Fail python-pip after upgrade on Debian 8 and 9

Some time after "pip install --upgrade pip" pip fail with error "ImportError: cannot import name main".

pip --version
Traceback (most recent call last):
File "/usr/bin/pip", line 9, in
from pip import main
ImportError: cannot import name main

Most easy way to fix is

apt-get purge python-pip
wget https://bootstrap.pypa.io/get-pip.py
python get-pip.py
ln -s /usr/local/bin/pip /usr/bin/pip
pip install --upgrade pip