Itertools is a Python module that provides a number of iterator building blocks. An iterator represents a container (collection) of data or objects and provides ways to iterate over the elements.
Besides native constructs available for iterating through data, the functions of this module are inspired from functional programming languages and others like SBML, Haskell etc. These functions act as simple building blocks allowing one to perform highly specialized actions in easy way.
For example,
>>> from itertools import *
>>> product('ABCD', 'xy')
Ax Ay Bx By Cx Cy Dx Dy
>>> permutations('ABCD', 2)
AB AC AD BA BC BD CA CB CD DA DB DC
# square all numbers upto 9
>>> list(imap(pow, range(10), repeat(2)))
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
# and many more
The Python docs has a section containing useful recipes constructed using these high level functions. In fact, the whole document is a must read for any Pythonista.
I used a combinations of product and permutations today to generate signed
permutations. The Wikipedia article of signed permutations, however, is
difficult to comprehend.