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

[Nick Coghlan]

Michael Hudson wrote:
> David Hopwood <david.nospam.hopwood at blueyonder.co.uk> writes:
> 
>> Armin Rigo wrote:
>>> Hi,
>>>
>>> There is an oversight in the design of __index__() that only just
>>> surfaced :-(  It is responsible for the following behavior, on a 32-bit
>>> machine with >= 2GB of RAM:
>>>
>>>     >>> s = 'x' * (2**100)       # works!
>>>     >>> len(s)
>>>     2147483647
>>>
>>> This is because PySequence_Repeat(v, w) works by applying w.__index__ in
>>> order to call v->sq_repeat.  However, __index__ is defined to clip the
>>> result to fit in a Py_ssize_t.
>> Clipping the result sounds like it would *never* be a good idea. What was
>> the rationale for that? It should throw an exception.
> 
> Why would you expect range(10)[:2**32-1] and range(10)[:2**32] to do
> different things?

In that case, I believe it is the slice object that should be suppressing the 
overflow error (via PyErr_Occurred and PyErr_Matches) when calculating the 
indices for a given length, rather than having silent clipping be part of the 
basic implementation of long.__index__().

Cheers,
Nick.

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