Add asyncio.windows_utils.Popen

[JelleZijlstra]

As requested in #7356, cc @AlexWaygood.

[AlexWaygood]

Thank you!!

[JelleZijlstra]

Suggested change

	stdin: PipeHandle | IO[AnyStr] | None
	stdout: PipeHandle | IO[AnyStr] | None
	stderr: PipeHandle | IO[AnyStr] | None
	stdin: PipeHandle | IO[AnyStr] | None # type: ignore[override]
	stdout: PipeHandle | IO[AnyStr] | None # type: ignore[override]
	stderr: PipeHandle | IO[AnyStr] | None # type: ignore[override]

[AlexWaygood]

Needs to be [assignment] rather than [override]

[JelleZijlstra]

Huh, thanks. That's a weird code to use there; I'm not even assigning anything.

[Akuli]

Are you sure this is correct? The attributes are set to file objects in super().__init__(), but later replaced in the else: below.

[JelleZijlstra]

I might be misreading the code; I don't have access to Windows myself. But I think it sets these attrs to a PipeHandle only if stdin == PIPE, so if you pass stdin=<some file object>, that file object ends up in self.stdin.

[AlexWaygood]

Looks like @Akuli is correct. If I apply this patch(!) to asyncio/windows_utils.py in my local clone of CPython, on my Windows machine:

diff --git a/Lib/asyncio/windows_utils.py b/Lib/asyncio/windows_utils.py
index ef277fac3e..24dda3263d 100644
--- a/Lib/asyncio/windows_utils.py
+++ b/Lib/asyncio/windows_utils.py
@@ -171,3 +171,4 @@ def __init__(self, args, stdin=None, stdout=None, stderr=None, **kwds):
                 os.close(stdout_wfd)
             if stderr == PIPE:
                 os.close(stderr_wfd)
+        print(f'{type(self.stdin)=}')

... then this is what I get in an interactive shell:

Python 3.11.0a5+ (main, Feb 11 2022, 10:54:07) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> from asyncio.windows_utils import Popen
>>> with open('foo.txt', 'w') as f:
...     p = Popen(sys.executable, stdin=f)
...
type(self.stdin)=<class 'NoneType'>

[Akuli]

I'm starting to think that maybe you aren't supposed to pass in file objects at all, only PipeHandles or None or PIPE.

For testing this on windows, maybe print each of self.stdin, self.stdout and self.stderr, and then run a subprocess with some high level function? As the file name suggests, this class seems to be mostly a platform-specific lower level implementation detail.

[JelleZijlstra]

@AlexWaygood Interesting, I'll have to look at the code to figure out how that's possible.

@Akuli Interestingly, Popen is not actually used or documented anywhere else in asyncio. It looks more like they kept it in by accident.

[AlexWaygood]

We could always just cop out and use Any...

[Akuli]

It is used, at least in Python 3.9 where windows_events.py has this code:

class _WindowsSubprocessTransport(base_subprocess.BaseSubprocessTransport):

    def _start(self, args, shell, stdin, stdout, stderr, bufsize, **kwargs):
        self._proc = windows_utils.Popen(
            args, shell=shell, stdin=stdin, stdout=stdout, stderr=stderr,
            bufsize=bufsize, **kwargs)

[AlexWaygood]

Yes, the use is still there in 3.11 https://github.com/python/cpython/blob/df9f7597559b6256924fcd3a1c3dc24cd5c5edaf/Lib/asyncio/windows_events.py#L890

[JelleZijlstra]

Right, sorry for missing that usage. I looked in subprocess.py and it seems like it only sets the stdin attribute if it's a PIPE, so the comment in asyncio was right after all.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[Akuli]

This matches the docstring, which is the best we can make things in asyncio as the source code is complicated.