Unsound typing using isinstance

[zacharie-moughanim]

isinstance make mypy assume a too strong hypothesis on the type of the value, for example:

def foo(t : list[int | str]) -> int :
  assert(isinstance(t[0], int))
  t[0] = "coucou"
  return t[0]

x : list[int | str] = [1, 2]
foo(x)

typechecks correctly while the type annotation is not correct.

[JelleZijlstra]

Another way to describe the issues is that the indexed assignment doesn't narrow the inferred type for t[0].

Mypy also errors for this:

def foo(t : list[int | str]) -> str:
  t[0] = "coucou"
  return t[0]

x: list[int | str] = [1, 2]
foo(x)

main.py:3: error: Incompatible return value type (got "int | str", expected "str") [return-value]. When of course, t[0] is known to be a string here.

[ilevkivskyi]

Yes, this is pretty bad inconsistency, we should either narrow an expression, or not. If we narrow it with isinstance() it should be also narrowed by an assignment, or at least invalidate the previous narrowing by putting "declared type" in binder.

[Shubhwtf]

Maybe one way to avoid the inconsistency is to bind the indexed value to a local variable before narrowing, like

def foo(t: list[int | str]) -> int:
    x = t[0]
    assert isinstance(x, int)
    t[0] = "coucou"
    return x

Here the narrowing applies to x, which is immutable with respect to the container mutation, so the reasoning remains sound