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

Tim Peters tim.peters at gmail.com
Tue Sep 22 18:47:02 CEST 2015

[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.

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 ;-)

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