# 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 the 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)`

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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))`

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Output:

`True`

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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))`

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Output:

`True`

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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))`

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Output:

`False`

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## 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) # True`

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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) # False`

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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.