[issue41619] Subprocesses created with DETACHED_PROCESS can pop up a console window

[Eryk Sun]

Eryk Sun  added the comment:

I suppose that, in addition to fixing the bug with si.dwFlags, code could be 
added to use DETACHED_PROCESS if GetConsoleCP() returns 0 (i.e. the launcher 
isn't attached to a console).

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[issue41619] Subprocesses created with DETACHED_PROCESS can pop up a console window

[Paul Moore]

Paul Moore  added the comment:

Confirmed, CREATE_NO_WINDOW works - in my test I used it *with* 
DETACHED_PROCESS.

Is it not a bug that the launcher doesn't respect the value of DETACHED_PROCESS 
when calling the Python interpreter, though? If nothing else, it means that 
there are differences between running Python via the launcher and not doing so 
(which are also evident as differences between a virtual environment and a 
non-virtual interpreter, as the venv machinery uses the launcher internally).

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[issue41619] Subprocesses created with DETACHED_PROCESS can pop up a console window

[Eryk Sun]

Eryk Sun  added the comment:

Creating the py.exe process with creationflags=DETACHED_PROCESS sets a special 
ConsoleHandle value in its ProcessParameters that makes the base API skip 
allocating a console session at process startup. However, the launcher itself 
spawns python.exe normally, without the DETACHED_PROCESS flag. So the base API 
in the python.exe process allocates a new console session that creates a window.

Using creationflags=CREATE_NO_WINDOW resolves the problem. This flag makes the 
base API in the py.exe process allocate a new console session that doesn't 
create a window. The child python.exe process inherits this windowless console 
session. Note that CREATE_NO_WINDOW is ignored if combined with 
CREATE_NEW_CONSOLE or DETACHED_PROCESS, since the combination makes no sense.

You can also use creationflags=CREATE_NEW_CONSOLE with 
startupinfo=STARTUPINFO(dwFlags=STARTF_USESHOWWINDOW), in which case py.exe 
allocates a console session with a hidden window. This option is useful if a 
console application should be able to show the console window later on via 
ShowWindow(GetConsoleWindow(), SW_SHOW). With CREATE_NO_WINDOW, in contrast, 
there is no window to show.

It should work if creationflags=DETACHED_PROCESS is combined with 
startupinfo=STARTUPINFO(dwFlags=STARTF_USESHOWWINDOW) because the launcher is 
supposed to clone its startup information to the child python.exe process, 
including dwFlags. But there's a bug in run_child in PC/launcher.c. It sets 
si.dwFlags to STARTF_USESTDHANDLES instead of using a bitwise OR to set the 
flag value.

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nosy: +eryksun

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[issue41619] Subprocesses created with DETACHED_PROCESS can pop up a console window

[Bernat Gabor]

Change by Bernat Gabor :


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nosy: +Bernat Gabor

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[issue41619] Subprocesses created with DETACHED_PROCESS can pop up a console window

[Paul Moore]

Paul Moore  added the comment:

Originally discovered in https://github.com/pypa/virtualenv/issues/1928 - see 
that issue for more context.

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[issue41619] Subprocesses created with DETACHED_PROCESS can pop up a console window

[Paul Moore]

New submission from Paul Moore :

The following code pops up an extra console window when run on Windows 10, 
Python 3.8:

from subprocess import DETACHED_PROCESS, Popen
p = Popen(["py", "-c", "from time import sleep; sleep(5)"], 
creationflags=DETACHED_PROCESS)

Adding CREATE_NO_WINDOW doesn't help, nor does adding a STARTUPINFO with 
SW_HIDE.

The problem occurs whether the Python interpreter is started via the py.exe 
wrapper or the actual python.exe. However, changing the process being run from 
"py" to sys.executable makes the problem go away. I've seen similar issues when 
using sys.executable, but the script is being called from a virtualenv.

I can't find any set of options for Popen that reliably avoids showing a 
console window when invoked like this.

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components: Windows
messages: 375811
nosy: paul.moore, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: Subprocesses created with DETACHED_PROCESS can pop up a console window
type: behavior
versions: Python 3.8

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