Simple thread-safe counter?

[Tim Peters]

[Paul Rubin]
> I'd like to have a function (or other callable object) that returns
> 0, 1, 2, etc. on repeated calls.  That is:
>
>    print f()   # prints 0
>    print f()   # prints 1
>    print f()   # prints 2
>    # etc.
>
> There should never be any possibility of any number getting returned
> twice, or getting skipped over, even if f is being called from
> multiple threads.
> 
> What's the simplest and most natural way to do this?  I can think of a
> few but am not sure that they work.  And I can think of some ways that
> are sure to work, but are messier than I'd like.

The GIL is your friend here:

    import itertools
    f = itertools.count().next

A similar thing can be done with xrange.  But either way sucks if you
call it often enough to exceed the size of a Python short int
(platform C long).  The obvious way with an explicit mutex doesn't
have that problem.