Self does not work with delegation and an explicit return type

[voidspace]

Bug Report

typing.Self should be used where a class method returns an instance of the class. If the class method delegates to a function that has an explicit return type then mypy reports an error for the use of self.

To Reproduce

This Python code in "test.py":

from typing import Self


class Thing:

    @classmethod
    def constructor(cls) -> Self:
        return function()
        
def function() -> Thing:
    return Thing()

Expected Behavior

No Errors

Actual Behavior

Produces this report from mypy:

$ mypy test.py
test.py:8: error: Incompatible return value type (got "Thing", expected "Self")  [return-value]

Your Environment

• Mypy version used:

$ mypy --version
mypy 1.13.0 (compiled: yes)

Python 3.12, Ubuntu 24.04

[brianschubert]

Mypy is correct to report an error here. Self isn't equivalent to the type of current class. Rather, it works like a type variable with an upper bound of the current class. Returning a Thing from a function with a return type of Self isn't allowed, since that would be invalid in cases where Self gets bound to a subtype of Thing. Consider:

class SubThing(Thing):
    def foo():
        pass

# Here, 'Self' is bound to 'SubThing', so 'x' has type 'SubThing'
x = SubThing.constructor()

# The type system says this should be ok...
x.foo()  # boom! AttributeError: 'Thing' object has no attribute 'foo'

See also Generic - Self from the typing spec for more details on how Self works.

[voidspace]

You're right, the code plays terribly with subclasses. (It's from an example for a course.) I've moved the code into the method and am using cls() instead of the hardcoded type in the function. Much better. I was surprised it didn't work though - thanks for the explanation.