How to type hint a TypeVar that can be either bound to A or to list[A]

[randolf-scholz]

I don't quite understand why the example below fails, and how to fix it. Isn't list[<T>] supposed to be covariant?

from typing import *

class A: pass
class B(A): pass

Z = TypeVar("Z", bound=A | list[A])

class Foo(Generic[Z]):
    inst_or_list: Z

    def __init__(self, x: Z):
        self.inst_or_list = x

a_inst: Foo[A] = Foo(A())               # ✔
a_list: Foo[list[A]] = Foo([A(), A()])  # ✔
b_inst: Foo[B] = Foo(B())               # ✔
b_list: Foo[list[A]] = Foo([B(), B()])  # ✔

test: Foo[list[B]] = Foo([B(), B()])    # ✘ raises type-var error

[sobolevn]

No, List in Python is invariant: https://mypy.readthedocs.io/en/stable/common_issues.html#invariance-vs-covariance