How to refer to the class-type of self, without bound TypeVars?

[randolf-scholz]

Code sample in pyright playground

from collections.abc import Sequence

class Expr:
    def simplify(self) -> "Expr": ...

class ExprSequence[E: Expr](Sequence[E], Expr):
    def __init__(self, exprs: list[E]) -> None: ...

    def simplify(self) -> Expr:
        return self.__class__([e.simplify() for e in self]) # ❌
        # "list[Expr]" cannot be assigned to list[E]

Inside a method like def simplify(self), how can I refer to the runtime type of type(self), i.e. the class of self without E being already bound? Both mypy and pyright infer both type(self) and self.__class__ essentially as type[typing.Self], which leads to the typing error in the example above.

I am not looking for ExprSequence([e.simplify() for e in self]), because this has different runtime behavior.

[Daverball]

You can probably annotate self with either ExprSequence[Expr] or ExprSequence[Any] if ExprSequence is invariant in E.

You are giving up some type safety, especially in the latter case, but that already goes out of the window to some degree due to the implicit reliance on ExprSequence.__init__ never violating LSP in a subclass. You could instead rely on an alternate constructor classmethod, which is checked for LSP violations, to improve safety.

You could even try to use a method-scoped TypeVar for this alternate constructor, so it can be bound independently of the class scoped E. Although that would make the constructor less convenient to use, since you can't use Self as the return type in that case.