Using a user-defined object as a specified parameter type in Python functions

After defining my `Player` class in Python3, I found myself struggling to create a method in another class that specifically takes an object of type `Player` as a parameter. Here is what my code looks like:

def change_player_activity(self, player: Player):
    pass

However, this implementation results in an error. While I am aware of the following workaround:

if not isinstance(player, Player):
        raise TypeError

I am curious if there exists a built-in method in Python3 that would allow me to restrict parameter input to objects of a specific class?

pythonfunctionclassoopparameter-passing

Answer №1

Can Python3 be instructed to only accept objects of a specific class?

Unfortunately, no.

In Python, there is no built-in mechanism for restricting function parameters to a certain class since Python follows the concept of Duck Typing. The idea is that if an object behaves like a particular class (e.g., a Player), then it should be treated as such. Otherwise, a TypeError would be raised.

It's important to avoid raising a TypeError unless the object is unable to perform actions expected of a Player. What if someone wants to pass their own custom Player class instance to your function? It won't be valid in that case.

Regarding your actual issue: If you need to type-hint a class before defining it, you can use a forward reference approach by placing the class name within quotes:

def change_player_activity(self, player: "Player"):
    pass

In future releases (__future__), Python may stop immediately looking up type hints to address these kind of challenges. To learn more about this proposal, see PEP 0563.

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