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Comment by jetrink | Hacker News Reader

It's useful for working with permutations. E.g,

    [f(a, b) for a in some_list for b in some_list]

or even

    [f(a, b) for a in some_list for b in some_list if a is not b]

itertools has functions that make that sort of nesting unnecessary (and some would say make the code clearer), eg:

    import itertools as it
    [f(a, b) for a, b in it.product(some_list, some_list) if a is not b]

Oh, or more directly:

    [f(a, b) for a, b in it.permutations(some_list, r=2)]

And then you could also use itertools.starmap instead of using a comprehension at all, if you wanted.