The most efficient way to join multiple sets (stored in a list of sets), use the Python one-liner set().union(*list) that creates a new set object, calls the union() method on the new object, and unpacks all sets from the list of sets into the union() method’s argument list.
A set is a unique collection of unordered elements. The union operation combines all elements of two sets into a new set—removing all duplicate entries in the process.
In the following video, you can watch me explain how to union multiple sets in Python:
So, let’s dive into the formal problem formulation, shall we?
Problem: Given a list or a collection of sets. How to join those sets using the union operation?
Example: You’ve got a list of sets [{1, 2, 3}, {1, 4}, {2, 3, 5}] and you want to calculate the union {1, 2, 3, 4, 5}.
Solution: To union a list of sets, use the following strategy:
- Create a new set using the
set()constructor. - Call the
union()method on the new set object. - Pass all sets as arguments into the
union()method by unpacking the list with the asterisk operator*list. - The result of the
union()method is a new set containing all elements that are in at least one of the sets.
Code: Here’s the one-liner code that unions a collection of sets.
# Create the list of sets
lst = [{1, 2, 3}, {1, 4}, {2, 3, 5}]
# One-Liner to union a list of sets
print(set().union(*lst))
The output of this code is the union of the three sets {1, 2, 3}, {1, 4}, {2, 3, 5}:
{1, 2, 3, 4, 5}
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Try it yourself: Here’s the code in an interactive code shell that runs it in your browser:
Exercise: Change the code to calculate the intersection of the sets in the list!
Where to Go From Here?
Enough theory. Let’s get some practice!
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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
To help students reach higher levels of Python success, he founded the programming education website Finxter.com that has taught exponential skills to millions of coders worldwide. He’s the author of the best-selling programming books Python One-Liners (NoStarch 2020), The Art of Clean Code (NoStarch 2022), and The Book of Dash (NoStarch 2022). Chris also coauthored the Coffee Break Python series of self-published books. He’s a computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.
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