pytutorial/python_basics/set_frozenset

set and frozenset

{:.no_toc}

* TOC {:toc}

Top

Questions to David Rotermund

{: .topic-optional} This is an optional topic!

Set Types — set, frozenset

A set object is an unordered collection of distinct hashable objects. Common uses include membership testing, removing duplicates from a sequence, and computing mathematical operations such as intersection, union, difference, and symmetric difference.

Like other collections, sets support x in set, len(set), and for x in set. Being an unordered collection, sets do not record element position or order of insertion. Accordingly, sets do not support indexing, slicing, or other sequence-like behavior.

There are currently two built-in set types, set and frozenset.

• The set type is mutable — the contents can be changed using methods like add() and remove(). Since it is mutable, it has no hash value and cannot be used as either a dictionary key or as an element of another set.
• The frozenset type is immutable and hashable — its contents cannot be altered after it is created; it can therefore be used as a dictionary key or as an element of another set.

class set([iterable])
class frozenset([iterable])

Return a new set or frozenset object whose elements are taken from iterable. The elements of a set must be hashable. To represent sets of sets, the inner sets must be frozenset objects. If iterable is not specified, a new empty set is returned.

Sets can be created by several means:

• Use a comma-separated list of elements within braces: {'jack', 'sjoerd'}
• Use a set comprehension: {c for c in 'abracadabra' if c not in 'abc'}
• Use the type constructor: set(), set('foobar'), set(['a', 'b', 'foo'])

The following table lists operations available for set that do not apply to immutable instances of frozenset:

update(*others)	Update the set, adding elements from all others.
set |= other | ...	Update the set, adding elements from all others.
intersection_update(*others)	Update the set, keeping only elements found in it and all others.
set &= other & ...	Update the set, keeping only elements found in it and all others.
difference_update(*others)	Update the set, removing elements found in others.
set -= other | ...	Update the set, removing elements found in others.
symmetric_difference_update(other)	Update the set, keeping only elements found in either set, but not in both.
set ^= other	Update the set, keeping only elements found in either set, but not in both.
add(elem)	Add element elem to the set.
remove(elem)	Remove element elem from the set. Raises KeyError if elem is not contained in the set.
discard(elem)	Remove element elem from the set if it is present.
pop()	Remove and return an arbitrary element from the set. Raises KeyError if the set is empty.
clear()	Remove all elements from the set.