problems after a pip upgrade, python package installation always fails now
 

I was urged to upgrade pip from 8.1 to 18.1 and then every time I run pip, it gives an error message like the following,

john@vb ~/work/software/nlopt-master $ pip --version
/home/john/.local/lib/python2.7/site-packages/pip/_vendor/requests /__init__.py:83: RequestsDependencyWarning: Old version of cryptography ([1, 2, 3]) may cause slowdown.
warnings.warn(warning, RequestsDependencyWarning)

So, I followed the advice to upgrade cryptography by doing the following, and get a direct error message,

john@vb ~/work/software/nlopt-master $ sudo pip install --upgrade cryptography
Traceback (most recent call last):
File "/usr/bin/pip", line 9, in <module>
from pip import main
ImportError: cannot import name main

don't quite know whether the issue is due to the recent upgrade of pip. If so, is there way to wind back to my old version? How to further fix these issues? My os is linux mint 18.2.

find the answer myself.

(1) downgrade from 18.1 to 8.1.1
sudo python -m pip install pip==8.1.1

(2) fix broken links
hash -r

then everything comes back to life, or at least pip freeze works as expected