Description

jmarshall

opened on Nov 11, 2026

Issue body actions

Bug report

On macOS, various adjustments are made to sysconfig.get_config_vars() values via Lib/_osx_support.py. In particular, if $ARCHFLAGS is set in the environment, any -arch XYZ options are removed from CFLAGS and similar entries and are replaced by the value of $ARCHFLAGS.

For example, with Python 3.9 (the macOS system Python, one of the few non-brew hence universal2 Pythons I have locally installed):

$ python3.9 -c 'import sysconfig; print(sysconfig.get_config_var("CFLAGS"))'
-Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -iwithsysroot/System/Library/Frameworks/System.framework/PrivateHeaders -iwithsysroot/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.9/Headers -arch arm64 -arch x86_64 -Werror=implicit-function-declaration -Wno-error=unreachable-code -Wno-error=unused-but-set-variable -Wno-error=cast-function-type-mismatch -Wno-unknown-warning-option

ARCHFLAGS='-arch foo' python3.9 -c 'import sysconfig; print(sysconfig.get_config_var("CFLAGS"))'
-Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -iwithsysroot/System/Library/Frameworks/System.framework/PrivateHeaders -iwithsysroot/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.9/Headers   -Werror=implicit-function-declaration -Wno-error=unreachable-code -Wno-error=unused-but-set-variable -Wno-error=cast-function-type-mismatch -Wno-unknown-warning-option -arch foo

Notice that -arch arm64 -arch x86_64 has been removed in the middle (leaving several adjacent spaces where they were removed) and -arch foo has been appended.

With Python 3.13 installed from python.org, we get:

$ python3.13 -c 'import sysconfig; print(sysconfig.get_config_var("CFLAGS"))'
-fno-strict-overflow -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -O3 -Wall -arch arm64 -arch x86_64

$ ARCHFLAGS='-arch foo' python3.13 -c 'import sysconfig; print(sysconfig.get_config_var("CFLAGS"))'
-fno-strict-overflow -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -O3 -Wall  -arch x86_64 -arch foo

Notice that -arch arm64 has been removed (leaving two adjacent spaces marking the location) but -arch x86_64 was not removed, and -arch foo has been appended.

Cibuildwheel sets ARCHFLAGS to build single-architecture wheels on a universal2 Python. Runs such as this pysam wheel build (see the macOS jobs in particular) exercise this functionality on a variety of Python versions. They show the -arch flags being modified correctly on versions up to 3.12, but incorrectly on 3.13, 3.14, and 3.15.

Up to version 3.12 the original CFLAGS value has these -arch options in the middle of the string. From version 3.13 onwards, the original CFLAGS value has them at the end of the string, and the last one (right at the end of the string) should be removed but is not.

CPython versions tested on:

3.13, 3.14, 3.15 releases, CPython main branch

Operating systems tested on:

macOS

Activity

1. jmarshall

   added

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

   on Nov 11, 2026
2. jmarshall commented on Nov 11, 2026

   

   jmarshall

   on Nov 11, 2026

   ContributorAuthor

   More actions

   This has been tested on macOS on 3.13, 3.14, and 3.15 (and observed to be working correctly on 3.12 and prior), and the same code bug has been observed on the main branch.

   This is caused by the regexes in Lib/_osx_support.py:

   cpython/Lib/_osx_support.py

   Lines 315 to 327 in de9a4fb

   	def _override_all_archs(_config_vars):
   	"""Allow override of all archs with ARCHFLAGS env var"""
   	# NOTE: This name was introduced by Apple in OSX 10.5 and
   	# is used by several scripting languages distributed with
   	# that OS release.
   	if 'ARCHFLAGS' in os.environ:
   	arch = os.environ['ARCHFLAGS']
   	for cv in _UNIVERSAL_CONFIG_VARS:
   	if cv in _config_vars and '-arch' in _config_vars[cv]:
   	flags = _config_vars[cv]
   	flags = re.sub(r'-arch\s+\w+\s', ' ', flags)
   	flags = flags + ' ' + arch
   	_save_modified_value(_config_vars, cv, flags)

   Notice the regex on line 325 ends in \s so requires whitespace after the -arch option and its argument. Thus it matches these ‑arch XYZ options within the string but not at the end of the string.

   There are several other similar regexes in Lib/_osx_support.py with similar problems.
3. picnixz

   added

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

   topic-sysconfig

   on Nov 11, 2026
4. picnixz commented on Nov 11, 2026

   

   picnixz

   on Nov 11, 2026

   Member

   More actions

   cc @ned-deily
5. jmarshall

   changed the title [-]ARCHFLAGS does not properly override ‑arch since Python 3.13[/-] [+]ARCHFLAGS does not properly override ‑arch in sysconfig.get_config_vars() values since Python 3.13[/+] on Nov 11, 2026
6. ned-deily

   self-assigned this

   on Nov 11, 2026

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