[Python-Dev] Re: Why do binary arithmetic operators care about differing method implementations but rich comparisons don't?

On Sun, Sep 27, 2020 at 2:58 PM Guido van Rossum wrote:

Hm... IIRC the reason why we did this for `__r*__` is because the more derived class might want to return an instance of that class, and we can't assume that the less derived class knows how to create an instance of the more derived class (the `__init__` signatures might differ).

Yep, that's what the data model docs suggest (see the note at https://docs.python.org/3/reference/datamodel.html#object.__ror__). But the interesting bit is skipping the call of __r*__ when `lhs.__r*__ == rhs.__r*__` (as long as the derived class requirements are met). That's the difference that I'm really curious about compared to rich comparisons and their inverse which don't have this call avoidance. To help visualize all of this, you can see https://github.com/brettcannon/desugar/blob/066f16c00a2c78784bfb18eec31476df... for binary arithmetic operators compared to https://github.com/brettcannon/desugar/blob/066f16c00a2c78784bfb18eec31476df... for rich comparisons. [SNIP]

I think we could try to change it but it would require a very careful risk analysis.

I'm not sure how critical it is to change. I'm sure there's some potential perf gain by avoiding the (potentially) unnecessary call, but I also don't know if people have implemented these functions in such a way that skipping the inverse operation on the right-hand side object would break something. Would abuse of the syntax make a difference (e.g. making `>` do something magical)? -Brett

On Sun, Sep 27, 2020 at 1:41 PM Brett Cannon wrote:

...

When you do a binary arithmetic operation, one of the things that dictates whether the left-hand side's __*__ method is called before the right-hand side's __r*__ method is if the left-hand side's __r*__ differs (there's also the fact __r*__ methods are not called if. the types are the same). Presumably this is because you only care about giving precedence to the right-hand side when it would actually matter due to a difference in implementation (with the assumption that there isn't a specific need to get the right-hand side special dispensation to participate in the operation).

But with rich comparisons there doesn't seem to be an equivalent check for a difference in method implementation. Why is that? Is it because we don't want to assume that if someone bothered to implement both __gt__ and __lt__ that they would not necessarily be the inverse of each other like __add__ and __radd__? _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-leave@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/7NZUCODE... Code of Conduct: http://python.org/psf/codeofconduct/

-- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-c...