Python - Check if Set is a subset of another Set

In this tutorial, we will look at how to check whether a set in Python is a subset of another set with the help of some examples.

What is a subset?

Let’s say we have two sets, A and B. Now, if all the elements of set A are present in set B then A is said to be a subset of B. Here’s an example –

Image showing set A as subset of set B with a venn diagram

You can see that all the elements of set A are present in set B, thus, we can say that set A is a subset of set B. However, the converse is not true, B is not a subset of A since there are elements in B that are not in A.

Check if a set is a subset of another set in Python

The Python set data structure comes with a number of built-in functions to accomplish common set operations like union, intersection, difference, etc. You can use the Python set issubset() function to check whether a set is a subset of another set. The following is the syntax:

# check if a is a subset of b
a.issubset(b)

We call the issubset() function from set a and pass the set b as an argument to check whether set a is a subset of set b. It returns a boolean value. Let’s look at an example.

# create two sets
a = {1, 2, 3}
b = {1, 2, 3, 4}
# check if a is subset of b
a.issubset(b)

Output:

True

We get True as the output since a is in fact, a subset of set b.

Let’s look at another example.

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# create two sets
a = {1, 2, 3}
b = {1, 3, 4, 5}
# check if a is subset of b
a.issubset(b)

Output:

False

Here we get False as the output because not all elements of set a are present in set b (2 from set a is not present in b). Thus, set a is not a subset of set b.

Alternatively, you can use the <= operator to check if a set is a subset of another set. For example,

# create two sets
a = {1, 2, 3}
b = {1, 2, 3, 4}
# check if a is subset of b
a <= b

Output:

True

We get the same result as we did with the set issubset() function.

Special Cases

There are also some special cases to keep in mind when checking for subsets.

Every set is a subset of itself.

# create a set
a = {1, 2, 3}
# check if a is subset of itself
a.issubset(a)

Output:

True

Empty set is subset of every set.

# create a set
a = set()
b = {1, 2, 3}
# check if a is subset of b
a.issubset(b)

Output:

True

Author

Piyush Raj

Piyush is a data professional passionate about using data to understand things better and make informed decisions. He has experience working as a Data Scientist in the consulting domain and holds an engineering degree from IIT Roorkee. His hobbies include watching cricket, reading, and working on side projects.

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