On Mon, Mar 22, 2021 at 5:37 PM Ben Rudiak-Gould <benrudiak@gmail.com> wrote:
>
> On Sun, Mar 21, 2021 at 11:10 PM Chris Angelico <rosuav@gmail.com> wrote:
>>
>> At what point does the process_objects list cease to be referenced?
>> After the last visible use of it, or at the end of the function?
>
>
> In Python as it stands, at the end of the function, as you say.
>
> Skip Montanaro's PEP suggested that in his register machine, locals would be dereferenced after their last visible use. I don't think that's intrinsically a bad idea, but it's not backward compatible. The thing with the process objects was just an example of currently working code that would break.
>
> The example has nothing to do with PyQt5 really. I just happen to know that QProcess objects kill the controlled process when they're collected. I think it's a bad design, but that's the way it is.
>
> Another example would be something like
>
>     td =  tempfile.TemporaryDirectory()
>     p = subprocess.Popen([..., td.name, ...], ...)
>     p.wait()
>
> where the temporary directory will hang around until the process exits with current semantics, but not if td is deleted after the second line. Of course you should use a with statement in this kind of situation, but there's probably a lot of code that doesn't.
>

Thanks for the clarification. I think the tempfile example will be a
lot easier to explain this with, especially since it requires only the
stdlib and isn't implying that there's broken code in a third-party
library.

I don't like this. In a bracey language (eg C++), you can declare that
a variable should expire prior to the end of the function by including
it in a set of braces; in Python, you can't do that, and the normal
idiom is to reassign the variable or 'del' it. Changing the semantics
of when variables cease to be referenced could potentially break a LOT
of code. Maybe, if Python were a brand new language today, you could
define the semantics that way (and require "with" blocks for anything
that has user-visible impact, reserving __del__ for resource disposal
ONLY), but as it is, that's a very very sneaky change that will break
code in subtle and hard-to-debug ways.

(Not sure why this change needs to go alongside the register-based VM,
as it seems to my inexpert mind to be quite orthogonal to it; but
whatever, I guess there's a good reason.)

ChrisA
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