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

Guido van Rossum guido at python.org
Tue Sep 22 18:55:35 CEST 2015

On Tue, Sep 22, 2015 at 9:47 AM, Tim Peters <tim.peters at gmail.com> wrote:

[Tim] >>> ... >>> The >>> top-level operation on the RHS is datetime.fromtimestamp(). However, >>> it didn't pass a tzinfo, so it creates a naive datetime. Assuming dt >>> was aware to begin with, the attempt to compare will always (gap or >>> not) raise an exception.

[Tim] >> Oops! In current Python, comparing naive and aware via == just >> returns False. That's even more confusing ;-) [Guido] > Hm, but that's in general how == is supposed to work between objects of > incompatible types. < and > are supposed to fail but == is supposed to > return False (the eq should return NotImplemented). If == ever raises an > exception, having two different objects as dict keys can cause random, > hard-to-debug failures. 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.

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

That's why, whenever anyone is confused by anything they see in a Python program, they should post all their code verbatim to Python-Dev, prefaced with a "Doesn't work! Fix it." comment ;-)

Oh, it would be so much better if they posted their code!

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