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

[Bartosz Golaszewski <brgl@xxxxxxxx>]

On Fri, Sep 27, 2024 at 8:57 PM Vincent Fazio <vfazio@xxxxxxxxxxx> wrote:
>
> Since `Chip._chip` can be `None`, it's necessary to inform type checkers
> of the state of the object to silence the union-attr errors.
>
> Type checkers may not be able to infer that an object is not `None` from
> an earlier call (such as `_check_closed`).
>
> Instead of littering the code with "# type: ignore" comments, use casts
> to inform type checkers that objects are not `None`.
>
> Using `assert` is another option, however this duplicates the logic in
> `_check_closed` so is redundant at best and, at worst, is not a safe
> replacement as `assert` can be elided in optimized Python environments
> and these checks need to be runtime enforced.
>
> Signed-off-by: Vincent Fazio <vfazio@xxxxxxxxxxx>
> ---
>  bindings/python/gpiod/chip.py | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/bindings/python/gpiod/chip.py b/bindings/python/gpiod/chip.py
> index 4aa5677f94caf8c5d863aa6d75915a5b650de137..fe7bcfe082d6e9f6220093d3fc45ff232b5d0d17 100644
> --- a/bindings/python/gpiod/chip.py
> +++ b/bindings/python/gpiod/chip.py
> @@ -7,7 +7,7 @@ from collections import Counter
>  from datetime import timedelta
>  from errno import ENOENT
>  from types import TracebackType
> -from typing import Optional, Union
> +from typing import Optional, Union, cast
>
>  from . import _ext
>  from ._internal import poll_fd
> @@ -97,6 +97,7 @@ class Chip:
>          longer be used after this method is called.
>          """
>          self._check_closed()
> +        self._chip = cast(_ext.Chip, self._chip)
>          self._chip.close()
>          self._chip = None
>

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?

Bart