Example in PEP 544 seems to be wrong

[MaxG87]

I am on Python 3.8 and use MyPy 0.740

I consider the following example from PEP 544:

from typing import Any, Protocol

class ProtoA(Protocol):
    def meth(self, x: int) -> int: ...
class ProtoB(Protocol):
    def meth(self, obj: Any, x: int) -> int: ...

class C:
    def meth(self, x: int) -> int: ...

a: ProtoA = C  # Type check error, signatures don't match!
b: ProtoB = C  # OK

In the final two lines a class is assigned to a instance. MyPy thus reports

/tmp/protocol-test.py:19: error: Incompatible types in assignment (expression has type "Type[C]", variable has type "ProtoA")
/tmp/protocol-test.py:20: error: Incompatible types in assignment (expression has type "Type[C]", variable has type "ProtoB")
Found 2 errors in 1 file (checked 1 source file)

If I fix that (i.e. adding the missing '()'), than it seems as if the assignment to b should cause a type error, not the assignment to a. Mypy`s report is consistent to my presumption.

I hope it is ok to ask this, since I am here anyways: What is the difference between ... and pass? RTFM with link would be fine.

[ilevkivskyi]

This is just a mypy bug (or rather a missing feature), it doesn't support class objects and modules as implementations of protocols, see python/mypy#4536

[MaxG87]

I am sorry to insist, but it still holds that the signatures for ProtoA and C match while these for ProtoB and C are in conflict. Thus, at least the comments must be swapped.

[ilevkivskyi]

You can just try examples in REPL:

a: ProtoA = C  # You say this should be accepted
a.meth(42)  # Oops TypeError here

You can also read some tutorials about how bound and unbound methods work in Python.