Build isolation doesn't work with Python installation that requires PYTHONPATH for stdlib

Environment

• pip version: 19.2.3
• Python version: 2.7 / 3.7
• OS: Windows (will probably fail on nix as well?)

Custom env, with PYTHONPATH, PYTHONHOME set.

Description When installing PEP518/517 package based on setuptools, the install fails with an ImportError: No module named site on py 2.7, or ModuleNotFoundError: No module named 'encodings' on py3.7.

Expected behavior Install completes successfully.

How to Reproduce Repro on Windows:

1. Take a normal/fresh CPython install with pip installed in e.g. C:\testpython, put it in PATH (without any other py installs) (SET PATH=C:\testpython).
2. SET PYTHONHOME=C:\testpython (to ensure no other system python is picked up via registry).
3. Move C:\testpython\Lib to C:\testpython\hidden\Lib.
4. See that calling python fails with the above error.
5. SET PYTHONPATH=C:\testpython\hidden\Lib;C:\testpython\hidden\Lib\site-packages.
6. See that python now works again
7. Upgrade pip if needed with python -m pip install --upgrade pip --target=C:\testpython\hidden\Lib\site-packages
8. Do a source install of a a package with pyproject.toml that uses setuptools under the hood. python -m pip install path_to_src_package --target=C:\testpython\hidden\Lib\site-packages. It fails since pip internally overwrites PYTHONPATH for the build env, so its subprocess call to sys.executable fails. This is where it overwrites PYTHONPATH:

https://github.com/pypa/pip/blob/53ee380d95b49a66c87eea6bda41b1c1fef7c600/src/pip/_internal/build_env.py#L123-L127

Output

C:\testpython>python -m pip install path_to_src_package
Processing path_to_src_package
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  ERROR: Command errored out with exit status 3221226505:
   command: 'C:\pytest\python.exe' 'C:\testpython\hidden\Lib\site-packages\pip\_vendor\pep517\_in_process.py' get_requires_for_build_wheel 'C:\Users\username\AppData\Local\Temp\tmpr4n0ozpb'
       cwd: C:\Users\username\AppData\Local\Temp\pip-req-build-o60yaw8r
  Complete output (4 lines):
  Fatal Python error: initfsencoding: unable to load the file system codec
  ModuleNotFoundError: No module named 'encodings'

  Current thread 0x000027b8 (most recent call first):
  ----------------------------------------
ERROR: Command errored out with exit status 3221226505: 'C:\testpython\python.exe' 'C:\testpython\hidden\Lib\site-packages\pip\_vendor\pep517\_in_process.py' get_requires_for_build_wheel 'C:\Users\username\AppData\Local\Temp\tmpr4n0ozpb' Check the logs for full command output.

[vidartf]

All I’d ask is that if at any point, you hit any pip issues, you demonstrate that they happen in a vanilla environment before asking for help here. (Or at a minimum, you make it clear that you’ve got a weird setup and link back here in the issue report).

Of course. I was hoping the OP was proof of me understanding what a proper MWE is 😉

[pradyunsg]

Thanks @vidartf for a basically excellent bug report; and for being understanding on why we view this as a wont-fix.

I’ll go ahead and close this now. 😃