What issues are you running into? I tried your example in the mypy playground (https://mypy-play.net/?mypy=latest&python=3.9&flags=verbose&gist=880f6fefd0d6a1b06af03a2c815ddfc3) and got no errors related to T_co.

El mar, 16 mar 2021 a las 20:16, Caleb Donovick (<donovick@cs.stanford.edu>) escribió:

I am trying to type check a persistent (immutable) data structure. However I am stumbling on restrictions on using covariant types as arguments. Is there a standard work around for this?

Simplified Example:

class Node(Generic[T_co]):
    def __init__(self, val: T_co, succ: Optional[Node[T_co]]) -> None:
        self.val: Final[T_co] = val
        self.succ: Final[Optional[Node[T_co]]] = succ

class List(Sequence[T_co]):
    def __init__(self, *, _head: Optional[Node[T_co]] = None) -> None:
        self.head: Final[Optional[Node[T_co]]] = _head

    # The problem method
    def push_front(self, val: T_co) -> List[T_co]:
        new_node = Node(val, self.head)
        return List(_head=new_node)

    def __iter__(self) -> Iterator[T_co]:
        ptr = self.head
        while ptr is not None:
            yield ptr.val
            ptr = ptr.succ
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