GeezerVison is showing github.com — New site.py implementation fails to load .pth files in subprocesses · Issue #149819 · python/cpython
New site.py implementation fails to load .pth files in subprocesses #149819DescriptionBug report
Bug description:
After #149583, .pth files are no longer loaded in Python subprocesses, as started with subprocess.run([sys.executable, …]) and the like.
The new implementation deliberately ignores .pth files in sys.path directories that are already known, and Python subprocesses already know all the paths of their parent process. Thus, all .pth files in those paths are ignored by subprocesses now.
I specifically noticed this when running setuptools in a build subprocess, which now fails to install its distutils replacement hack.
CPython versions tested on:
CPython main branch
Operating systems tested on:
Linux
Linked PRs
Activitychanged the title [-]New site.py implementations fails fails to load .pth files in subprocesses[/-] [+]New site.py implementations fails to load .pth files in subprocesses[/+] on May 14, 2026warsaw commented on May 15, 2026 It works for me with the following test script.
#!/bin/sh
#
# Usage:
# ./149819.sh # uses ./python.exe
# ./149819.sh /usr/bin/python3 # uses a specific interpreter
PYTHON=${1:-./python.exe}
VENV=$(mktemp -d)
"$PYTHON" -m venv "$VENV"
SITE=$("$VENV"/bin/python -c 'import sysconfig; print(sysconfig.get_paths()["purelib"])')
cat > "$SITE"/pp.pth <<EOF
import sys; print('pp.pth')
EOF
"$VENV/bin/python" -c "import sys, subprocess; print('parent'); subprocess.run([sys.executable, '-c', 'print(\'child\')'])"This outputs:
pp.pth
parent
pp.pth
child
warsaw commented on May 15, 2026 This is using the head of the 3.15 branch 21909e8 warsaw commented on May 18, 2026 Closing as Can't Reproduce. scoder commented on May 19, 2026 ContributorAuthor More actions
Closing as Can't Reproduce.
Don't expect everyone else to work 24/7 on fixing other people's code. :]
Here's the easy (for me) reproducer:
git clone https://github.com/cython/cython
cd cython
python3.16 -m venv venv
. venv/bin/activate
python3 -m pip install setuptools
python3 runtests.py --debug "compile\.declarations" Fails in a file that the test run generates with
Traceback (most recent call last):
File "…/TEST_TMP/compile/declarations/setup.py", line 2, in <module>
from distutils.core import setup
ModuleNotFoundError: No module named 'distutils'
I was using CPython's main branch at f1a47e7 for this (that's shortly after #149659 was merged). warsaw commented on May 20, 2026 Could you please try again with 87a879f from main or ad2f0cb from the 3.15 branch? I tried your recipe in both and it still WFM. It probably doesn't matter, but I'm on macOS.
% python3 runtests.py --debug "compile\.declarations"
Python 3.15.0b1+ (heads/3.15:ad2f0cb997a, May 20 2026, 10:11:23) [Clang 21.0.0 (clang-2100.1.1.101)]
Running tests against Cython 3.3.0a0 7eeaf92d5d0c5ee1c2964566856b4d0568e8532c
Using Cython language level 2.
Test dependency not found: 'numpy'
Test dependency not found: 'pythran'
Test dependency not found: 'setuptools.sandbox'
Test dependency found: 'asyncio' version 3.15.0b1+
Test dependency found: 'pstats' version 3.15.0b1+
Test dependency found: 'posix' version 3.15.0b1+
Test dependency found: 'array' version ?.?
Test dependency not found: 'Cython.Coverage'
Test dependency not found: 'Cython.Coverage'
Test dependency not found: 'IPython.testing.globalipapp'
Test dependency not found: 'jedi_BROKEN_AND_DISABLED'
Test dependency found: 'test.support' version 3.15.0b1+
Backends: c,cpp
[etoe.declarations: SLOWTEST 0.88 sec] ----------------------------------------------------------------------
Ran 1 test in 0.888s
OK
Most expensive pipeline stages: 'AnalyseDeclarationsTransform': 0.15 / 8 (0.019 / run, 54.2%), 'parse': 0.03 / 8 (0.004 / run, 12.4%), 'generate_pyx_code_stage': 0.03 / 1 (0.031 / run, 11.1%), 'AnalyseExpressionsTransform': 0.01 / 8 (0.001 / run, 3.7%), 'PostParse': 0.01 / 8 (0.001 / run, 1.9%), 'OptimizeBuiltinCalls': 0.00 / 8 (0.001 / run, 1.6%), 'RemoveUnreachableCode': 0.00 / 16 (0.000 / run, 1.4%), 'InterpretCompilerDirectives': 0.00 / 8 (0.000 / run, 1.0%), 'MarkClosureVisitor': 0.00 / 8 (0.000 / run, 0.9%), 'ControlFlowAnalysis': 0.00 / 8 (0.000 / run, 0.9%)
Times:
etoe-build : 0.66 sec ( 1, 0.663 / run) - slowest: 'c:declarations(1)' (0.66s)
etoe-run : 0.22 sec ( 1, 0.217 / run) - slowest: 'c:declarations(2)' (0.22s)
ALL DONE
scoder commented on May 20, 2026 ContributorAuthor More actions I tried with latest main (87a879f) and it fails for me, as before. This is on Ubuntu Linux. We don't have "latest main" integration tests running on anything else, so I can't compare right now. But your failure to reproduce it speaks for a platform specific issue. Possibly some spawn/fork/exec kind of difference? warsaw commented on May 21, 2026 Please review the latest linked PR scoder commented on May 21, 2026 ContributorAuthor More actions Tried it, works for me, resolves this issue. changed the title [-]New site.py implementations fails to load .pth files in subprocesses[/-] [+]New site.py implementation fails to load .pth files in subprocesses[/+] on May 21, 2026added 3 commits that reference this issue on May 21, 2026gh-149819: fix .pth and .start file processing in subprocess when inh… warsaw commented on May 21, 2026 Fixed in main and 3.15 (it should show up in the next beta). Thanks again!
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