83 lines
4.7 KiB
Markdown
83 lines
4.7 KiB
Markdown
# [set](https://docs.python.org/3/library/stdtypes.html#set) and [frozenset](https://docs.python.org/3/library/stdtypes.html#frozenset)
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{:.no_toc}
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<nav markdown="1" class="toc-class">
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* TOC
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{:toc}
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## Top
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Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
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{: .topic-optional}
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This is an optional topic!
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# [Set Types — set, frozenset](https://docs.python.org/3/library/stdtypes.html#set-types-set-frozenset)
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> 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.
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>
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> 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.**
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>
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> There are currently two built-in set types, set and frozenset.
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> * 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.
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> * 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.
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```python
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class set([iterable])
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class frozenset([iterable])
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```
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> 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.
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>
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> Sets can be created by several means:
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> * Use a comma-separated list of elements within braces: {'jack', 'sjoerd'}
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> * Use a set comprehension: {c for c in 'abracadabra' if c not in 'abc'}
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> * Use the type constructor: set(), set('foobar'), set(['a', 'b', 'foo'])
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Instances of **set and frozenset** provide the following operations:
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|len(s)|Return the number of elements in set s (cardinality of s).|
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|x in s|Test x for membership in s.|
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|x not in s|Test x for non-membership in s.|
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|isdisjoint(other)|Return True if the set has no elements in common with other. Sets are disjoint if and only if their intersection is the empty set.|
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|issubset(other)|Test whether every element in the set is in other.|
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|set <= other|Test whether every element in the set is in other.|
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|set < other|Test whether the set is a proper subset of other, that is, set <= other and set != other.|
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|issuperset(other)|Test whether every element in other is in the set.|
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|set >= other|Test whether every element in other is in the set.|
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|set > other|Test whether the set is a proper superset of other, that is, set >= other and set != other.|
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|union(*others)|Return a new set with elements from the set and all others. |
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|set \| other \| ...|Return a new set with elements from the set and all others. |
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|intersection(*others)|Return a new set with elements common to the set and all others.|
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|set & other & ...|Return a new set with elements common to the set and all others.|
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|difference(*others)|Return a new set with elements in the set that are not in the others.|
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|set - other - ...|Return a new set with elements in the set that are not in the others.|
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|symmetric_difference(other)|Return a new set with elements in either the set or other but not both.|
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|set ^ other|Return a new set with elements in either the set or other but not both.|
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|copy()|Return a shallow copy of the set.|
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The following table lists operations **available for set** that do not apply to immutable instances of frozenset:
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| update(*others)| Update the set, adding elements from all others. |
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| set \|= other \| ... | Update the set, adding elements from all others.|
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| intersection_update(*others) | Update the set, keeping only elements found in it and all others.|
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| set &= other & ... | Update the set, keeping only elements found in it and all others.|
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|difference_update(*others)| Update the set, removing elements found in others. |
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|set -= other \| ...| Update the set, removing elements found in others.|
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|symmetric_difference_update(other)|Update the set, keeping only elements found in either set, but not in both.|
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|set ^= other|Update the set, keeping only elements found in either set, but not in both.|
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|add(elem)|Add element elem to the set.|
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|remove(elem)|Remove element elem from the set. Raises KeyError if elem is not contained in the set.|
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|discard(elem)|Remove element elem from the set if it is present.|
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|pop()|Remove and return an arbitrary element from the set. Raises KeyError if the set is empty.|
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|clear() | Remove all elements from the set.|
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