Sets

Sets are containers that share properties and operations with sets in mathematics. A mathematical set is a collection of distinct objects. Here are some mathematical set expressions:

And their Python counterparts:

A = {1,2,3,4}
B = {5}
C = A.union(B)   # returns set([1,2,3,4,5])
D = A.intersection(C)   # returns set([1,2,3,4])
E = C.difference(A)   # returns set([5])
5 in C   # returns True

Sets contain an element only once, corresponding to the aforementioned definition:

A = {1,2,3,3,3}
B = {1,2,3}
A == B # returns True

And a set is unordered; that is, the order of the elements in the set is not defined:

A = {1,2,3}
B = {1,3,2}
A == B # returns True

Sets in Python can contain all kinds of hashable objects, that is, numeric objects, strings, and Booleans.

There are union and intersection methods:

A={1,2,3,4}
A.union({5})
A.intersection({2,4,6}) # returns set([2, 4])

Also, sets can be compared using the methods issubset and issuperset :

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