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{packaging} enable site in python._pth instead of deleting it #33419

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{packaging} enable site in python._pth instead of deleting it #33419

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17 changes: 14 additions & 3 deletions build_scripts/windows/scripts/build.cmd
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Original file line number Diff line number Diff line change
Expand Up @@ -110,10 +110,21 @@ if not exist %PYTHON_DIR% (
del python-archive.zip
echo Python downloaded and extracted successfully

REM Delete _pth file so that Lib\site-packages is included in sys.path
REM Append `import site` to python*._pth so site.py runs at startup
REM (which adds Lib\site-packages to sys.path). We keep the file (rather
REM than deleting it) because on Python 3.14+ removing it breaks pip's
REM PEP 517 isolated BuildEnvironment subprocess: the child python.exe
REM can no longer locate the stdlib zip and dies in init_fs_encoding
REM with ModuleNotFoundError: No module named 'encodings'.
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@jiasli jiasli May 22, 2026 •
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Could you find evidence that Python 3.14 specifically changed the behavior when _pth file is deleted?

The del python*._pth logic was there since long ago as a manual process and was added to build.cmd in #21746 (comment).

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@YangAn-microsoft YangAn-microsoft May 22, 2026 •
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No there isn't a specific change introduced by Python 3.14 that caused the issue. In fact, only Python 3.14 win32 embedded has trouble importing from python*.zip.
I came up with this fix after experimenting the official way of enabling site packages for embedded python. (https://docs.python.org/3/using/windows.html#finding-modules)
Since this solves the win32 issue and using a more officially recommended solution than existing del python*._pth, I believe it's a proper change.

REM Keeping an explicit _pth makes stdlib + site-packages discoverable
REM in both the parent and any spawned subprocess.
REM https://github.com/pypa/pip/issues/4207#issuecomment-297396913
REM https://docs.python.org/3.10/using/windows.html#finding-modules
del python*._pth
REM https://docs.python.org/3/using/windows.html#finding-modules
if exist python*._pth (
for %%f in (python*._pth) do (
findstr /x "import site" "%%f" >nul || echo import site>> %%f
)
)

echo Installing pip
curl --output get-pip.py %GET_PIP_DOWNLOAD_URL%
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