[Python-Dev] PEP 495 accepted (original) (raw)

Tim Peters tim.peters at gmail.com
Tue Sep 22 19:34:19 CEST 2015

[Tim]

Sure - no complaint. I was just saying that in the specific, complicated, contrived expression Nick presented, that it always returns False (no matter which aware datetime he starts with) would be more of a head-scratcher than if it raised a "can't compare naive and aware datetimes" exception instead.

[Guido]

And yet I think the desired behavior of == requires us to return False.

Yes - we remain in violent agreement on all points here.

I think we should change this in the PEP, except I can't find where the PEP says == should raise an exception in this case.

It doesn't - the only comparison behavior changed by the PEP is in case of interzone comparison when at least one comparand is a "problem time" (which can only happen with a post-495 tzinfo). Then "==" is always False. That hack is the ugliest part of the PEP, but was needed to preserve the hash invariant (d1 == d2 implies hash(d1) == hash(d2)).

BTW, while the PEP doesn't spell this out, trichotomy can fail in some such cases (those where "==" would have returned True had it not been forced to return False - then "<" and ">" will also be False).

In any case, nothing changes for any case of aware-vs-naive comparison.

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