Foundations Updated 2026-09 View as Markdown

Sets

Sets are lists with no duplicate entries.

Sets are lists with no duplicate entries. Let’s say you want to collect a list of words used in a paragraph:

print(set("my name is Eric and Eric is my name".split()))

This will print out a list containing “my”, “name”, “is”, “Eric”, and finally “and”. Since the rest of the sentence uses words which are already in the set, they are not inserted twice.

Sets are a powerful tool in Python since they have the ability to calculate differences and intersections between other sets. For example, say you have a list of participants in events A and B:

a = set(["Jake", "John", "Eric"])
print(a)
b = set(["John", "Jill"])
print(b)

To find out which members attended both events, you may use the “intersection” method:

a = set(["Jake", "John", "Eric"])
b = set(["John", "Jill"])

print(a.intersection(b))
print(b.intersection(a))

To find out which members attended only one of the events, use the “symmetric_difference” method:

a = set(["Jake", "John", "Eric"])
b = set(["John", "Jill"])

print(a.symmetric_difference(b))
print(b.symmetric_difference(a))

To find out which members attended only one event and not the other, use the “difference” method:

a = set(["Jake", "John", "Eric"])
b = set(["John", "Jill"])

print(a.difference(b))
print(b.difference(a))

To receive a list of all participants, use the “union” method:

a = set(["Jake", "John", "Eric"])
b = set(["John", "Jill"])

print(a.union(b))

a = ["Jake", "John", "Eric"]
b = ["John", "Jill"]

A = set(a)
B = set(b)

print(A.difference(B))