Set
Sets are a recent addition to the language that are neither mappings nor sequences; rather, they are unordered collections of unique and immutable objects. You create sets by calling the built-in set function or using new set literals and expressions in 3.X and 2.7, and they support the usual mathematical set operations (the choice of new {…} syntax for set literals makes sense, since sets are much like the keys of a valueless dic-tionary):
>>> X = set('spam') # Make a set out of a sequence in 2.X and 3.X
>>> Y = {'h', 'a', 'm'} # Make a set with set literals in 3.X and 2.7
>>> X, Y # A tuple of two sets without parentheses
({'m', 'a', 'p', 's'}, {'m', 'a', 'h'})
>>> X & Y # Intersection
{'m', 'a'}
>>> X | Y # Union
{'m', 'h', 'a', 'p', 's'}
>>> X - Y # Difference
...
