typing.Optional with not None default, should not type check None value until it mutate.

[sileht]

The example is self explicit.
It looks weird that we need to check for final_params is None when we just set it explicitly to something else.

import typing

def local_optional_default_not_working(
  params: typing.Optional[typing.Dict[str, str]] = None,
) -> None:
    final_params: typing.Optional[typing.Dict[str, str]] = {"per_page": "100"}
    if params is not None:
        final_params.update(params)


def local_optional_default_workaround1(
  params: typing.Optional[typing.Dict[str, str]] = None,
) -> None:
    final_params: typing.Optional[typing.Dict[str, str]]
    final_params = {"per_page": "100"}

    if params is not None:
        final_params.update(params)

def local_optional_default_workaround2(
  params: typing.Optional[typing.Dict[str, str]] = None,
) -> None:
    final_params: typing.Optional[typing.Dict[str, str]] = {"per_page": "100"}
    if final_params is None:
        raise RuntimeError("final_params can be None really ?")

    if params is not None:
        final_params.update(params)

Expected Behavior

No mypy error

Actual Behavior

$ mypy --strict optional_local_default.py
optional_local_default.py:8: error: Item "None" of "Optional[Dict[str, str]]" has no attribute "update"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files): strict
• Python version used: 3.9
• Operating system and version: arch

[hauntsaninja]

Duplicate of #2008 (also see the typing-sig thread linked at the end)