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.