The index that the builtin function enumerate() returns
is a mere integer and will wrap back to a negative
number when it overflows. This seems to be by design
(unless I'm misunderstanding the point of
PyInt_FromLong), but is sufficiently surprising that
I'd suggest it's a bug anyway. I've attached a test
case - it takes a while to execute, but the results are
so deeply exciting that it's worth doing just for fun!

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Raymond, any comments?

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You're correct.  The behavior was by design, emphasizing 
speed and simplicity over a toy limiting case.  If some 
app actually requires enumeration of over 2**31 objects, 
it is trivial to write a generator that gives the desired 
flexibility.  Of course, on 64 bit machines, the limit is 
a bit higher ;-)

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Fair enough, I take your points both about the ease of
making a new generator. Is it possible to make the existing
one a bit less surprising, though, by having it raise an
exception when it wraps? I'm at a loss to think of any
algorithm that would work correctly when it wraps, so a new
exception wouldn't be a change in interface reeeeeeeeally. ;)

Failing that, is it possible to document it?

Nows raises an OverflowError instead of silently wrapping around.  See revisions 53665 and 53666.