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

[Michael Hudson]

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?

Cheers,
mwh

-- 
  This makes it possible to pass complex object hierarchies to
  a C coder who thinks computer science has made no worthwhile
  advancements since the invention of the pointer.
                                       -- Gordon McMillan, 30 Jul 1998