Python Language Set
Internal
Overview
A set is an unordered collection of unique elements. Like dictionary keys, set elements must be immutable and hashable.
Sets and the __hash__() Function
Creation and Initialization
A set can be created in two ways, via the set() function or with a set literal.
A set can be created and initialization with a curly braces set literal:
s = {1, 2, 3}
Sets can also be created with the set() function:
Set from a tuple:
s = set((1, 2, 3))
Set from a list:
s = set([1, 2, 3])
Set from a string:
s = set('ABC')
print(s) # {'C', 'B', 'A'}
Set Equality
Sets are equal only if their contents are equal:
assert {1, 2, 3} == {3, 2, 1}
Shallow Copy
s = set()
s.add('a')
s2 = s.copy()
Add Elements
Add One Element
s = set()
s.add('a')
assert 'a' in s
Add Multiple Elements
Update the set with the union of itself and others, where others can be any iterable. update() eliminates duplicates.
s = set()
s.update(['a', 'b', 'a'])
assert len(s) == 2
assert 'a' in s
assert 'b' in s
Add Multiple Elements with union()
union() merges the elements of the iterable given as argument into the current set, and returns a new set, which is the union of the current set and the argument.
a = {1, 2}
b = {5, 6}
assert a | b == {1, 2, 5, 6}
assert a.union(b) == {1, 2, 5, 6}
Remove an Element from a Set
s.remove('a')
Does not return anything, simply removes the element.
If the element does not exist, throws KeyError: 'a'.
s.pop()
Remove an arbitrary element from the set, raising KeyError if the set is empty.
Convert to List
s=set()
l=list(s)
Set Operations
Union
See above.
Inclusion
Check whether a set is included in another set with issubset():
set_a = {'a', 'b', 'c', 'd'}
set_b = {'a', 'c'}
assert set_b.issubset(set_a)
The function also works for non-homogenous sets, where the elements have different types:
set_a = {'a', 'b', 'c', 1, 5, 9}
set_b = {1, 'c'}
assert set_b.issubset(set_a)
Intersection
a = {'a', 'b', 'c', 1, 2, 3}
b = {'x', 'y', 'c', 1, 10, 20}
assert a.intersection(b) == {1, 'c'}
assert a & b == {1, 'c'}
Clear
Reset the set to an empty state, discarding all elements:
a.clear()
frozenset()
The fronzenset() returns an immutable frozenset object initialized with the elements from a given iterable. frozenset objects can be used as dictionary keys.
If the iterable contains duplicate elements, they are ignored (the iterable is handled like a set):
f = frozenset(['a', 'b', 'c'])
f2 = frozenset(['a', 'b', 'c', 'a'])
assert f == f2