Trouble with type variance and factory functions

[rdong8]

from abc import ABC
from dataclasses import dataclass


@dataclass(frozen=True, kw_only=True, slots=True)
class Balance(ABC):
    value: int


@dataclass(frozen=True, kw_only=True, slots=True)
class USD(Balance):
    pass


@dataclass(frozen=True, kw_only=True, slots=True)
class EUR(Balance):
    pass


@dataclass(frozen=True, kw_only=True, slots=True)
class Account[BalanceType: Balance](ABC):
    balance: BalanceType


@dataclass(frozen=True, kw_only=True, slots=True)
class USDAccount(Account[USD]):
    pass


@dataclass(frozen=True, kw_only=True, slots=True)
class EURAccount(Account[EUR]):
    pass


def create[BalanceType: Balance](balance: BalanceType) -> Account[BalanceType]:
    match balance:
        case USD():
            return USDAccount(balance=balance)
        case EUR():
            return EURAccount(balance=balance)
        case _:
            assert False

Pyright gives me an error:

Type "USDAccount" is not assignable to return type "Account[BalanceType@create]"
  "USDAccount" is not assignable to "Account[BalanceType@create]"
    Type parameter "BalanceType@Account" is invariant, but "USD" is not the same as "BalanceType@create"Pylance[reportReturnType](https://github.com/microsoft/pyright/blob/main/docs/configuration.md#reportReturnType)

If I understand it correctly, by making everything frozen, it should automatically be covariant in BalanceType, so this shouldn't be an error right?

[erictraut]

The problem is not with variance. The issue is that when you call USDAccount(balance=balance), the resulting type is USDAccount. The information about the type variable has been lost, and this type is no longer assignable to Account[BalanceType]. Not surprisingly, mypy generates the same error here.

You can fix this by making USDAccount and EURAccount generic.

@dataclass(frozen=True, kw_only=True, slots=True)
class USDAccount[T: USD](Account[T]):
    test: int = 4


@dataclass(frozen=True, kw_only=True, slots=True)
class EURAccount[T: EUR](Account[T]):
    pass

[rdong8]

This works, however I'm now getting an exception (but pyright passes) when I have a classmethod like this:

from __future__ import annotations
from abc import ABC
from dataclasses import dataclass


@dataclass(frozen=True, kw_only=True, slots=True)
class Balance(ABC):
    value: int


@dataclass(frozen=True, kw_only=True, slots=True)
class USD(Balance):
    pass


@dataclass(frozen=True, kw_only=True, slots=True)
class Account[BalanceType: Balance](ABC):
    balance: BalanceType


@dataclass(frozen=True, kw_only=True, slots=True)
class USDAccount[T: USD](Account[T]):
    @classmethod
    def from_usd(cls, usd: T) -> USDAccount[T]:
        return cls(balance=usd)

USDAccount.from_usd(USD(value=3))

TypeError: <class '__main__.USDAccount'> is not a generic class

I also tried giving from_usd its own generic parameter U: USD:

@dataclass(frozen=True, kw_only=True, slots=True)
class USDAccount[T: USD](Account[T]):
    @classmethod
    def from_usd[U: USD](cls, usd: U) -> USDAccount[U]:
        return cls(balance=usd)

USDAccount.from_usd(USD(value=3))

This runs but there is a pyright error:

Type "Self@USDAccount[T@USDAccount]" is not assignable to return type "USDAccount[U@from_usd]"
  "USDAccount[T@USDAccount]*" is not assignable to "USDAccount[U@from_usd]"
    Type parameter "T@USDAccount" is invariant, but "T@USDAccount" is not the same as "U@from_usd"Pylance[reportReturnType](https://github.com/microsoft/pyright/blob/main/docs/configuration.md#reportReturnType)

[erictraut]

Hmm, I don't see a runtime exception when I run this code in Python 3.12. It doesn't work in older versions of Python because they don't support the new generics syntax.

[rdong8]

Ah you're right, must've been something weird with my jupyter notebook. Thanks!