[Python-Dev] Bad interaction of __index__ and sequence repeat

[Nick Coghlan]

Tim Peters wrote:
> OTOH, in the long discussion about PEP 357, I'm not sure anyone except
> Travis was clear on whether nb_index was meant to apply only to
> sequence /slicing/ or was meant to apply "everywhere an object gets
> used in an index-like context".  Clipping makes sense only for the
> former, but it looks like the implementation treats it more like the
> latter.  This was probably exacerbated by:
> 
>     http://mail.python.org/pipermail/python-dev/2006-February/060663.html
> 
>     [Travis]
>     There are other places in Python that check specifically for int objects
>     and long integer objects and fail with anything else.  Perhaps all of
>     these should aslo call the __index__ slot.
> 
>     [Guido]
>     Right, absolutely.
> 
> This is a mess :-)

I've been trawling through the code a bit, and I don't think it's as bad as 
all that.

All I believe is really needed is to:
  - remove the PyErr_Occurred() check and its body from long_index in longobject.c
  - add a PyErr_Occurred() check to force a -1 return from PyNumber_Index in 
abstract.c
  - add a PyErr_Occurred() and PyErr_ExceptionMatches(PyOverflowError) check 
to invoke PyErr_Clear() in _PyEval_SliceIndex in ceval.c.

Add test cases to test_index.py to check that:

   (2**100).__index__() == 2**100
   (-2**100).__index__() == -2**100
   slice(-2**100, 2**100).indices(sys.maxint) == (0, sys.maxint, 1)
   "a" * 2**100 raises OverflowError

Add test cases to test_operator.py to check that:

   operator.index(2**100) == 2**100
   operator.index(-2**100) == -2**100

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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