The last container object we introduce in this section is defined by the data type set. 

Sets are containers that share properties and operations with sets in mathematics. A mathematical set is a collection of distinct objects. Like in mathematics, in Python the elements of a set are also listed within a pair of braces.

Here are some mathematical set expressions:

And here are their Python counterparts:

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

Moreover, 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 immutable objects, that is, numeric objects, strings...