288 lines
14 KiB
ReStructuredText
288 lines
14 KiB
ReStructuredText
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:mod:`sets` --- Unordered collections of unique elements
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========================================================
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.. module:: sets
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:synopsis: Implementation of sets of unique elements.
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.. moduleauthor:: Greg V. Wilson <gvwilson@nevex.com>
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.. moduleauthor:: Alex Martelli <aleax@aleax.it>
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.. moduleauthor:: Guido van Rossum <guido@python.org>
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.. sectionauthor:: Raymond D. Hettinger <python@rcn.com>
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.. versionadded:: 2.3
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.. deprecated:: 2.6
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The built-in ``set``/``frozenset`` types replace this module.
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The :mod:`sets` module provides classes for constructing and manipulating
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unordered collections of unique elements. Common uses include membership
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testing, removing duplicates from a sequence, and computing standard math
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operations on sets such as intersection, union, difference, and symmetric
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difference.
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Like other collections, sets support ``x in set``, ``len(set)``, and ``for x in
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set``. Being an unordered collection, sets do not record element position or
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order of insertion. Accordingly, sets do not support indexing, slicing, or
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other sequence-like behavior.
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Most set applications use the :class:`Set` class which provides every set method
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except for :meth:`__hash__`. For advanced applications requiring a hash method,
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the :class:`ImmutableSet` class adds a :meth:`__hash__` method but omits methods
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which alter the contents of the set. Both :class:`Set` and :class:`ImmutableSet`
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derive from :class:`BaseSet`, an abstract class useful for determining whether
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something is a set: ``isinstance(obj, BaseSet)``.
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The set classes are implemented using dictionaries. Accordingly, the
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requirements for set elements are the same as those for dictionary keys; namely,
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that the element defines both :meth:`__eq__` and :meth:`__hash__`. As a result,
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sets cannot contain mutable elements such as lists or dictionaries. However,
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they can contain immutable collections such as tuples or instances of
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:class:`ImmutableSet`. For convenience in implementing sets of sets, inner sets
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are automatically converted to immutable form, for example,
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``Set([Set(['dog'])])`` is transformed to ``Set([ImmutableSet(['dog'])])``.
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.. class:: Set([iterable])
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Constructs a new empty :class:`Set` object. If the optional *iterable*
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parameter is supplied, updates the set with elements obtained from iteration.
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All of the elements in *iterable* should be immutable or be transformable to an
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immutable using the protocol described in section :ref:`immutable-transforms`.
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.. class:: ImmutableSet([iterable])
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Constructs a new empty :class:`ImmutableSet` object. If the optional *iterable*
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parameter is supplied, updates the set with elements obtained from iteration.
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All of the elements in *iterable* should be immutable or be transformable to an
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immutable using the protocol described in section :ref:`immutable-transforms`.
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Because :class:`ImmutableSet` objects provide a :meth:`__hash__` method, they
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can be used as set elements or as dictionary keys. :class:`ImmutableSet`
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objects do not have methods for adding or removing elements, so all of the
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elements must be known when the constructor is called.
