Missing error when an attribute is a variant generic

[parched]

The following produces no errors but I expected it to be treated the same as if B.a was a get/set property (which causes a variance error)

class A:
    pass

class A2(A):
    something: str = "hello"
    
T_co = TypeVar('T_co', covariant=True)

class B(Generic[T_co]):
    def __init__(self, a: T_co) -> None:
        self.a: T_co = a  # EXPECTED: typing error here like "public attributes must have invariant type"
        
def set_a(b: B[A]) -> None:
    b.a = A()
    

b2: B[A2] = B(A2())

set_a(b2)

print(b2.a.something) # ACTUAL: runtime error here

Python 3.10, mypy 0.961 with --strict

[erictraut]

The problem here is that you have defined T_co as covariant, but given how you are using it in B, it should be declared as invariant.

[parched]

Yes, but shouldn't mypy tell me that? It does if I explicitly create a setter for a.

[parched]

Or is variance something that the user needs to check is correct and it's use should be flagged like Any as potentially "unsafe"?

[erictraut]

Mypy validates the variance for protocol classes but not for other standard (nominal) classes.