>>> import itertools
>>> itertools.islice('abc', 0)
<itertools.islice object at 0x00A397B0>
>>> itertools.islice('abc', -2)

Traceback (most recent call last):
  File "<pyshell#37>", line 1, in -toplevel-
    itertools.islice([1,2,3], -2)
ValueError: Indices for islice() must be positive.

Error message is changed to "Indices for islice() must be 
non-negative." since 0 is allowed.

>>> itertools.islice('abc', -1)

Traceback (most recent call last):
  File "<pyshell#38>", line 1, in -toplevel-
    itertools.islice([1,2,3], -1)
ValueError: Stop argument must be an integer or None.

Clearly -1 is an integer. This was due to some 
optimization in itertoolsmodule.c. If PyInt_AsLong 
returned -1 it was taken as a conversion error. I added 
an additional else statement to distinguish the two cases 
that change the variable to -2 if it was in fact -1 to 
begin with. A little 'hacky' but it seems like the best 
soloution and it doesn't undo the optimization.

...johahn

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Let me add that the code has not been compiled. I just 
noticed two missing { in the first patch and uploaded a new 
version.

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Fixed the error messages.  See Modules/itertoolsmodule.c 1.25