Python Set intersection() Method

Q: How do I find common elements between two lists in Python?

Convert both to sets and intersect them: `set(a) & set(b)`. The result is unordered, so use `sorted()` if you need an order.

Q: What does intersection() return if there are no common elements?

It returns an empty set, printed as `set()`, not `None` and not an error.

By Yogesh Sharma (LinkedIn) · Every code example is run and checked on Python 3.13 (how we test) · Last updated 21 Oct 2026

Short answer

a.intersection(b) returns a new set with only the elements that appear in both a and b. The original sets are unchanged.

Use intersection to find what groups have in common: students enrolled in both Python and Maths, products that appear in two shops, or tags shared by two posts. It is one of the main set operations.

The result is empty (set()) when the sets share nothing. If you want to ignore the shared items instead, look at difference().

Quick facts
Syntax	`a.intersection(*others)` or `a & b`
Parameters	Zero or more iterables
Returns	A new set
Changes the original?	No (`intersection_update()` does)
Time complexity	About O(min(len(a), len(b))) for two sets

How set.intersection() works

Python walks the smaller set and keeps each element that is also in the other, which is fast because membership tests on sets are O(1) on average.

Python

python_class = {"Asha", "Ravi", "Meera"}
maths_class = {"Ravi", "Meera", "Kabir"}
print(sorted(python_class.intersection(maths_class)))
print(sorted(python_class & maths_class))

Intersection of more than two sets

Pass several arguments and only elements present in all of them survive. With no argument at all, a.intersection() returns a copy of a.

Python

a = {1, 2, 3, 4}
b = {2, 3, 4, 5}
c = {3, 4, 9}
print(sorted(a.intersection(b, c)))

intersection() vs intersection_update() vs &

Form	Accepts	Result	Modifies the original?
`a.intersection(b)`	Any iterable	New set	No
`a & b`	Sets only	New set	No
`a.intersection_update(b)`	Any iterable	`None`	Yes, keeps only common items
`a &= b`	Sets only	Updates `a`	Yes
`a.isdisjoint(b)`	Any iterable	`True` or `False`	No

Examples you can run

Press Run, then edit the code and run it again.

Students in both classes

Only the names present in both sets are kept.

Students in both classes

python_class = {"Asha", "Ravi", "Meera"}
maths_class = {"Ravi", "Meera", "Kabir"}
both = python_class.intersection(maths_class)
print(sorted(both))
print(len(both))

Expected output['Meera', 'Ravi']
2

No overlap gives an empty set

Using a list as the argument works too, and no overlap yields set().

No overlap gives an empty set

cart = {"pen", "book"}
wishlist = ["laptop", "mouse"]
common = cart.intersection(wishlist)
print(common)
print(bool(common))

Expected outputset()
False

Common mistakes

Expecting the original to shrink

✗ Wrong

a = {1, 2, 3}
a.intersection({2, 3})
print(a)

{1, 2, 3} (the result was discarded)

✓ Right

a = {1, 2, 3}
a = a.intersection({2, 3})
print(sorted(a))

Assign the result, or use intersection_update() to modify a in place.

Using & with a list

✗ Wrong

a = {1, 2, 3}
print(a & [2, 3])

TypeError: unsupported operand type(s) for &: 'set' and 'list'

✓ Right

a = {1, 2, 3}
print(sorted(a & set([2, 3])))

Use the method, or wrap the list with set().

Python Set intersection() Method: questions people ask

How do I find common elements between two lists in Python?

Convert both to sets and intersect them: set(a) & set(b). The result is unordered, so use sorted() if you need an order.

What is the difference between intersection() and the & operator?

They give the same result for sets, but the method accepts any iterable and several arguments while & needs sets on both sides.

What does intersection() return if there are no common elements?

It returns an empty set, printed as set(), not None and not an error.

Keep learning

Step-by-step lessons:
Python Set Operations Python Sets

More in set methods:
Python Set add() Method Python Set remove() Method Python Set discard() Method Python Set pop() Method Python Set union() Method Python Set difference() Method Python Set symmetric_difference() Method Python Set update() Method See all →