def pop(self, key, *args):
            'Remove *key* from maps[0] and return its value. Raise KeyError if *key* not in maps[0].'
            try:
                return self.maps[0].pop(key, *args)
            except KeyError:
                raise KeyError('Key not found in the first mapping: {!r}'.format(key))
Remove *key* from maps[0] and return its value. Raise KeyError if *key* not in maps[0].
stackoverflow/venv/lib/python3.6/site-packages/pip-19.0.3-py3.6.egg/pip/_vendor/distlib/compat.py