Pip problem when Python2 and Python3 coexist

Python2 and Python3 are installed on the Raspberry Pi at the same time, the initial pip is version 9.01, with the PIP install Django can only be installed to version 1.11, but I need 2.0 of Django.
Then upgrade PIP:

python -m pip install --upgrade pip

PIP upgrades to the latest version 18.0 and installs Django 2.0 successfully.

At this point, however, PIP3 can be used normally, and PIP2 will go wrong:

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

Because two versions of Python use the same pip, they affect each other and need to reinstall their version of PIP:

python3-m pip install --upgrade pip --force-reinstall

python2-m pip install --upgrade pip --force-reinstall

The use of PIP2,PIP3 is normal.

Pip problem when Python2 and Python3 coexist