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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