Re: [libgpiod][PATCH 07/22] bindings: python: fix Chip union-attr type errors

[Vincent Fazio <vfazio@xxxxxxxxx>]

On Tue, Oct 8, 2024 at 8:16 AM Bartosz Golaszewski <brgl@xxxxxxxx> wrote:
>
>
> Ok, so I don't really understand what is happening here. We're going
> to re-assign _chip in every function? What happens to cast() if _chip
> IS None?

In this scenario, self._chip cannot be None. The self._check_closed() guard
ensures this, however, type checkers (at least mypy) cannot infer that from the
current code pattern.

`cast` is informing the type checker that past this point, self._chip should
not be considered as None and it's safe to reference attributes off the object

This seemed like the cleanest alternative, though others are:

* use a local variable for the cast result. This may be less confusing but
  incurs more changed lines

    self._check_closed()
    chip = cast(_ext.Chip, self._chip)
    return chip.path

* use asserts. These aren't always enforced during runtime so we cannot replace
  _check_closed but it does inform the type checker that it can narrow the type.
  Using both is ok, just slightly redundant.

    self._check_closed()
    assert self._chip is not None
    return self._chip.path

* add a `if self._chip is None` check that duplicates _check_closed
(or replace it completely)

* other "creative" solutions like a wrapper that returns a Chip or
throws an exception if it's None.

    def _chip_or_exc(self) -> Chip:
        if self._chip is None:
            raise Exception
        return self._chip

    chip = self._chip_or_exc()
    return chip.path

>
> Bart