0000 | [diff] [blame] | 923 | |
Benjamin Peterson | d319ad5 | 2010-07-18 14:27:02 +0000 | [diff] [blame] | 924 | A real dictionary used to store the contents of the :class:`UserDict` |
| 925 | class. |
Raymond Hettinger | 53dbe39 | 2008-02-12 20:03:09 +0000 | [diff] [blame] | 926 | |
| 927 | |
| 928 | |
| 929 | :class:`UserList` objects |
| 930 | ------------------------- |
| 931 | |
| 932 | This class acts as a wrapper around list objects. It is a useful base class |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 933 | for your own list-like classes which can inherit from them and override |
Raymond Hettinger | 53dbe39 | 2008-02-12 20:03:09 +0000 | [diff] [blame] | 934 | existing methods or add new ones. In this way, one can add new behaviors to |
| 935 | lists. |
| 936 | |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 937 | The need for this class has been partially supplanted by the ability to |
Raymond Hettinger | 53dbe39 | 2008-02-12 20:03:09 +0000 | [diff] [blame] | 938 | subclass directly from :class:`list`; however, this class can be easier |
| 939 | to work with because the underlying list is accessible as an attribute. |
| 940 | |
| 941 | .. class:: UserList([list]) |
| 942 | |
| 943 | Class that simulates a list. The instance's contents are kept in a regular |
| 944 | list, which is accessible via the :attr:`data` attribute of :class:`UserList` |
| 945 | instances. The instance's contents are initially set to a copy of *list*, |
| 946 | defaulting to the empty list ``[]``. *list* can be any iterable, for |
| 947 | example a real Python list or a :class:`UserList` object. |
| 948 | |
Benjamin Peterson | d319ad5 | 2010-07-18 14:27:02 +0000 | [diff] [blame] | 949 | In addition to supporting the methods and operations of mutable sequences, |
| 950 | :class:`UserList` instances provide the following attribute: |
Raymond Hettinger | 53dbe39 | 2008-02-12 20:03:09 +0000 | [diff] [blame] | 951 | |
Benjamin Peterson | d319ad5 | 2010-07-18 14:27:02 +0000 | [diff] [blame] | 952 | .. attribute:: data |
Raymond Hettinger | 53dbe39 | 2008-02-12 20:03:09 +0000 | [diff] [blame] | 953 | |
Benjamin Peterson | d319ad5 | 2010-07-18 14:27:02 +0000 | [diff] [blame] | 954 | A real :class:`list` object used to store the contents of the |
| 955 | :class:`UserList` class. |
Raymond Hettinger | 53dbe39 | 2008-02-12 20:03:09 +0000 | [diff] [blame] | 956 | |
| 957 | **Subclassing requirements:** Subclasses of :class:`UserList` are expect to |
| 958 | offer a constructor which can be called with either no arguments or one |
| 959 | argument. List operations which return a new sequence attempt to create an |
| 960 | instance of the actual implementation class. To do so, it assumes that the |
| 961 | constructor can be called with a single parameter, which is a sequence object |
| 962 | used as a data source. |
| 963 | |
| 964 | If a derived class does not wish to comply with this requirement, all of the |
| 965 | special methods supported by this class will need to be overridden; please |
| 966 | consult the sources for information about the methods which need to be provided |
| 967 | in that case. |
Raymond Hettinger | b3a65f8 | 2008-02-21 22:11:37 +0000 | [diff] [blame] | 968 | |
| 969 | :class:`UserString` objects |
Christian Heimes | c3f30c4 | 2008-02-22 16:37:40 +0000 | [diff] [blame] | 970 | --------------------------- |
Raymond Hettinger | b3a65f8 | 2008-02-21 22:11:37 +0000 | [diff] [blame] | 971 | |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 972 | The class, :class:`UserString` acts as a wrapper around string objects. |
| 973 | The need for this class has been partially supplanted by the ability to |
Raymond Hettinger | b3a65f8 | 2008-02-21 22:11:37 +0000 | [diff] [blame] | 974 | subclass directly from :class:`str`; however, this class can be easier |
| 975 | to work with because the underlying string is accessible as an |
| 976 | attribute. |
| 977 | |
| 978 | .. class:: UserString([sequence]) |
| 979 | |
| 980 | Class that simulates a string or a Unicode string object. The instance's |
Georg Brandl | 48310cd | 2009-01-03 21:18:54 +0000 | [diff] [blame] | 981 | content is kept in a regular string object, which is accessible via the |
| 982 | :attr:`data` attribute of :class:`UserString` instances. The instance's |
Raymond Hettinger | b3a65f8 | 2008-02-21 22:11:37 +0000 | [diff] [blame] | 983 | contents are initially set to a copy of *sequence*. The *sequence* can |
