Unknown variable reported with .format but not with f-string?

[Holt59]

Describe the bug

Using .format() instead of a format string shows a different behavior about an unknown variable type that does not make sense (to me).

Code or Screenshots
The code below shows:

Argument type is partially unknown
  Argument corresponds to parameter "args" in function "format"
  Argument type is "str | Unknown" Pylance(reportUnknownArgumentType)

...with .format(), but not with f-string.

from __future__ import annotations

class C:
    name: str
    outer: C | None

    def f(self) -> str:
        name = self.name
        oc = self.outer
        while oc is not None:
            # name = f"{oc.name}.{name}"
            name = "{}.{}".format(oc.name, name)
            oc = oc.outer

        return name

VS Code extension or command-line

Both:

• VS Code extension, Pylance v2023.9.12.
• pyright version 1.1.327

[erictraut]

Thanks for the bug report. This is due to a bug that occurs when an overloaded function (such as format) is called within a loop and the narrowed type of an argument (name) depends on the return type of that call. It doesn't affect f-string type evaluations because there are no overloaded calls involved in that case.

This will be addressed in the next release.

[erictraut]

This is addressed in pyright 1.1.328, which I just published. It will also be included in a future release of pylance.