Comment 5 for bug 1373254

From the forum for the package team responsible for PIP comes the following comments on this issue:
-------- 1st comment --------
dstufft commented on May 26, 2014 <Python Packaging Authority member>

Ah I see.

So pip itself bundles some software, html5lib being one of those, on various Linux distros they unbundle that software and cause pip to import it normally instead of using it's own bundled copy.

What this means is that if you install an incompatible version of one of pip's dependencies into the global Python, then you'll break pip because pip will import that and attempt to use that instead of the correct one installed by apt-get.

-------- 2nd comment (a clarification of the 1st comment above) --------
Ivoz commented on Jun 4, 2014 < Python Packaging Authority member>

@lrossouw try pip installing packages in a virtualenv or using the --user flag instead of sudo installing them as system packages.

The issue seems to be caused by debian's modification of its pip, so we can't address it ourselves.

-------- Source for above comments: --------
https://github.com/pypa/pip/issues/1742

I'm including these comments as a record that the dev's in charge of PIP suggested that this issue belongs here. This may not be necessary but it is a record of the events that led here and as such I'm hoping it is at least informative if not helpful to some people.