| 984 | be an instance of :class:`bytes`, :class:`str`, :class:`UserString` (or a |
| 985 | subclass) or an arbitrary sequence which can be converted into a string using |
| 986 | the built-in :func:`str` function. |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 987 | |
Éric Araujo | fa088db | 2011-06-04 18:42:38 +0200 | [diff] [blame] | 988 | .. _collections-abstract-base-classes: |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 989 | |
| 990 | ABCs - abstract base classes |
| 991 | ---------------------------- |
| 992 | |
Ezio Melotti | 9b2e67c | 2011-03-28 13:50:41 +0300 | [diff] [blame] | 993 | The collections module offers the following :term:`ABCs <abstract base class>`: |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 994 | |
| 995 | ========================= ===================== ====================== ==================================================== |
Ezio Melotti | 9b2e67c | 2011-03-28 13:50:41 +0300 | [diff] [blame] | 996 | ABC Inherits from Abstract Methods Mixin Methods |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 997 | ========================= ===================== ====================== ==================================================== |
| 998 | :class:`Container` ``__contains__`` |
| 999 | :class:`Hashable` ``__hash__`` |
| 1000 | :class:`Iterable` ``__iter__`` |
| 1001 | :class:`Iterator` :class:`Iterable` ``__next__`` ``__iter__`` |
| 1002 | :class:`Sized` ``__len__`` |
| 1003 | :class:`Callable` ``__call__`` |
| 1004 | |
Georg Brandl | e951e91 | 2011-02-03 07:08:25 +0000 | [diff] [blame] | 1005 | :class:`Sequence` :class:`Sized`, ``__getitem__`` ``__contains__``, ``__iter__``, ``__reversed__``, |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 1006 | :class:`Iterable`, ``index``, and ``count`` |
| 1007 | :class:`Container` |
| 1008 | |
Ezio Melotti | 9b2e67c | 2011-03-28 13:50:41 +0300 | [diff] [blame] | 1009 | :class:`MutableSequence` :class:`Sequence` ``__setitem__``, Inherited :class:`Sequence` methods and |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 1010 | ``__delitem__``, ``append``, ``reverse``, ``extend``, ``pop``, |
Ezio Melotti | 9b2e67c | 2011-03-28 13:50:41 +0300 | [diff] [blame] | 1011 | ``insert`` ``remove``, and ``__iadd__`` |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 1012 | |
| 1013 | :class:`Set` :class:`Sized`, ``__le__``, ``__lt__``, ``__eq__``, ``__ne__``, |
Georg Brandl | e951e91 | 2011-02-03 07:08:25 +0000 | [diff] [blame] | 1014 | :class:`Iterable`, ``__gt__``, ``__ge__``, ``__and__``, ``__or__``, |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 1015 | :class:`Container` ``__sub__``, ``__xor__``, and ``isdisjoint`` |
| 1016 | |
Ezio Melotti | 9b2e67c | 2011-03-28 13:50:41 +0300 | [diff] [blame] | 1017 | :class:`MutableSet` :class:`Set` ``add``, Inherited :class:`Set` methods and |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 1018 | ``discard`` ``clear``, ``pop``, ``remove``, ``__ior__``, |
| 1019 | ``__iand__``, ``__ixor__``, and ``__isub__`` |
| 1020 | |
| 1021 | :class:`Mapping` :class:`Sized`, ``__getitem__`` ``__contains__``, ``keys``, ``items``, ``values``, |
| 1022 | :class:`Iterable`, ``get``, ``__eq__``, and ``__ne__`` |
| 1023 | :class:`Container` |
| 1024 | |
Ezio Melotti | 9b2e67c | 2011-03-28 13:50:41 +0300 | [diff] [blame] | 1025 | :class:`MutableMapping` :class:`Mapping` ``__setitem__``, Inherited :class:`Mapping` methods and |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 1026 | ``__delitem__`` ``pop``, ``popitem``, ``clear``, ``update``, |
| 1027 | and ``setdefault`` |
| 1028 | |
| 1029 | |
| 1030 | :class:`MappingView` :class:`Sized` ``__len__`` |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 1031 | :class:`ItemsView` :class:`MappingView`, ``__contains__``, |
| 1032 | :class:`Set` ``__iter__`` |
Ezio Melotti | 9b2e67c | 2011-03-28 13:50:41 +0300 | [diff] [blame] | 1033 | :class:`KeysView` :class:`MappingView`, ``__contains__``, |
| 1034 | :class:`Set` ``__iter__`` |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 1035 | :class:`ValuesView` :class:`MappingView` ``__contains__``, ``__iter__`` |
| 1036 | ========================= ===================== ====================== ==================================================== |
| 1037 | |
Ezio Melotti | 9b2e67c | 2011-03-28 13:50:41 +0300 | [diff] [blame] | 1038 | |
| 1039 | .. class:: Container |
| 1040 | Hashable |
| 1041 | Sized |
| 1042 | Callable |
| 1043 | |
