On Fri, Sep 27, 2019 at 10:11:15AM -0700, Andrew Barnert wrote: > On Sep 27, 2019, at 09:05, Steven D'Aprano <st...@pearwood.info> wrote: > > > > What > > I'd really like to do is use the collections.abc module to do the check: > > > > if isinstance(obj, collections.abc.Subscriptable): ... > > What about isinstance(obj, (Sequence, Mapping))?
No, they require more than just being subscriptable. py> class Test(object): ... def __getitem__(self, idx): ... return idx**2 ... py> squares = Test() py> squares[5] 25 py> from collections.abc import Sequence, Mapping py> isinstance(squares, (Sequence, Mapping)) False One of the tests I want is for something which is subscriptable but not sized, like squares above. To be a (virtual) subclass of Sequence, the object has to provide __getitem__ and __len__, and to be a Mapping it also has to provide __iter__. Neither __len__ nor __iter__ are necessary for my purposes, and in fact __len__ may be undesirable. > That isn’t quite the same thing, since you can have types that are > subscriptable, but its subscripts don’t mean either index or key. Its not even close to the same thing :-( > _GenericAlias is probably not the best example here, but it is an > example. Where is that from? > Plus, it seems like it’s actually a more direct LBYL translation of > what you’re EAFPing with your except (IndexError, KeyError) test. Sorry, that doesn't fly. There's nothing in my except test which requires the existence of __len__. All I need is an object that supports subscripting, but the current state of the ABCs requires that I either roll my own test which will probably be wrong, or test for methods that I don't need or want. -- Steven _______________________________________________ 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/QTZUKQNRDBXCRZORD2QZX7ZUAWPHW3EI/ Code of Conduct: http://python.org/psf/codeofconduct/
