Python Set Difference

Summary: in this tutorial, you’ll learn about the Python Set difference and how to use it to find the difference between two or more sets.

Introduction to the Python Set difference

The difference between the two sets results in a new set that has elements from the first set which aren’t present in the second set.

Suppose you have the following s1 and s2 sets:

s1 = {'Python', 'Java', 'C++'}
s2 = {'C#', 'Java', 'C++'}Code language: JavaScript (javascript)

The difference between s1 and s2 sets results in the following set with one element:

{'Python'}
Code language: JavaScript (javascript)

…because there is only 'Python' element from the first set that doesn’t exist in the second set.

The set difference isn’t commutative. The difference between the s2 and s1 sets returns the following set:

{'C#'}Code language: JavaScript (javascript)

The following Venn diagram illustrates the difference between the s1 and s2 sets:

And the following Venn diagram illustrates the difference between s2 and s1 sets:

In Python, you can use the set difference() method or set difference operator (-) to find the difference between sets.

1) Using Python Set difference() method to find the difference between sets

The Set type has a difference() method that returns the difference between two or more sets:

set1.difference(s2, s3, ...)Code language: CSS (css)

For example, you can use the set difference() method to find the difference between s1 and s2 sets:

s1 = {'Python', 'Java', 'C++'}
s2 = {'C#', 'Java', 'C++'}
s = s1.difference(s2)

print(s)Code language: PHP (php)

Output:

{'Python'}Code language: JavaScript (javascript)

And this example shows how to use the set difference() method to find the difference between s2 and s1 sets.

s1 = {'Python', 'Java', 'C++'}
s2 = {'C#', 'Java', 'C++'}
s = s2.difference(s1)

print(s)Code language: PHP (php)

Output:

{'C#'}Code language: JavaScript (javascript)

Note that the difference() method returns a new set. It doesn’t change the original sets.

2) Using Python set difference operator (-) to find the difference between sets

Besides the difference() method, Python provides you with the set difference operator (-) that allows you to find the difference between sets.

s = s1 - s2

The following example uses the difference operator (-) to find the difference between the s1 and s2 sets:

s1 = {'Python', 'Java', 'C++'}
s2 = {'C#', 'Java', 'C++'}
s = s1 - s2

print(s)Code language: PHP (php)

And this example use the set difference operator to return the difference between s2 and s1:

s1 = {'Python', 'Java', 'C++'}
s2 = {'C#', 'Java', 'C++'}
s = s2 - s1

print(s)
Code language: PHP (php)

Output:

{'C#'}Code language: JavaScript (javascript)

The set difference() method vs set difference operator (-)

The set difference() method can accept one or more iterables (e.g., strings, lists, dictionaries) while the set difference operator (-) only allows sets.

When you pass iterables to the set difference() method, it’ll convert the iterables to sets before performing the difference operation.

The following shows how to use the set difference() method with a list:

scores = {7, 8, 9}
numbers = [9, 10]
new_scores = scores.difference(numbers)

print(new_scores)
Code language: PHP (php)

However, if you use the set difference operator (-) with iterables, you’ll get an error:

scores = {7, 8, 9}
numbers = [9, 10]
new_scores = scores - numbers

print(new_scores)
Code language: PHP (php)

Error:

TypeError: unsupported operand type(s) for -: 'set' and 'list'Code language: JavaScript (javascript)

Summary

• A difference between two sets results in a new set containing elements in the first set that aren’t present in the second set.
• Use the set difference() method or set difference operator (-) to find the difference between sets.