I get an error when I execute python2 -m pip freeze this command. 
The error message is as below:
/usr/bin/python: cannot import name HashMissing; 'pip' is a package and cannot be directly executed
I had used apt-get remove --purge python python-pip fo removing the python2. 
Then, I do these path checking for knowing pip and python command path.  

    john@mymachine:~$ whereis python
    python: /usr/bin/python /usr/bin/python3.4m /usr/bin/python3.4 /usr/bin/python2.7 /usr/bin/python2.7-config /usr/lib/python3.4     /usr/lib/python2.7 /etc/python3.4 /etc/python2.7 /usr/local/lib/python3.4 /usr/local/lib/python2.7 /usr/include/python2.7 /usr/share/python /usr/share/man/man1/python.1.gz
    john@mymachine:~$ which -a pip
    /usr/local/bin/pip
    jonh@mymachine:~$ whereis pip
    pip: /usr/local/bin/pip /usr/local/bin/pip3.5 /usr/local/bin/pip2.7 /usr/local/bin/pip3.4
    john@mymachine:~$ which -a python
    /usr/bin/python

Wish some help.
Don't know the reason. But, I install the pip again.
wget https://bootstrap.pypa.io/get-pip.py
and
python get-pip.py
The problem is resolved.
python2 -m pip freeze showing an error