Invalid Protocol

What it does

Checks for protocol classes that are invalid at runtime or do not satisfy the typing specification.

Why is this bad?

An invalidly defined protocol class may lead to the type checker inferring unexpected things or accepting unsafe operations. Some invalid protocol definitions also raise TypeError at runtime.

Examples

A Protocol class cannot inherit from a non-Protocol class; this raises a TypeError at runtime:

>>> from typing import Protocol
>>> class Foo(int, Protocol): ...
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    class Foo(int, Protocol): ...
TypeError: Protocols can only inherit from other protocols, got <class 'int'>

A generic protocol's declared type-variable variance must match how that variable is used by its protocol members. For example, a type variable that appears only in a method's return type must be covariant:

from typing import Protocol, TypeVar

T = TypeVar("T")


class Source(Protocol[T]):  # error: [invalid-protocol]
    def read(self) -> T: ...

Although Python constructs this protocol successfully at runtime, it is invalid for static typing. Declare the type variable with TypeVar("T", covariant=True) instead.