argparse: Check if stderr is defined before writing to it

[simaoafonso-pwt]

Bug report

argparse tries to write to stderr inconditionally, even when it's None. This is only a problem on Windows. Here:

https://github.com/python/cpython/blob/3.11/Lib/argparse.py#L2596-L2600

I detected this when using PyInstaller to create a binary on Windows using --noconsole, which triggers using pythonw.exe. In this case, sys.stderr is None.

This is similar to #89057

Your environment

• Any version of Python on Windows, up to 3.11

Linked PRs

• gh-101640: Make argparse working in pythonw by dropping stderr writes there #101802
• [3.11] gh-101640: Make argparse _print_message catch any write error (GH-101802) #104250

[terryjreedy]

@simaoafonso-pwt The title of the 3.11 backport says specifically what was done. Without a stacktrace, we cannot be sure that this was the site of the error. And even if it was, whether this is sufficient to fix this issue for you. If you can, please make a new binary (after the 3.11 merge) and see if it now works. And report here so we can either close or look further (given a traceback and message).

[simaoafonso-pwt]

Sorry, is there some kind of nightly build I could use to test this?
From what I can tell, none of the alpha versions include the changes, either 3.11 or the main branch.

Compiling Python on Windows is not something I'm very comfortable with.

[arhadthedev]

Sorry, is there some kind of nightly build I could use to test this?

You can follow https://devguide.python.org/getting-started/ to get the latest sources and build them.

The prebuilt executable will be availible as CPython 3.12b1 on May 22.

[simaoafonso-pwt]

The prebuilt executable will be availible as CPython 3.12b1 on May 22.

I tried to check this, but it seems PyInstaller is not yet 3.12-ready, it might take a longer time: pyinstaller/pyinstaller#7601

[terryjreedy]

I am closing for now. This can be reopened or a new issue opened if later found necessary.

[terryjreedy]

Compiling on Windows is pretty easy now if one has git installed. Clone python/cpython. Install VS 2022 with python extension exactly as detailed in devguide. Give command line, I believe PCbuild\build.bat -e.

[simaoafonso-pwt]

This is already closed, but only now I got time to check this, now that PyInstaller published wheels for 3.12, and can confirm this is fixed in 3.12b1. Thanks.

[simaoafonso-pwt]

FYI, in case you need a workaround for this issue on lower Python versions, you can use this:

# Workaround for https://github.com/python/cpython/issues/101640
if sys.version_info >= (3, 12):
    ArgumentParser = argparse.ArgumentParser
else:
    class ArgumentParser(argparse.ArgumentParser):
        def _print_message(self, message, file=None):
            if file is None:
                pass  # Don't print if the file is unavailable
            else:
                super()._print_message(message, file=file)

Then use ArgumentParser instead.