[Python-ideas] Structural type checking for PEP 484

[Matthias Kramm]

I like this proposal; given Python's flat nominal type hierarchy, it will 
be useful to have a parallel subtyping mechanism to give things finer 
granularity without having to resort to ABCs.

Are the return types of methods invariant or variant under this proposal?

I.e. if I have

  class A(Protocol):
    def f() -> int: ...

does

  class B:
    def f() -> bool:
      return True

implicitly implement the protocol?

Also, marking Protocols using subclassing seems confusing and error-prone.
In your examples above, one would think that you could define a new 
protocol using

class SizedAndClosable(Sized):
    pass

instead of

class SizedAndClosable(Sized, Protocol):
    pass

because Sized is already a protocol.

Maybe the below would be a more intuitive syntax:

  @protocol
  class SizedAndClosable(Sized):
      pass

Furthermore, I strongly agree with #7. Typed, but optional, attributes are 
a bad idea.

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