If you have "foo==0.1" installed, and foo has the dep "bar~=0.2" (current vers of bar 0.2.1).

Then bar releases version 0.2.2

So your deps want bar 0.2.1, but foo now wants bar 0.2.2

This breaks your pip install.

EDIT: there are a few other gotchas (please respond to this post if you know of any more)

e.g. from https://medium.com/knerd/the-nine-circles-of-python-dependen...

"If two of your dependencies are demanding overlapping versions of a library, pip will not necessarily install a version of this library that satisfies both requirements" e.g. https://github.com/pypa/pip/issues/2775