In set theory, the union of a collection of sets is the set of all the elements in the collection of the sets. Following image depicts the set union operations between a collection of sets.

Set Union
Python Set Union
Python set class provides union() function to get the union of a collection of sets. The result is a new set with all the elements from the collection of sets.
Let’s look at some examples of Python set union() function.
set1 = {1, 2, 3, 4}
set2 = {2, 3, 5, 6}
set3 = {3, 4, 6, 7}
print(set1.union(set2))
print(set2.union(set3))
print(set3.union(set1))
Output:
{1, 2, 3, 4, 5, 6}
{2, 3, 4, 5, 6, 7}
{1, 2, 3, 4, 6, 7}

Python Set Union
Union of Multiple Sets
We can create the union of multiple sets through two ways.
- By passing multiple sets as argument in union() function.
- Since union() returns a new set, we can create a chain of union() function calls.
Below code snippet shows above two ways implementation.
print(set1.union(set2, set3))
# OR
print(set1.union(set2).union(set3))
Output:
{1, 2, 3, 4, 5, 6, 7}
{1, 2, 3, 4, 5, 6, 7}

Python Multiple Sets Union
Reference: Official Documentation