Unable to infer attributes of constrained generic type parameter outside of the class

[rdong8]

I was going to open an issue for this, but I realize that I might just be wrong about what to expect. It seems that Pyright is unable to infer the constraints from a generic type parameter when it is referred to outside of the class, unless you specify a concrete type for it.

from abc import ABC, abstractmethod


class Currency(ABC):
    @abstractmethod
    def to_int(self) -> int:
        return 0


class Account[BaseCurrency: Currency]:
    def __init__(self, balance: BaseCurrency):
        self.balance = balance

    def get_balance(self) -> int:
        return self.balance.to_int()  # Works as expected


def get_balance(account: Account) -> int:
    return account.balance.to_int()  # to_int is "Unknown" to pyright

This only works if you do get_balance[BaseCurrency: Currency](account: Account[BaseCurrency]) -> int. Is this the expected behavior?

[erictraut]

Pyright is working correctly here. The Account class is generic and accepts one type argument. In the function get_balance, you have annotated the account parameter with the type Account. Since you have omitted the type argument, type checkers will assume Any (or in the case of pyright, Unknown, which is an implicit Any). That means the instance variable balance has the type Unknown.

This behavior is all prescribed by the Python typing spec, and pyright is conforming to the spec.