Use typing.AbstractSet for the ABC (there was an unfortunate naming conflict -- we have dict and Mapping, list and Sequence, but set and Set...)
On Wed, Aug 26, 2020 at 2:16 PM Alex Hall <alex.moj...@gmail.com> wrote: > On Wed, Aug 26, 2020 at 10:10 PM Christopher Barker <python...@gmail.com> > wrote: > >> Honestly, I haven't delved into type checking much, so maybe I'm making >> this up, but what if you write a function that is type hinted to take a >> Mapping, and uses | or |=, and all the test code uses a built in dict, and >> all seems to be well. >> > > This made me curious so I did an experiment, but with Set because I > couldn't be bothered to worry about Python versions. This code: > > ``` > > import typing > import collections > import builtins > > def foo(t: typing.Set, c: collections.Set, b: builtins.set): > print(t + t) > print(t | t) > print(t.union(t)) > > print(c + c) > print(c | c) > print(c.union(c)) > > print(b + b) > print(b | b) > print(b.union(b)) > > ``` > > complains about `+` (as it should) and `c.union` in both PyCharm and mypy, > and nothing else. So typing.Set is based on the builtin and not the ABC. If > you want to support any subclass of collections.Set, your type hint needs > to be collections.Set. Using typing.Set will fail to show warnings when you > use a builtin only method, and will show warnings when you try to assign an > ABC set (e.g. I get a warning with `foo(t=<instance of > collections.abc.Set>)`. The problem then is that `collections.abc.Set[int]` > doesn't work, i.e. you need to use typing.Set if you want generics. > -- --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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