PEX_PYTHON_PATH impacts interpreter selection for Pex installed from pip, but not the pex release file

[Eric-Arellano]

Using pex installed via pip install pex, I get this behavior:

pex --interpreter-constraint='CPython>=3.5' -- -c "import sys, os; print(os.path.realpath(sys.executable))"
/Users/eric/.pyenv/versions/3.6.10/bin/python3.6

PEX_PYTHON_PATH=/Users/eric/.pyenv/versions/3.7.7/bin pex --interpreter-constraint='CPython>=3.5' -- -c "import sys, os; print(os.path.realpath(sys.executable))"
/Users/eric/.pyenv/versions/3.7.7/bin/python3.7

But using pex installed from the releases page, I get this behavior:

./pex --interpreter-constraint='CPython>=3.5' -- -c "import sys, os; print(os.path.realpath(sys.executable))"
/Users/eric/.pyenv/versions/3.6.10/bin/python3.6

PEX_PYTHON_PATH=/Users/eric/.pyenv/versions/3.7.7/bin ./pex --interpreter-constraint='CPython>=3.5' -- -c "import sys, os; print(os.path.realpath(sys.executable))"
/Users/eric/.pyenv/versions/3.6.10/bin/python3.6

[Eric-Arellano]

I now understand why setting PEX_PYTHON_PATH works when running Pex when installed via pip. Variables(rc=options.rc_file) ends up reading from os.environ still:

https://github.com/pantsbuild/pex/blob/20a67c3e95e4d15b9e200dc15e9b01aad2cbb83e/pex/variables.py#L184-L192

This means that unless the user explicitly has set PEX_IGNORE_RCFILES, PEX_PYTHON_PATH is already being used, regardless of RC files having been created. I'm not sure we were aware this happens.

--

It's not clear to me yet why the behavior is different when running Pex via the release page.

[Eric-Arellano]

It's not clear to me yet why the behavior is different when running Pex via the release page.

Found it: #741. We strip PEX_PYTHON_PATH from os.environ when running ./pex. By the time we're building a new PEX, this env var has been stripped. Atm, the sole way to load PEX_PYTHON_PATH when building a new Pex by running ./pex is to use an RC File, which won't get stripped.