Covariant subtyping of mutable protocol members is rejected

Mypy rejects this because this is potentially unsafe.Consider this example:

from typing_extensions import Protocol
 
class P(Protocol):
    x: float
 
def fun(arg: P) -> None:
    arg.x = 3.14
 
class C:
    x = 42
c = C()
fun(c)  # This is not safe
c.x << 5  # Since this will fail!

To work around this problem consider whether “mutating” is actually partof a protocol. If not, then one can use a @property inthe protocol definition:

from typing_extensions import Protocol
 
class P(Protocol):
    @property
    def x(self) -> float:
       pass
 
def fun(arg: P) -> None:
    ...
 
class C:
    x = 42
fun(C())  # OK