[Python-Dev] PEP 495 accepted

[Guido van Rossum]

On Tue, Sep 22, 2015 at 10:34 AM, Tim Peters <tim.peters at gmail.com> wrote:

> [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.
>

And I guess we can't make < and > raise an exception for backward
compatibility reasons. :-(

-- 
--Guido van Rossum (python.org/~guido)
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