### How to find the intersection and union of two lists in Python

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My friend Bill had previously alerted me to the coolness of Python
`set`

s. However I hadn't found opportunity to use them
until now. Here are three functions using `set`

s to
remove duplicate entries from a list, find the intersection of two
lists, and find the union of two lists. Note, `set`

s were
introduced in Python 2.4, so Python 2.4 or later is required. Also,
the items in the list must be hashable and order of the lists is not
preserved.

For more information on Python `set`

s, see the
Library Reference.

""" NOTES: - requires Python 2.4 or greater - elements of the lists must be hashable - order of the original lists is not preserved """ def unique(a): """ return the list with duplicate elements removed """ return list(set(a)) def intersect(a, b): """ return the intersection of two lists """ return list(set(a) & set(b)) def union(a, b): """ return the union of two lists """ return list(set(a) | set(b)) if __name__ == "__main__": a = [0,1,2,0,1,2,3,4,5,6,7,8,9] b = [5,6,7,8,9,10,11,12,13,14] print unique(a) print intersect(a, b) print union(a, b)

Results:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [8, 9, 5, 6, 7] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]

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