Description

geofft

opened on Oct 17, 2025

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Bug report

Bug description:

We noticed that python-build-standalone's build-details.json contains "platform": "" on non-cross builds.

I think I know why. For consistency (I think), python-build-standalone always uses ./configure --with-build-python on both cross builds and normal builds.

This causes configure.ac to do

    PYTHON_FOR_BUILD='_PYTHON_PROJECT_BASE=$(abs_builddir) _PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) PYTHONPATH=$(srcdir)/Lib _PYTHON_SYSCONFIGDATA_NAME=_sysconfigdata_$(ABIFLAGS)_$(MACHDEP)_$(MULTIARCH) _PYTHON_SYSCONFIGDATA_PATH=$(shell test -f pybuilddir.txt && echo $(abs_builddir)/`cat pybuilddir.txt`) '$with_build_python

(line 167 in 3.15.0a1), as opposed to PYTHON_FOR_BUILD='./$(BUILDPYTHON) -E' in the without-build-python case.

However, $_PYTHON_HOST_PLATFORM is only actually set to something in the cross-compile case (line 820 in 3.15.0a1), meaning that $PYTHON_FOR_BUILD is effectively doing _PYTHON_HOST_PLATFORM= python3.

The fact that this variable is an empty string, in turn, causes sysconfig.get_platform() to return an empty string:

$ uvx python3.15
Python 3.15.0a1 (main, Oct 14 2025, 21:28:08) [Clang 20.1.4 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sysconfig
>>> sysconfig.get_platform()
'linux-x86_64'
>>> import os
>>> os.environ['_PYTHON_HOST_PLATFORM'] = ''
>>> sysconfig.get_platform()
''

which makes generate-build-details.py output an empty string.

Arguably this is two bugs: configure.ac should not set _PYTHON_HOST_PLATFORM to an empty string, and sysconfig should not misbehave if _PYTHON_HOST_PLATFORM is set to an empty string (and should treat that as equivalent to being unset). Or maybe only one of these is a bug and the other is intended behavior. Also, of course, generate-build-details.py could work around this itself. I'm happy to provide a PR for any or all of these given some feedback on which direction to take this.

This applies to 3.14+ (all versions with build-details.json). If it's helpful, here are our full build logs for 3.14 and 3.15 in GHA; expand the "Build" section and search for generate-build-details. I've patched generate-build-details.py with print(f"generate-build-details debug: {sysconfig=} {sysconfig.get_platform()=}", file=sys.stderr).

(FYI I am probably about to file another bug about build-details.json generation in a moment, which might have a related cause and influence what the fix should be. EDIT: was already reported, #136267, left some analysis there)

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-140266: Fix empty platform in build-details.json when using --with-build-python #142375
• gh-140266: Only pass _PYTHON_HOST_PLATFORM when cross compiling #161530

Reacted by Itamar Oren

Reacted by msmojtabafar

Activity

1. geofft

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Oct 17, 2025
2. sergey-miryanov

   added

   buildThe build process and cross-buildThe build process and cross-build

   on Oct 17, 2025
3. Krishna-web-hub commented on Dec 6, 2025

   

   Krishna-web-hub

   on Dec 6, 2025

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   Hi @geofft ,

   I have been investigating this issue and already reproduced the problem locally. I also verified that the root cause is _PYTHON_HOST_PLATFORM being set to an empty string when using --with-build-python during a normal (non-cross) build.

   I have a working fix where an empty _PYTHON_HOST_PLATFORM is treated the same as unset, and the platform falls back to the correct value (e.g. linux-x86_64).

   I want to prepare a PR with tests to confirm the expected behavior.
   Let me know if there are any specific preferences for where the check should be applied (configure, sysconfig, or generate-build-details), and I can adjust accordingly.

   Thanks!
4. picnixz commented on Dec 6, 2025

   

   picnixz

   on Dec 6, 2025

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   If this is a duplicate of #136267, please close this one as such and forward your comment out there. TiA.
5. Krishna-web-hub commented on Dec 6, 2025

   

   Krishna-web-hub

   on Dec 6, 2025

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   Thanks @picnixz for the feedback!

   I agree that #136267 describes the broader/root issue with build-details.json mixing data from the build Python and the target Python during cross-builds.

   However, I think #140266 exposes a more specific failure mode:

   When using --with-build-python on a non-cross build, _PYTHON_HOST_PLATFORM is set to an empty string.

   sysconfig.get_platform() interprets the empty variable literally and returns "".

   This produces "platform": "" in build-details.json.

   My patch treats an empty _PYTHON_HOST_PLATFORM the same as unset, so sysconfig.get_platform() falls back to the correct detected value (e.g. linux-x86_64).
   This fixes the incorrect "platform": "" behavior and also aligns with normal expectations.

   I’m happy for the PR to reference #136267, and if maintainers prefer to track everything under that issue, I’m okay with closing #140266 as a duplicate.

   Just let me know which workflow you prefer
6. picnixz commented on Dec 6, 2025

   

   picnixz

   on Dec 6, 2025

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   I actually don't know if this is the correct patch (cc @AA-Turner).
7. Krishna-web-hub

   added 2 commits that reference this issue on Dec 6, 2025

   Fix build-details platform handling and adjust traceback test(fixes p…

   This issue will close once commit 7e29c1c is merged into the 'main' branch.

   7e29c1c

   Fix build-details platform handling and adjust traceback test(fixes p…

   This issue will close once commit b92000e is merged into the 'main' branch.

   b92000e
8. Krishna-web-hub commented on Dec 7, 2025

   

   Krishna-web-hub

   on Dec 7, 2025

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   Thanks! I’ve opened the PR at #142375. Looking forward to feedback from @AA-Turner.
9. picnixz commented on Dec 7, 2025

   

   picnixz

   on Dec 7, 2025

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   Please do not rush. I have no idea whether this is the correct fix and so I would prefer that we first hear it from Adam instead.
10. Krishna-web-hub

    added a commit that references this issue on Mar 5, 2026

    pythongh-140266: Fix data race when accessing func_code in PyFunction…

    d7c99b0
11. manueljacob commented on Apr 7, 2026

    

    manueljacob

    on Apr 7, 2026

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    I agree with the analysis. _PYTHON_HOST_PLATFORM is set in configure only if cross-compiling. If not cross-compiling, _PYTHON_HOST_PLATFORM will therefore be substituted in the Makefile with the empty string. However, if --with-build-python is passed, PYTHON_FOR_BUILD will include passing _PYTHON_HOST_PLATFORM no matter whether cross-compiling or not.

    _PYTHON_HOST_PLATFORM is used in other places to determine cross-compilation (e.g.

    cpython/Tools/build/check_extension_modules.py

    Line 112 in 8a73478

    default="_PYTHON_HOST_PLATFORM" in os.environ,

    ). I think that that is too implicit, as the name of the variable does not tell that it’s about whether cross-compiling or not, but that’s a separate issue. Given the current usage of _PYTHON_HOST_PLATFORM, I think the right fix would be to pass _PYTHON_HOST_PLATFORM based on whether cross-compiling or not instead of whether --with-build-python is passed or not.

    I don’t see any reason to ever set environment variable _PYTHON_HOST_PLATFORM to the empty string. Therefore, I do not think that that should be special-cased anywhere (that was done e.g. in the rejected PR).
12. itamaro

    added a commit that references this issue on Nov 7, 2026

    pythongh-140266: Only pass _PYTHON_HOST_PLATFORM when cross compiling

    c7bf53b

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