>> On Apr 15, 2020, at 14:08, Caleb Donovick <donov...@cs.stanford.edu> wrote: >> >> Besides performance, I don’t think it fits with Guido’s conception of the >> protocols as being more minimal than the builtin types—e.g., set has not >> just a & operator, but also an intersection method that takes 0 or more >> arbitrary iterables; the set protocol has no such method, so >> collections.abc.Set neither specifies nor provides an intersection method). >> It’s a bit muddy of a conception at the edges, but I think this goes over >> the line, and maybe have been explicitly thought about and rejected for the >> same reason as Set.intersection. > > Making __missing__ a first class part of how __getitem__ seems more analogous > to __getattr__ and __getattribute__ than the intersection method.
That’s a good point, but… > Is there any other dunder that is only implicitly called on builtin types? Actually, IIRC, __getattr__ is itself another example. Python never calls __getattr__, only __getattribute__. However, object.__getattribute__ (and type.__getattribute__) calls __getattr__ on failure, the exact same way dict.__getitem__ calls __missing__ on failure. (Obviously there are differences in practice—e.g., every object is-a object but not every mapping is-a dict, and it’s pretty rare to implement __getattribute__ without super… but if you do implement __getattribute__ without super, or do the equivalent in a C extension class, __getattr__ doesn’t get called, just like __missing__; I’m pretty sure the docs explain this somewhere.) But there are fallbacks that really are a “first class part of the protocol” as you intended: iteration falls back to __getitem__, construction calls __init__ if __new__ returns an instance, addition tries __radd__ before and/or after __add__, etc., and that’s all definitely part of the protocol, baked into the interpreter itself, not just part of how some class implements the protocol. So your proposal is perfectly coherent and reasonable (at least if you change it from “like __getattr__” to “like __radd__” or something), and this is really just a footnote. But I think I’m still against it, both for the “protocols as minimal as possible” reason, and because there’s no obvious way to make it a first class part of the protocol for mappings without making it a first class part of the protocol for sequences (because at the level of the interpreter, and the syntax, they’re both the same subscription protocol). As a side note to the footnote, check out the pickle protocol. IIRC, the fallback from __reduce_ex__ to __reduce__ is a first-class part of the protocol (like iteration, construction, and arithmetic), but the fallback from __getnewargs__ or __getstate__ is just part of some builtin types’ implementations (like attribution and subscription). _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/AOJA2LRC4Z3S3CTZGHXXKAVKPMJUXSLM/ Code of Conduct: http://python.org/psf/codeofconduct/
