Disallow instantiating a Protocol class #12261

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1. adriangb

   added

   bugmypy got something wrongmypy got something wrong

   on Feb 28, 2022
2. A5rocks commented on Mar 1, 2022

   

   A5rocks

   on Mar 1, 2022

   Last edited by A5rocks

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   My main question here would be whether this fails too:

   x = Foo
   x()

   Because if so, it disallows perfectly valid code:

   x: Foo = ConcreteFoo
   x()

   and if it doesn't, it's a really special case that probably doesn't help much (in my opinion. Perhaps you've run into this and think it would be worth it, which is perfectly valid). Note that mypy has historically gone the all-the-way route with abstract base classes and this prompted #5374 (#5374 (comment) for reference).
3. adriangb commented on Mar 1, 2022

   

   adriangb

   on Mar 1, 2022

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   Interesting, that is a good point.

   For your first example, Pyright flags it with the same error.
   For your second example, Pyright allows it without errors.

   But I don't think your second example is valid. It should be:

   x: Type[Foo] = Foo

   Which Pyright disallows with:

   Expression of type "Type[Foo]" cannot be assigned to declared type "Type[Foo]"
   "Foo" is not a concrete class type and cannot be assigned to type

   For the example presented in #5374 (comment):

   from typing import Protocol, Type
   
   class Foo(Protocol):
       ...
   
   class ConcreteFoo(Foo):
       ...
   
   
   def fun(x: Type[Foo]) -> Foo:
       return x()
   
   
   fun(Foo)  # error: Argument of type "Type[Foo]" cannot be assigned to parameter "x" of type "Type[Foo]" in function "fun" "Foo" is not a concrete class type and cannot be assigned to type
   fun(ConcreteFoo)  # fine

   So basically Pyright seems to prohibit using a protocol class as a type (even as an argument to a function). This makes sense to me: I often want to pass in a concrete class as a parameter, but I don't I ever have had a use case for passing in an abstract class as a parameter.

   As additional context, the reason I brought this up is https://bugs.python.org/issue44807

   from typing import Protocol
   
   class Foo(Protocol):
       x: int
       def __init__(self, x: int) -> None:
           self.x = x
   
   
   class ConcreteFoo(Foo):
       x: int
   
   ConcreteFoo(x=1)  # ConcreteFoo.__init__() takes exactly one argument (the instance to initialize)

   Currently this fails at runtime with quite confusing behavior/errors because of the monkey patching that Protocol does to __init__.
   My hope was that this check could be moved from runtime monkey patching to mypy.
4. Rahix commented on Mar 17, 2022

   

   Rahix

   on Mar 17, 2022

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   but I don't I ever have had a use case for passing in an abstract class as a parameter.

   There are usecases, see #4717.
5. JelleZijlstra

   added

   topic-protocols

   on Mar 19, 2022
6. hauntsaninja commented on May 1, 2023

   

   hauntsaninja

   on May 1, 2023

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   mypy complains about the original example. Looks like this got changed a while back, in 0.950
7. hauntsaninja

   closed this as completedon May 1, 2023