On Tue, Sep 1, 2020 at 8:20 AM Christopher Barker <python...@gmail.com> wrote:
> Thanks -- good to get this written down! > > Question: it strikes me that the use case of [] for type hints is > logically quite different than for indexing. So do they need to use the > same syntax / dunder? > > I think that type hints are used in specific places, and thus the > interpreter could know if a given [] was a type hint or an indexing > operation, and thus could dispatch it differently. > > Would that be desirable? I'm not sure -- but as type hints' use of [] is > logically quite different, it might make sense to have it use different > rules. > > There is precedent -- after all, () used to create a tuple follows > different rules than () used to call a function. > > If it's not technically possible to make the distinction, then it's a > non-issue, but if it is, it may be worth considering. > Type hints are indeed dispatched differently, but this is done based on information that is only available at runtime. Since PEP 560, for `x[y]`, if no `__getitem__` method is found, and `x` is a type (class) object, and `x` has a class method `__class_getitem__`, that method is called. Extending this with keyword args is straightforward. Modifying the compiler to generate different bytecode for this case is essentially impossible. See https://github.com/python/cpython/blob/6844b56176c41f0a0e25fcd4fef5463bcdbc7d7c/Objects/abstract.c#L181-L198 for the code (it's part of PyObject_GetItem). -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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