On 20 March 2017 at 22:11, Matthias Kramm <kr...@google.com> wrote: > I'm a big fan of this. I really want structural subtyping for > http://github.com/google/pytype. > > I am glad you like it.
> > On Mon, Mar 20, 2017 at 5:00 AM, Ivan Levkivskyi <levkivs...@gmail.com> > wrote: > >> Explicitly declaring implementation >> ----------------------------------- >> >> To explicitly declare that a certain class implements the given protocols, >> > > Why is this necessary? The whole point of ducktyping is that you *don't* > have to declare what you implement. > > I get that it looks convenient to have your protocol A also supply some of > the methods you'd expect classes of type A to have. But completing an > implementation in that way should be done explicitly (via including a > utility class or using a decorator like functools.total_ordering), not as > side-effect of an (unnecessary) protocol declaration. > I would put the question backwards: do we need to *prohibit* explicit subclassing? I think we shouldn't. Mostly for two reasons: 1. Backward compatibility: People are already using ABCs, including generic ABCs from typing module. If we prohibit explicit subclassing of these ABCs, then quite a lot of code will break. 2. Convenience: There are existing protocol-like ABCs (that will be turned into protocols) that have many useful "mix-in" (non-abstract) methods. For example in case of Sequence one only needs to implement __getitem__ and __len__ in an explicit subclass, and one gets __iter__, __contains__, __reversed__, index, and count for free. Another example is Mapping, one needs to implement only __getitem__, __len__, and __iter__, and one gets __contains__, keys, items, values, get, __eq__, and __ne__ for free. If you think it makes sense to add a note that implicit subtyping is preferred (especially for user defined protocols), then this certainly could be done. -- Ivan
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