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.. _set-objects:
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Set Objects
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-----------
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Instances of :class:`Set` and :class:`ImmutableSet` both provide the following
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operations:
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+-------------------------------+------------+---------------------------------+
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| Operation | Equivalent | Result |
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+===============================+============+=================================+
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| ``len(s)`` | | cardinality of set *s* |
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+-------------------------------+------------+---------------------------------+
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| ``x in s`` | | test *x* for membership in *s* |
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+-------------------------------+------------+---------------------------------+
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| ``x not in s`` | | test *x* for non-membership in |
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| | | *s* |
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+-------------------------------+------------+---------------------------------+
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| ``s.issubset(t)`` | ``s <= t`` | test whether every element in |
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| | | *s* is in *t* |
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+-------------------------------+------------+---------------------------------+
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| ``s.issuperset(t)`` | ``s >= t`` | test whether every element in |
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| | | *t* is in *s* |
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+-------------------------------+------------+---------------------------------+
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| ``s.union(t)`` | ``s | t`` | new set with elements from both |
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| | | *s* and *t* |
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+-------------------------------+------------+---------------------------------+
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| ``s.intersection(t)`` | ``s & t`` | new set with elements common to |
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| | | *s* and *t* |
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+-------------------------------+------------+---------------------------------+
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| ``s.difference(t)`` | ``s - t`` | new set with elements in *s* |
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| | | but not in *t* |
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+-------------------------------+------------+---------------------------------+
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| ``s.symmetric_difference(t)`` | ``s ^ t`` | new set with elements in either |
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| | | *s* or *t* but not both |
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+-------------------------------+------------+---------------------------------+
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| ``s.copy()`` | | new set with a shallow copy of |
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| | | *s* |
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+-------------------------------+------------+---------------------------------+
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Note, the non-operator versions of :meth:`union`, :meth:`intersection`,
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:meth:`difference`, and :meth:`symmetric_difference` will accept any iterable as
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an argument. In contrast, their operator based counterparts require their
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arguments to be sets. This precludes error-prone constructions like
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``Set('abc') & 'cbs'`` in favor of the more readable
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``Set('abc').intersection('cbs')``.
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.. versionchanged:: 2.3.1
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Formerly all arguments were required to be sets.
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In addition, both :class:`Set` and :class:`ImmutableSet` support set to set
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comparisons. Two sets are equal if and only if every element of each set is
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contained in the other (each is a subset of the other). A set is less than
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another set if and only if the first set is a proper subset of the second set
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(is a subset, but is not equal). A set is greater than another set if and only
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if the first set is a proper superset of the second set (is a superset, but is
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not equal).
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The subset and equality comparisons do not generalize to a complete ordering
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function. For example, any two disjoint sets are not equal and are not subsets
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of each other, so *all* of the following return ``False``: ``a<b``, ``a==b``,
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or ``a>b``. Accordingly, sets do not implement the :meth:`__cmp__` method.
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Since sets only define partial ordering (subset relationships), the output of
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the :meth:`list.sort` method is undefined for lists of sets.
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The following table lists operations available in :class:`ImmutableSet` but not
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found in :class:`Set`:
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+-------------+------------------------------+
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| Operation | Result |
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+=============+==============================+
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| ``hash(s)`` | returns a hash value for *s* |
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+-------------+------------------------------+
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The following table lists operations available in :class:`Set` but not found in
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:class:`ImmutableSet`:
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+--------------------------------------+-------------+---------------------------------+
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| Operation | Equivalent | Result |
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+======================================+=============+=================================+
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| ``s.update(t)`` | *s* \|= *t* | return set *s* with elements |
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| | | added from *t* |
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+--------------------------------------+-------------+---------------------------------+
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| ``s.intersection_update(t)`` | *s* &= *t* | return set *s* keeping only |
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| | | elements also found in *t* |
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+--------------------------------------+-------------+---------------------------------+
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| ``s.difference_update(t)`` | *s* -= *t* | return set *s* after removing |
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| | | elements found in *t* |
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+--------------------------------------+-------------+---------------------------------+
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| ``s.symmetric_difference_update(t)`` | *s* ^= *t* | return set *s* with elements |
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| | | from *s* or *t* but not both |
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+--------------------------------------+-------------+---------------------------------+
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| ``s.add(x)`` | | add element *x* to set *s* |
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+--------------------------------------+-------------+---------------------------------+
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| ``s.remove(x)`` | | remove *x* from set *s*; raises |
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| | | :exc:`KeyError` if not present |
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+--------------------------------------+-------------+---------------------------------+
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| ``s.discard(x)`` | | removes *x* from set *s* if |
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| | | present |
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+--------------------------------------+-------------+---------------------------------+
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| ``s.pop()`` | | remove and return an arbitrary |
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| | | element from *s*; raises |
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| | | :exc:`KeyError` if empty |
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+--------------------------------------+-------------+---------------------------------+
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| ``s.clear()`` | | remove all elements from set |
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| | | *s* |
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+--------------------------------------+-------------+---------------------------------+
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Note, the non-operator versions of :meth:`update`, :meth:`intersection_update`,
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:meth:`difference_update`, and :meth:`symmetric_difference_update` will accept
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any iterable as an argument.
