In its __doc__ string and in the documentation, str.join() is described
as taking a sequence.  This is not general enough; it actually takes any
iterable of strings:

    >>> ','.join(str(x) for x in range(5))
    '0,1,2,3,4'

Maybe this is a small nit to pick, but it slowed me down for a few
minutes, and I already vaguely remembered that str.join() could handle
iterables.