| 1044 | ABCs for classes that provide respectively the methods :meth:`__contains__`, |
| 1045 | :meth:`__hash__`, :meth:`__len__`, and :meth:`__call__`. |
| 1046 | |
| 1047 | .. class:: Iterable |
| 1048 | |
| 1049 | ABC for classes that provide the :meth:`__iter__` method. |
| 1050 | See also the definition of :term:`iterable`. |
| 1051 | |
| 1052 | .. class:: Iterator |
| 1053 | |
| 1054 | ABC for classes that provide the :meth:`__iter__` and :meth:`next` methods. |
| 1055 | See also the definition of :term:`iterator`. |
| 1056 | |
| 1057 | .. class:: Sequence |
| 1058 | MutableSequence |
| 1059 | |
| 1060 | ABCs for read-only and mutable :term:`sequences <sequence>`. |
| 1061 | |
| 1062 | .. class:: Set |
| 1063 | MutableSet |
| 1064 | |
| 1065 | ABCs for read-only and mutable sets. |
| 1066 | |
| 1067 | .. class:: Mapping |
| 1068 | MutableMapping |
| 1069 | |
| 1070 | ABCs for read-only and mutable :term:`mappings <mapping>`. |
| 1071 | |
| 1072 | .. class:: MappingView |
| 1073 | ItemsView |
| 1074 | KeysView |
| 1075 | ValuesView |
| 1076 | |
| 1077 | ABCs for mapping, items, keys, and values :term:`views <view>`. |
| 1078 | |
| 1079 | |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 1080 | These ABCs allow us to ask classes or instances if they provide |
| 1081 | particular functionality, for example:: |
| 1082 | |
| 1083 | size = None |
| 1084 | if isinstance(myvar, collections.Sized): |
| 1085 | size = len(myvar) |
| 1086 | |
| 1087 | Several of the ABCs are also useful as mixins that make it easier to develop |
| 1088 | classes supporting container APIs. For example, to write a class supporting |
| 1089 | the full :class:`Set` API, it only necessary to supply the three underlying |
| 1090 | abstract methods: :meth:`__contains__`, :meth:`__iter__`, and :meth:`__len__`. |
| 1091 | The ABC supplies the remaining methods such as :meth:`__and__` and |
| 1092 | :meth:`isdisjoint` :: |
| 1093 | |
| 1094 | class ListBasedSet(collections.Set): |
| 1095 | ''' Alternate set implementation favoring space over speed |
| 1096 | and not requiring the set elements to be hashable. ''' |
| 1097 | def __init__(self, iterable): |
| 1098 | self.elements = lst = [] |
| 1099 | for value in iterable: |
| 1100 | if value not in lst: |
| 1101 | lst.append(value) |
| 1102 | def __iter__(self): |
| 1103 | return iter(self.elements) |
| 1104 | def __contains__(self, value): |
| 1105 | return value in self.elements |
| 1106 | def __len__(self): |
| 1107 | return len(self.elements) |
| 1108 | |
| 1109 | s1 = ListBasedSet('abcdef') |
| 1110 | s2 = ListBasedSet('defghi') |
| 1111 | overlap = s1 & s2 # The __and__() method is supported automatically |
| 1112 | |
| 1113 | Notes on using :class:`Set` and :class:`MutableSet` as a mixin: |
| 1114 | |
| 1115 | (1) |
| 1116 | Since some set operations create new sets, the default mixin methods need |
| 1117 | a way to create new instances from an iterable. The class constructor is |
| 1118 | assumed to have a signature in the form ``ClassName(iterable)``. |
| 1119 | That assumption is factored-out to an internal classmethod called |
| 1120 | :meth:`_from_iterable` which calls ``cls(iterable)`` to produce a new set. |
| 1121 | If the :class:`Set` mixin is being used in a class with a different |
Raymond Hettinger | e5820c6 | 2011-03-22 09:11:39 -0700 | [diff] [blame] | 1122 | constructor signature, you will need to override :meth:`_from_iterable` |
Raymond Hettinger | a6b76ba | 2010-08-08 00:29:08 +0000 | [diff] [blame] | 1123 | with a classmethod that can construct new instances from |
| 1124 | an iterable argument. |
| 1125 | |
| 1126 | (2) |
| 1127 | To override the comparisons (presumably for speed, as the |
| 1128 | semantics are fixed), redefine :meth:`__le__` and |
| 1129 | then the other operations will automatically follow suit. |
| 1130 | |
| 1131 | (3) |
| 1132 | The :class:`Set` mixin provides a :meth:`_hash` method to compute a hash value |
| 1133 | for the set; however, :meth:`__hash__` is not defined because not all sets |
| 1134 | are hashable or immutable. To add set hashabilty using mixins, |
| 1135 | inherit from both :meth:`Set` and :meth:`Hashable`, then define |
| 1136 | ``__hash__ = Set._hash``. |
| 1137 | |
| 1138 | .. seealso:: |
| 1139 | |
| 1140 | * `OrderedSet recipe <http://code.activestate.com/recipes/576694/>`_ for an |
| 1141 | example built on :class:`MutableSet`. |
| 1142 | |
| 1143 | * For more about ABCs, see the :mod:`abc` module and :pep:`3119`. |