To check if a set A is a subset of set B in Python, use the **issubset()** method.

For example:

A = {1, 2, 3} B = {1, 2, 3, 4, 5, 6} print(A.issubset(B))

Output:

True

Alternatively, you can use comparison operators to accomplish the same.

For example, to check if set A is a subset of B, use <= operator:

A = {1, 2, 3} B = {1, 2, 3, 4, 5, 6} print(A <= B)

Output:

True

## A Word on Set Theory

Given two sets A and B:

- A is a
**subset**of B if all the elements of set A are in set B. - A set is always a
**subset of itself**. - A is a
**proper subset**of B if the sets are not equal**and**set A is a subset of B.

Here is a Venn diagram illustration of set A being a subset of set B:

### How to Check If a Set Is a Subset in Python

In Python’s built-in set data type, there is an `issubset()`

method you can use to check if a set is a subset of another set.

The syntax is as follows:

set_A.issubset(set_B)

This returns `True`

if `set_A`

is a subset of `set_B`

. Otherwise, it returns `False`

.

Alternatively, you can also use comparison operators to do the same:

set_A <= set_B

For example:

A = {1, 2, 3} B = {1, 2, 3, 4, 5, 6} print(A.issubset(B)) print(A <= B)

Output:

True True

### A Set Is a Subset of Itself

A set is always a subset of itself.

You can use the `issubset()`

method to verify this:

A = {1, 2, 3} print(A.issubset(A)) # Alternatively print(A <= A)

Output:

True True

### How to Check If a Set Is a Proper Subset in Python

Set A is a proper subset of B if all elements of A are in B but B has at least one element that A does not.

In Python, there is no dedicated method to check if a set is a proper subset of another.

To check if a set is a proper subset of another one, use the greater than operator:

A = {1, 2, 3} B = {1, 2, 3, 4, 5, 6} # Proper subset print(A < B)

Output:

True

## Conclusion

Today you learned how to use `issubset()`

method in Python. You also learned some basic set theory.

- Set A is a
**subset**of set B if set B has all the elements of set A. - Set A is a
**proper subset**of B if all elements of A are in B but B has at least one element that A does not. - A set is always a
**subset of itself**.

In Python, you can check if set A is a subset of set B by:

**A.issubset(B)****A <= B**

To check if a set A is a proper subset of set B:

**A <= B**

Thanks for reading. I hope you find it useful.

Happy coding!