I just got bit by this strange issue. One of my Tox virtual environment creates a nested virtual environment which does not respect settings in
~/pip/pip.ini. This is problematic because the nested environment needs to install packages from our corporate DevPI installation.
I can "fix" this by adding
passenv, but I find it strange that the default behavior breaks pip and it's really hard to diagnose this unless you can make educated guesses about pip internals.