Shared via mypy Playground

main.py

from typing import Optional, Union, cast, Literal, NewType, TypeGuard
from collections import defaultdict
from dataclasses import dataclass

@dataclass
class Application:
    id: int
    parent_id: Optional[int] = None

ParentApplication = NewType('ParentApplication', Application)
ChildApplication = NewType('ChildApplication', Application)

def abort_if_application_is_child(application: 'Application') -> ParentApplication:
    if application.parent_id is not None:
        raise Exception('Application is Child')
    return cast(ParentApplication, application)
        

def function_only_valid_to_call_for_parents(parent_application: ParentApplication):
    ...
    
def function_only_valid_to_call_for_children(child_application: ChildApplication):
    ...
    
def existing_function_that_doesnt_care_parent_or_child(application: Application):
    ...
    
    
application = Application(id=123)
any_other_application = Application(id=456)
application = abort_if_application_is_child(application)

function_only_valid_to_call_for_parents(application) # no problem
function_only_valid_to_call_for_children(application) # problem
existing_function_that_doesnt_care_parent_or_child(application) # no problem

function_only_valid_to_call_for_children(any_other_application)