This morning, I noticed my Mercurial install isn't working either - failing with this error message:

$ hg pull
abort: couldn't find mercurial libraries in [/usr/local/Cellar/mercurial/4.0.1/lib/python2.7/site-packages /usr/local/Cellar/mercurial/4.0.1/bin /usr/local/Cellar/python/2.7.12_2/Frameworks/Python.framework/Versions/2.7/lib/python27.zip /usr/local/Cellar/python/2.7.12_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7 /usr/local/Cellar/python/2.7.12_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/plat-darwin /usr/local/Cellar/python/2.7.12_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/plat-mac /usr/local/Cellar/python/2.7.12_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/plat-mac/lib-scriptpackages /usr/local/Cellar/python/2.7.12_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-tk /usr/local/Cellar/python/2.7.12_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-old /usr/local/Cellar/python/2.7.12_2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-dynload /Users/jaraco/Library/Python/2.7/lib/python/site-packages /usr/local/lib/python2.7/site-packages /Library/Python/2.7/site-packages]
(check your install and PYTHONPATH)

That happens even though I was still able to "import mercurial" in my Python environment. I uninstalled and reinstalled the Mercurial (using Homebrew) and it failed to reinstall with the same error, suggesting strongly that it was the Python environment that was corrupted. I then uninstalled and reinstalled the Homebrew Python 2.7, and the problem was solved. I think this points to another manifestation of something deleting files that were symlinked by directories in the virtualenv.