Type hinting for class defined in a function

Question

We are adding type hints to our code base and there's one specific case where I don't know how to do proper type hinting. We have a function that will create an instance of a specific class if it didn't yet exist. The class of which it creates an object is defined in the function itself to make it a Singleton. An example of such a function is given in the following piece of code:

__GREETER = None

def get_greeter():
   global __GREETER

   class Greeter:
       def greet_user(name: str):
           print(f'Hello {name}')

   if not __GREETER:
       __GREETER = Greeter()

   return __GREETER

I was now wondering how we can add a type hint for the return type of the get_greeter() function. Can I just use get_greeter() -> 'Greeter' or should I do something different?

Note that there are indeed cleaner ways to define singletons since python 3 but refactoring that is something on the backlog.

Answer 1

I would suggest a slight refactoring of your code, to make it a more generic approach. You can make a singleton pattern applicable, even if you expose your class. For eg. see below.

class Greeter:
    _instance = None

    def __new__(cls):
        if cls._instance is None:
            cls._instance = super(Greeter, cls).__new__(cls)
        return cls._instance

    def greet_user(name: str):
        print(f'Hello {name}')

You can re-write your Greeter class, which makes sure its a singleton. Now for backward compatibility, keep the method you have as it is.

def get_greeter() -> Greeter:
    return Greeter()

It creates only one instance ever. And your hinting is also achieved.