On Mon, Mar 22, 2021 at 5:37 PM Ben Rudiak-Gould <benrud...@gmail.com> wrote: > > On Sun, Mar 21, 2021 at 11:10 PM Chris Angelico <ros...@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 _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/C3CUQYW3TQGJHC7SP5B4QJXFDV2XTEXB/ Code of Conduct: http://python.org/psf/codeofconduct/
