pip: Invalid values in boolean options in the environment cause traceback

Environment

• pip version: 10.0.1
• Python version: 3.6.6
• OS: Debian Unstable

Description

When setting an environment variable that is supposed to hold a boolean value to an invalid string that is not empty, a traceback happens.

Expected behavior

An error message along the lines of "‘blah’ is not a valid value for …, valid values

How to Reproduce

1. pip install pip==10.0.1
2. Run env PIP_REQUIRE_VIRTUALENV='blah' PYTHONPATH=src python -m pip install twisted
3. Observe a traceback

Output

$ env PIP_REQUIRE_VIRTUALENV='blah' PYTHONPATH=src python -m pip install twisted                                                       
Traceback (most recent call last):                                             
  File "/home/moshez/.pyenv/versions/3.6.4/lib/python3.6/runpy.py", line 193, in
 _run_module_as_main                                                           
...
    values = self.get_default_values()
  File "/home/moshez/src/pip/src/pip/_internal/baseparser.py", line 230, in get_default_values
    defaults = self._update_defaults(self.defaults.copy())  # ours             
  File "/home/moshez/src/pip/src/pip/_internal/baseparser.py", line 195, in _update_defaults
    val = strtobool(val)
  File "/home/moshez/.pyenv/versions/3.6.4/lib/python3.6/distutils/util.py", line 317, in strtobool
    raise ValueError("invalid truth value %r" % (val,))                        
ValueError: invalid truth value 'blah'

Note

The documentation of strtobool (seen in the traceback above) says:

“True values are y, yes, t, true, on and 1; false values are n, no, f, false, off and 0. Raises ValueError if val is anything else.”