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[Python-Dev] PEP 509: Add a private version to dict

Victor Stinner victor.stinner at gmail.com
Mon Jan 11 18:24:03 EST 2016 
2016-01-12 0:07 GMT+01:00 Gregory P. Smith <greg at krypto.org>:
>> Changes
>> =======
>>
>> (...)
>
> Please be more explicit about what tests you are performing on the values.
> setitem's "if the value is different" really should mean "if value is not
> dict['key']".  similarly for update, there should never be equality checks
> performed on the values.  just an "is" test of it they are the same object
> or not.

Ok, done. By the way, it's also explained below: values are compared
by their identify, not by their content.

For best dict efficiency, we can not implement this micro-optimization
(to avoid a potential branch misprediction in the CPU) and always
increase the version. But for guards, the micro-optimization can avoid
a lot of dictionary lookups, especially when a guard watches for a
large number of keys.

Victor
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