Error when starting Python GUI script

[cbrnr]

I get an error when starting a Python GUI script from git-bash (which I believe uses msys2):

This error does not occur when I start the same script from cmd or powershell.

Steps to Reproduce the Problem

1. Install Python 3.9.8 for Windows (https://www.python.org/ftp/python/3.9.8/python-3.9.8-amd64.exe)
2. Install PySide2 with pip install PySide2
3. Download and unzip the files required for a minimal example: https://github.com/cbrnr/stderr-duplication/archive/refs/heads/main.zip
4. In a terminal, change into that directory and type pip install .
5. Type mypackage to run the script and get the error message

[Biswa96]

@dscho may provide some info.

[dscho]

I have no idea where that is coming from.

[jeremyd2019]

pypa/pip#10444 but that got closed for no response

[jeremyd2019]

I do vaguely remember caveats with GetStdHandle in GUI apps. Seems odd that they would only have an issue with stderr though.

[cbrnr]

Thanks @jeremyd2019 for the find! That seems to be the same issue. It is definitely not related to some special install method because I have a package on PyPI that has the same behavior on Bash for Windows. I have also asked on SO but have not received any comments leading to a solution.

[jeremyd2019]

Some observations that may be interesting:

• all 3 handles (stdin, stdout, stderr) are 0 when GUI app is run from cmd.
• stdin and stdout are 0 but stderr is non-zero when GUI app is run from MSYS2 from Windows console, but DuplicateHandle fails on this non-zero handle with ERROR_NOT_SUPPORTED.
• all 3 handles are valid when GUI app is run from MSYS2 mintty.
• all 3 handles are 0 when GUI app is run via winpty (regardless of Windows console or mintty)
• enable_pcon option makes no difference.

[cbrnr]

I think this can be closed in favor of pypa/pip#10444; let's continue the discussion over there.