On 18/03/14 07:52, Maciej Fijalkowski wrote:
Hi
I have a question about calling __eq__ in some cases.
We're thinking about doing an optimization where say:
if x in d:
return d[x]
where d is a dict would result in only one dict lookup (the second one
being constant folded away). The question is whether it's ok to do it,
despite the fact that it changes the semantics on how many times
__eq__ is called on x.
Yes it is OK. The number of equality checks is not part of the specification of
the dictionary. In fact, it differs between a 32 and 64 bit build of the same
code.
Consider two objects that hash to 2**33+1 and 2**34+1 respectively.
On a 32 bit machine their truncated hashes are both 1, so they must be
distinguished
by an equality test. On a 64 bit machine their hashes are distinct and no
equality
check is required.
Cheers,
Mark.
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