pypa/pip — pull request #1543 by pfmoore (2014-02-07 20:48:33+00:00)
Title: Special case os.devnull for PIP_CONFIG_FILE to mean 'no config file'

Workaround for Python bug http://bugs.python.org/issue20053 (issue with ensurepip not being able to isolate itself from a user config file)

Longer term, a proper pip "isolated mode" is the better approach.