> It's a stronger statement than simply undoing > the declaration that it's a sequence. There would be no way to reset > to the default state.
How is this different from anything else that is inherited? The setting of a flag to `False` is not some irreversible process which permanently blocks child classes from setting that flag to `True`. If I want to give priority to `Seq` over `Parent` in Brandt's original example I need only switch the order of inheritance so that `Seq` is earlier in `Child` MRO or explicitly set the flag to `True` (or `Seq.__match_seq__`). In contrast Brandt's scheme does irreversibly set flags, there is no way to undo the setting of `__match_seq__` in a parent class. This really doesn't seem like an issue to me. I can't personally think of a use case for explicitly setting a flag to `False` but I also don't see why it should be forbidden. We get "- Otherwise, set the flag if any of the parents set have the flag set" for free through normal MRO rules except in the case where there is an explicit `False` (which I assume will be exceedingly rare and if it isn't there is clearly some use case). Why make it more complicated? On Wed, Mar 31, 2021 at 6:05 PM Chris Angelico <ros...@gmail.com> wrote: > On Thu, Apr 1, 2021 at 11:54 AM Caleb Donovick <donov...@cs.stanford.edu> > wrote: > > > > > Here, `Child` will *not* match as a sequence, even though it probably > should, > > > > Strong disagree, if I explicitly set `__match_seq__` to `False` in > `Parent` I probably have a good reason for it and would absolutely expect > `Child` to not match as a sequence. > > > > How much difference is there between: > > class Grandparent: > """Not a sequence""" > class Parent(Grandparent): > """Also not a sequence""" > class Child(Parent): > """No sequences here""" > > and this: > > class Grandparent(list): > """Is a sequence""" > class Parent(Grandparent): > """Explicitly not a sequence""" > __match_seq__ = False > class Child(Parent): > """Shouldn't be a sequence""" > > > ? Either way, Parent should function as a non-sequence. But if Child > inherits from both Parent and tuple, it is most definitely a tuple, > and therefore should be a sequence. > > With your proposed semantics, setting __match_seq__ to False is not > simply saying "this isn't a sequence", but it's saying "prevent this > from being a sequence". It's a stronger statement than simply undoing > the declaration that it's a sequence. There would be no way to reset > to the default state. > > Brandt's proposed semantics sound complicated, but as far as I can > tell, they give sane results in all cases. > > ChrisA > _______________________________________________ > Python-Dev mailing list -- python-dev@python.org > To unsubscribe send an email to python-dev-le...@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/GKOUSL2CPMO7NPPTK2E7XE7LXTPDVRDJ/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
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