Python issubset

Summary: in this tutorial, you’ll learn about how to use the Python issubset() method to check if a set is a subset of another set.

Introduction to the Python issubset() method

Suppose that you have two sets A and B. Set A is a subset of set B if all elements of A are also elements of B. Then, set B is a superset of set A.

The following Venn diagram illustrates that set A is a subset of the set B:

Set A and set B can be equal. If set A and set B are not equal, A is a proper subset of B.

In Python, you can use the Set issubset() method to check if a set is a subset of another:

set_a.issubset(set_b)Code language: CSS (css)

If the set_a is a subset of the set_b, the issubset() method returns True. Otherwise, it returns False.

The following example uses the issubset() method to check if the set_a is a subset of the set_b:

numbers = {1, 2, 3, 4, 5}
scores = {1, 2, 3}

print(scores.issubset(numbers))Code language: PHP (php)

Output:

TrueCode language: PHP (php)

By definition, a set is also a subset of itself. The following example returns True:

numbers = {1, 2, 3, 4, 5}

print(numbers.issubset(numbers))Code language: PHP (php)

Output:

TrueCode language: PHP (php)

The following example returns False because some elements in the numbers set aren’t in the scores set. In other words, the numbers set is not a subset of the scores set:

numbers = {1, 2, 3, 4, 5}
scores = {1, 2, 3}

print(numbers.issubset(scores))Code language: PHP (php)

Output:

FalseCode language: PHP (php)

Using subset operators

Besides using the issubset() method, you can use the subset operator (<=) to check if a set is a subset of another set:

set_a <= set_b

The subset operator (<=) returns True if set_a is a subset of the set_b. Otherwise, it returns False. For example:

numbers = {1, 2, 3, 4, 5}
scores = {1, 2, 3}

result = scores <= numbers
print(result)  # True

result = numbers <= numbers
print(result)  # TrueCode language: PHP (php)

The proper subset operator (<) check if the set_a is a proper subset of the set_b:

set_a < set_b

For example:

numbers = {1, 2, 3, 4, 5}
scores = {1, 2, 3}

result = scores < numbers
print(result)  # True

result = numbers < numbers
print(result)  # FalseCode language: PHP (php)

In this example, the set numbers is not a proper subset of itself, therefore, the < operator returns False.

Summary

• Set A is a subset of set B if all elements of the set A are also elements of the set B
• Use Set issubset() method returns True if a set is a subset of another set.
• Also, use the subset operator (<=) or the proper subset operator (<) to check if a set is a subset or proper subset of another set.