Editable install does not work with --prefix

Environment

• pip version: 20.0.1
• Python version: 3.8.1
• OS: macOS

Description Editable installs fail when using a custom --prefix. Independently, install appears to attempt to uninstall synonymous packages on the PYTHONPATH when installing to a different prefix, though this might be intentional.

Expected behavior That editable installs would work for arbitrary prefixes.

How to Reproduce

$ mkdir -p "$PWD/foo/lib/python3.8/site-packages"
$ env PYTHONPATH="$PWD/foo/lib/python3.8/site-packages" python -m pip install --prefix "$PWD/foo" -e git+https://github.com/pypa/pip#egg=pip

Output

Obtaining pip from git+https://github.com/pypa/pip#egg=pip
  Cloning https://github.com/pypa/pip to ./venv/src/pip
  Running command git clone -q https://github.com/pypa/pip /Users/layday/Desktop/venv/src/pip
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
Installing collected packages: pip
  Attempting uninstall: pip
    Found existing installation: pip 20.0.1
    Uninstalling pip-20.0.1:
      Successfully uninstalled pip-20.0.1
  Running setup.py develop for pip
    ERROR: Command errored out with exit status 1:
     command: /Users/layday/Desktop/venv/bin/python -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/Users/layday/Desktop/venv/src/pip/setup.py'"'"'; __file__='"'"'/Users/layday/Desktop/venv/src/pip/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' develop --no-deps --prefix /Users/layday/Desktop/foo/lib/python3.8/site-packages
         cwd: /Users/layday/Desktop/venv/src/pip/
    Complete output (33 lines):
    running develop
    Checking .pth file support in /Users/layday/Desktop/foo/lib/python3.8/site-packages/lib/python3.8/site-packages
    /Users/layday/Desktop/venv/bin/python -E -c pass
    TEST FAILED: /Users/layday/Desktop/foo/lib/python3.8/site-packages/lib/python3.8/site-packages does NOT support .pth files
    error: bad install directory or PYTHONPATH

    You are attempting to install a package to a directory that is not
    on PYTHONPATH and which Python does not read ".pth" files from.  The
    installation directory you specified (via --install-dir, --prefix, or
    the distutils default setting) was:

        /Users/layday/Desktop/foo/lib/python3.8/site-packages/lib/python3.8/site-packages

    and your PYTHONPATH environment variable currently contains:

        '/private/var/folders/18/r2nky_v137d3fwhtx0644m940000gn/T/pip-build-env-vjsgcmks/site'

    Here are some of your options for correcting the problem:

    * You can choose a different installation directory, i.e., one that is
      on PYTHONPATH or supports .pth files

    * You can add the installation directory to the PYTHONPATH environment
      variable.  (It must then also be on PYTHONPATH whenever you run
      Python and want to use the package(s) you are installing.)

    * You can set up the installation directory to support ".pth" files by
      using one of the approaches described here:

      https://setuptools.readthedocs.io/en/latest/easy_install.html#custom-installation-locations


    Please make the appropriate changes for your system and try again.
    ----------------------------------------
  Rolling back uninstall of pip
  Moving to /Users/layday/Desktop/venv/bin/pip
   from /private/var/folders/18/r2nky_v137d3fwhtx0644m940000gn/T/pip-uninstall-xp6y4cgw/pip
  Moving to /Users/layday/Desktop/venv/bin/pip3
   from /private/var/folders/18/r2nky_v137d3fwhtx0644m940000gn/T/pip-uninstall-xp6y4cgw/pip3
  Moving to /Users/layday/Desktop/venv/bin/pip3.8
   from /private/var/folders/18/r2nky_v137d3fwhtx0644m940000gn/T/pip-uninstall-xp6y4cgw/pip3.8
  Moving to /Users/layday/Desktop/venv/lib/python3.8/site-packages/pip-20.0.1.dist-info/
   from /Users/layday/Desktop/venv/lib/python3.8/site-packages/~ip-20.0.1.dist-info
  Moving to /Users/layday/Desktop/venv/lib/python3.8/site-packages/pip/
   from /Users/layday/Desktop/venv/lib/python3.8/site-packages/~ip
ERROR: Command errored out with exit status 1: /Users/layday/Desktop/venv/bin/python -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/Users/layday/Desktop/venv/src/pip/setup.py'"'"'; __file__='"'"'/Users/layday/Desktop/venv/src/pip/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' develop --no-deps --prefix /Users/layday/Desktop/foo/lib/python3.8/site-packages Check the logs for full command output.

[layday]

I bisected it to 744b8cf96eb3c3d4504048621c9406dd648fb71f.

[layday]

I don’t think this is relevant anymore, closing.