Clarify documentation for redirected stdout/stderr when using subprocess in Linux

[tanty]

BPO	44158
Nosy	@tanty, @slateny
Files	test.py: Example test

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-05-17.19:46:00.282>
labels = ['easy', 'type-bug', '3.9', '3.10', '3.11', 'docs']
title = 'Clarify documentation for redirected stdout/stderr when using subprocess in Linux'
updated_at = <Date 2022-04-05.09:31:13.894>
user = 'https://github.com/tanty'

bugs.python.org fields:

activity = <Date 2022-04-05.09:31:13.894>
actor = 'slateny'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2021-05-17.19:46:00.282>
creator = 'tanty'
dependencies = []
files = ['50048']
hgrepos = []
issue_num = 44158
keywords = ['easy']
message_count = 2.0
messages = ['393825', '416764']
nosy_count = 3.0
nosy_names = ['docs@python', 'tanty', 'slateny']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44158'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[tanty]

1. Redirect the stderr and stdout with contextlib's redirect_stderr, redirect_stdout or just by replacing the fd in sys.stderr and sys.stdout
2. We run a subprocess with subprocess.run()
3. The documentation for subprocess states:

https://docs.python.org/3/library/subprocess.html#frequently-used-arguments

"
With the default settings of None, no redirection will occur; the child’s file handles will be inherited from the parent.
"

4. The documentation for contextlib states:

https://docs.python.org/3/library/contextlib.html?highlight=contextlib#contextlib.redirect_stdout

"
Note that the global side effect on sys.stdout means that this context manager is not suitable for use in library code and most threaded applications. It also has no effect on the output of subprocesses.
"
5. The stdout and stderr for the subprocess-ed command is not redirected, as stated in the contextlib documentation.

It took me a while to realize that redirecting stdout and stderr didn't have an effect in the child subprocess.

It would be great if this could be improved so the subproccess-ed command also inherits the redirection. At the very least, this issue with the redirection should be added as a notice in the subprocess documentation.

The attached test.py script shows this behavior. The code is very simple and self-explanatory. You can run with:

# test.py

or

# test.py -n

Somehow, related to: https://bugs.python.org/issue1227748

[slateny]

Hmm, I'm not quite following - when you say that the subproccess-ed command should also inherit the redirection, which subprocess command are you referring to, and nested or unnested? I gave it a ran and everything seems to run how I'd expected it, so maybe put another way, what's the expected output from the files and stdout/stderr?