python raises error `Undefined class X` when you declaring parameter type or return type of method as the self of class:

class MyClass:
    def mymethod(self, a: MyClass): # parameter type is class self
        pass

# or

class MyClass:
    def mymethod(self) -> MyClass: # return type is class self
        pass
# or both of them

error `Undefined class ...` will be raised.

Add at the top of your script:

from __future__ import annotations

You can already write such code in 3.10. In earlier Python versions you can either write annotation explicitly as string or (since 3.7) use and import suggested by Irit.