On 1 April 2018 at 02:58, Jeroen Demeyer <j.deme...@ugent.be> wrote: > On 2018-03-31 18:09, Steven D'Aprano wrote: >> Seems to me that if you want a fast, exact (no subclasses) check, you >> should use "type(obj) is Class" rather than isinstance. If the *only* >> reason to prohibit subclassing is to make isinstance a bit faster, >> I don't think that's a good enough reason. > > I didn't really mean "isinstance" literally, I was mostly thinking of the C > API. I agree that it's not clear. > > Do you happen to know why the existing function classes in Python disallow > subclassing? I assumed that it was for exactly this reason.
Disallowing subclasses is a simplifying assumption for builtin types, since it means they don't need to account for subclasses potentially failing to enforce class invariants. That said, allowing Cython/CFFI/etc to use the existing fast paths for native Python functions and/or builtin C functions is a reasonable justification for flipping that switch and doing the extra work needed to make it robust - it's just likely to involve adding a number of `*_CheckExact()` calls in various places. Cheers, Nick. -- Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
