Type alias defined in parent seems to be lost in children?

[Guillaume Roger (guillaume-alliander)]

With this shortest example I could make up:

from typing import Literal, cast


type Cheese = Literal["camenbert"]


def melt_cheese(cheese: Cheese) -> str: ...


class Parent:
    def __init__(self):
        self.cheese = cast(Cheese, "camenbert") # SEE BELOW


class Child(Parent):
    def __init__(self):
        super().__init__()
        melt_cheese(self.cheese) # ERROR HERE

Running pyright tells me:

error: Argument of type "str" cannot be assigned to parameter "cheese" of type "Cheese" in function "melt_cheese"
  "str" is not assignable to type "Cheese" (reportArgumentType)

It is as if in Child, the fact that self.cheese is of type Cheese is lost. If I were to move the line with SEE BELOW in the init of Child, there would not be an issue, it only happens with inheritance.

The workaround is easy (add a cast), but by experience, I am probably not understanding something right.

Pyright 1.1.403, and python 3.13.0, via uv.

[Eric Traut (erictraut)]

The cast in this case does nothing because the type of the expression "camenbert" is already Literal["camenbert"]. Casting it to the same type is therefore a no-op.

You haven't provided a type declaration for the cheese instance variable, so its type is inferred based on the values assigned to it within the Parent class. The sole assignment is assigning a value of type Literal["camenbert"], but literals are widened to their non-literal counterpart when inferring types. After all, you would not generally expect the statement self.foo = 1 to imply that foo is limited to just 1; you'd expect that foo would accept any int.

For more details about inference, refer to this documentation.

If your intent is for cheese to be limited to a literal value, you should declare it as such:

class Parent:
    def __init__(self):
        self.cheese: Cheese = "camenbert"

[Guillaume Roger (guillaume-alliander)]

This makes sense now, thanks for the clear explanation.