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How to Check If a Set Is a Subset of Another Set in Python

In Python, a set is considered a subset of another if every element in the first set is also contained within the second set. Checking for this relationship is fundamental when validating data permissions, filtering categories, or performing mathematical set operations.

This guide explains how to check for subset relationships using the readable .issubset() method and the concise <= operator, while highlighting the distinction between standard subsets and proper (strict) subsets.

Understanding the Subset Relationship​

Set A is a subset of Set B if all elements of A are present in B.

  • A = {1, 2}
  • B = {1, 2, 3, 4}
  • Result: A ⊆ B is True.

If A contains even one element not found in B, it is not a subset.

Method 1: Using the issubset() Method​

The built-in set.issubset(other) method allows you to check if the set is a subset of other. This method is readable and flexible.

skills_junior = {"Python", "Data Analysis"}
skills_senior = {"Python", "Data Analysis", "Machine Learning", "SQL"}

# ✅ Check if junior skills are a subset of senior skills
is_subset = skills_junior.issubset(skills_senior)

print(f"Is subset? {is_subset}")

Output:

Is subset? True

Handling Non-Subset Cases​

If the first set has extra elements, the method returns False.

skills_cloud = {"Python", "AWS"} # AWS is not in senior skills
skills_senior = {"Python", "Data Analysis", "Machine Learning", "SQL"}

# ⛔️ False: 'AWS' is missing from the senior set
print(skills_cloud.issubset(skills_senior))

Output:

False
tip

The issubset() method accepts any iterable (lists, tuples) as an argument. It does not strictly require the second argument to be a set. For example, {'a'}.issubset(['a', 'b']) works perfectly.

Method 2: Using the <= Operator​

Python allows you to use mathematical comparison operators for sets. The <= operator checks if the left operand is a subset of the right operand.

set_a = {1, 2, 3}
set_b = {1, 2, 3, 4, 5}

# ✅ Concise syntax
if set_a <= set_b:
    print("Set A is a subset of Set B")
else:
    print("Set A is NOT a subset of Set B")

Output:

Set A is a subset of Set B
warning

Unlike issubset(), the <= operator requires both operands to be sets. If you try to compare a set with a list (e.g., set_a <= [1, 2]), Python will raise a TypeError.

Checking for Proper Subsets (Strict)​

A proper subset (or strict subset) means Set A is inside Set B, but A is not equal to B (i.e., B has at least one extra element).

  • Use <= for "Subset or Equal".
  • Use < for "Proper Subset".
set_x = {1, 2, 3}
set_y = {1, 2, 3} # Identical to set_x

# ✅ Standard Subset Check (True because sets are equal)
print(f"Is subset (<=): {set_x <= set_y}")

# ✅ Proper Subset Check (False because sets are equal)
print(f"Is proper subset (<): {set_x < set_y}")

Output:

Is subset (<=): True
Is proper subset (<): False

Conclusion​

To check subset relationships in Python:

  1. Use set_a.issubset(set_b) for readability and when comparing against non-set iterables (like lists).
  2. Use set_a <= set_b for concise, mathematical syntax when comparing two sets.
  3. Use set_a < set_b if you need to ensure set_a is smaller than and not equal to set_b.

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