Extending a Protocol leads to unsafe behavior #8005
Description
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Are you reporting a bug, or opening a feature request?
bug -
Please insert below the code you are checking with mypy, or a mock-up repro if the source is private. We would appreciate if you try to simplify your case to a minimal repro.
from typing import Protocol
class P(Protocol):
def foo(self) -> int: ...
class A(P): # mypy does not flag this
pass
class B:
pass
test: P = A() # mypy does not flag this
test = B() # mypy flags this
print(A().foo() + 5) # both error at runtime
print(B().foo() + 5)
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What is the actual behavior/output?
A().foo() + 5crashes at runtime withTypeError: unsupported operand type(s) for +: 'NoneType' and 'int'
Bis correctly type checked and rejected. -
What is the behavior/output you expect?
According to https://mypy.readthedocs.io/en/latest/protocols.html#defining-subprotocols-and-subclassing-protocols, "Explicitly including a protocol as a base class ... forces mypy to verify that your class implementation is actually compatible with the protocol."
I would expect extending a Protocol to be similar to implementing an interface in Java.
In this particular example, mypy should ideally detect thatAdoes not fill in the stub method fromP, like it does forB. Alternatively, if that is not possible, perhaps extending a protocol should not be allowed. -
What are the versions of mypy and Python you are using? Do you see the same issue after installing mypy from Git master?
Python 3.8.0
mypy 0.750+dev.d9dea5f3ad30bb449e4167b3e8476684ded0482e -
What are the mypy flags you are using? (For example --strict-optional)
mypy --strict test.py