"pkg_resources.DistributionNotFound: pip==1.5.6 Error with pip"

If you have an old version of pip, running pip install -U pip can break pip.

$ pip
Traceback (most recent call last):
 File "/usr/local/bin/pip", line 5, in <module>
 from pkg_resources import load_entry_point
 File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 2749, in <module>
 working_set = WorkingSet._build_master()
 File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 446, in _build_master
 return cls._build_from_requirements(__requires__)
 File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 459, in _build_from_requirements
 dists = ws.resolve(reqs, Environment())
 File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 628, in resolve
 raise DistributionNotFound(req)
pkg_resources.DistributionNotFound: pip==1.5.6

To fix this, use easy_install:

$ sudo easy_install --upgrade pip

I found this information here. Thank you.

How to update a broken pip · kamatari
https://kamatari.github.io/2014/06/29/how-to-update-pip/