Set operations

This reference documents all methods and operators available on Python set objects.

Adding and removing

add()

set.add(elem)

Add an element to the set. Has no effect if the element is already present.

colours = {"red", "green"}
colours.add("blue")

remove()

set.remove(elem)

Remove an element from the set. Raises KeyError if the element is not found.

colours = {"red", "green", "blue"}
colours.remove("green")

discard()

set.discard(elem)

Remove an element from the set if it is present. Does not raise an error if the element is missing.

colours = {"red", "green"}
colours.discard("blue")  # No error

pop()

set.pop()

Remove and return an arbitrary element. Raises KeyError if the set is empty.

colours = {"red", "green", "blue"}
item = colours.pop()

clear()

set.clear()

Remove all elements from the set.

Set algebra

Union

Items in either set (or both).

Form	Syntax
Operator	a | b
Method	a.union(b)
In-place	a |= b or a.update(b)

a = {1, 2, 3}
b = {3, 4, 5}
print(a | b)  # {1, 2, 3, 4, 5}

Intersection

Items in both sets.

Form	Syntax
Operator	a & b
Method	a.intersection(b)
In-place	a &= b or a.intersection_update(b)

a = {1, 2, 3}
b = {2, 3, 4}
print(a & b)  # {2, 3}

Difference

Items in the first set but not the second.

Form	Syntax
Operator	a - b
Method	a.difference(b)
In-place	a -= b or a.difference_update(b)

a = {1, 2, 3}
b = {2, 3, 4}
print(a - b)  # {1}

Symmetric difference

Items in either set, but not both.

Form	Syntax
Operator	a ^ b
Method	a.symmetric_difference(b)
In-place	a ^= b or a.symmetric_difference_update(b)

a = {1, 2, 3}
b = {2, 3, 4}
print(a ^ b)  # {1, 4}

Comparison

issubset()

set.issubset(other)

Return True if every element of the set is in the other set. Equivalent to set <= other.

Use set < other to test for a proper subset (subset but not equal).

a = {1, 2}
b = {1, 2, 3}
print(a.issubset(b))  # True
print(a < b)          # True (proper subset)

issuperset()

set.issuperset(other)

Return True if every element of the other set is in this set. Equivalent to set >= other.

Use set > other to test for a proper superset.

a = {1, 2, 3}
b = {1, 2}
print(a.issuperset(b))  # True

isdisjoint()

set.isdisjoint(other)

Return True if the two sets have no elements in common.

evens = {2, 4, 6}
odds = {1, 3, 5}
print(evens.isdisjoint(odds))  # True

Other operations

copy()

set.copy()

Return a shallow copy of the set.

frozenset

A frozenset is an immutable version of a set. It supports all set operations except those that modify the set.

vowels = frozenset({"a", "e", "i", "o", "u"})

Frozen sets can be used as dictionary keys or as elements of other sets.

Summary of operators versus methods

Operation	Operator	Method
Union	|	union()
Intersection	&	intersection()
Difference	-	difference()
Symmetric difference	^	symmetric_difference()
Subset	<=	issubset()
Proper subset	<	—
Superset	>=	issuperset()
Proper superset	>	—