[Python-ideas] Binary arithmetic does not always call subclasses first

[Stephan Hoyer]

On Sun, Apr 23, 2017 at 10:57 PM, Greg Ewing <greg.ewing at canterbury.ac.nz>
wrote:

> Stephan Hoyer wrote:
>
>> In practice, CPython requires that the right operand defines a different
>> method before it defers to it.
>>
>
> I'm not sure exactly what the rationale for this behaviour is,
> but it's probably something along the lines that the left
> method should already know how to deal with that combination
> of types, and right methods are only supposed to be called
> as a fallback if the left method can't handle the operands,
> so calling it in that situation would be wrong.
>
>>
Yes, this could makes sense. But in that case, why check for explicitly
overridden methods on subclasses at all? I can rationalize either not
treating subclasses differently or always trying subclasses first, but not
the current behavior.

Of these two options, I prefer always trying subclasses first because I
agree with the rationale in the docs: "This behavior allows subclasses to
override their ancestors’ operations."

In general, code should be written such that subclasses are aware of
super-classes, not the other way around.


> The 3.x docs don't have the "and overrides" language;
>>
>
> so arguably we would be changing the implementation to match
> the docs.
>

Based on the change in the documentation between 2.x and 3.x, I wonder if
this is something that someone intended to clean up as part of Python 3000
but never got around to. I would love to hear from anyone familiar with the
historical context here.

Nonetheless, at this point the behavior has been around for quite some
time. Almost assuredly, there is *someone* relying on this feature/bug,
though probably unintentionally. So I would also love to hear from anyone
who knows of code that this change would actually break (as opposed to fix
and/or allow for removing redundant methods).

More broadly: is this change significant enough that it needs a PEP?
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