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.. versionchanged:: 2.3.1
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Formerly all arguments were required to be sets.
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Also note, the module also includes a :meth:`union_update` method which is an
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alias for :meth:`update`. The method is included for backwards compatibility.
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Programmers should prefer the :meth:`update` method because it is supported by
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the builtin :class:`set()` and :class:`frozenset()` types.
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.. _set-example:
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Example
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-------
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::
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>>> from sets import Set
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>>> engineers = Set(['John', 'Jane', 'Jack', 'Janice'])
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>>> programmers = Set(['Jack', 'Sam', 'Susan', 'Janice'])
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>>> managers = Set(['Jane', 'Jack', 'Susan', 'Zack'])
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>>> employees = engineers | programmers | managers # union
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>>> engineering_management = engineers & managers # intersection
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>>> fulltime_management = managers - engineers - programmers # difference
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>>> engineers.add('Marvin') # add element
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>>> print engineers
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Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])
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>>> employees.issuperset(engineers) # superset test
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False
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>>> employees.update(engineers) # update from another set
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>>> employees.issuperset(engineers)
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True
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>>> for group in [engineers, programmers, managers, employees]:
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... group.discard('Susan') # unconditionally remove element
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... print group
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...
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Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])
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Set(['Janice', 'Jack', 'Sam'])
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Set(['Jane', 'Zack', 'Jack'])
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Set(['Jack', 'Sam', 'Jane', 'Marvin', 'Janice', 'John', 'Zack'])
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.. _immutable-transforms:
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Protocol for automatic conversion to immutable
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----------------------------------------------
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Sets can only contain immutable elements. For convenience, mutable :class:`Set`
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objects are automatically copied to an :class:`ImmutableSet` before being added
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as a set element.
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The mechanism is to always add a hashable element, or if it is not hashable, the
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element is checked to see if it has an :meth:`__as_immutable__` method which
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returns an immutable equivalent.
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Since :class:`Set` objects have a :meth:`__as_immutable__` method returning an
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instance of :class:`ImmutableSet`, it is possible to construct sets of sets.
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A similar mechanism is needed by the :meth:`__contains__` and :meth:`remove`
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methods which need to hash an element to check for membership in a set. Those
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methods check an element for hashability and, if not, check for a
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:meth:`__as_temporarily_immutable__` method which returns the element wrapped by
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a class that provides temporary methods for :meth:`__hash__`, :meth:`__eq__`,
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and :meth:`__ne__`.
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The alternate mechanism spares the need to build a separate copy of the original
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mutable object.
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:class:`Set` objects implement the :meth:`__as_temporarily_immutable__` method
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which returns the :class:`Set` object wrapped by a new class
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:class:`_TemporarilyImmutableSet`.
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The two mechanisms for adding hashability are normally invisible to the user;
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however, a conflict can arise in a multi-threaded environment where one thread
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is updating a set while another has temporarily wrapped it in
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:class:`_TemporarilyImmutableSet`. In other words, sets of mutable sets are not
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thread-safe.
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.. _comparison-to-builtin-set:
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Comparison to the built-in :class:`set` types
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---------------------------------------------
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The built-in :class:`set` and :class:`frozenset` types were designed based on
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lessons learned from the :mod:`sets` module. The key differences are:
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* :class:`Set` and :class:`ImmutableSet` were renamed to :class:`set` and
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:class:`frozenset`.
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* There is no equivalent to :class:`BaseSet`. Instead, use ``isinstance(x,
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(set, frozenset))``.
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* The hash algorithm for the built-ins performs significantly better (fewer
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collisions) for most datasets.
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* The built-in versions have more space efficient pickles.
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* The built-in versions do not have a :meth:`union_update` method. Instead, use
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the :meth:`update` method which is equivalent.
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* The built-in versions do not have a ``_repr(sorted=True)`` method.
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Instead, use the built-in :func:`repr` and :func:`sorted` functions:
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``repr(sorted(s))``.
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* The built-in version does not have a protocol for automatic conversion to
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immutable. Many found this feature to be confusing and no one in the community
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reported having found real uses for it.
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