Python virtual environments are a fantastic method of insulating your projects from each other, allowing each project to have different versions of their requirements.
They work (at a very high level) by making a lightweight copy of the system Python, which symlinks back to the real thing whenever necessary. You can then install whatever you want in
lib/pythonX.Y/site-packages (e.g. via
pip), and you are good to go.
Depending on what provides your source Python, however, upgrading it can break things. For example, I use Homebrew, which (under the hood) stores everything it builds in versioned directories:
$ readlink $(which python) ../Cellar/python/2.7.8_2/bin/python
Whenever there even a minor change to this Python, symlinks back to that versioned directory may not work anymore, which breaks my virtual environments:
$ python dyld: Library not loaded: @executable_path/../.Python Referenced from: /Users/mikeboers/Documents/Flask-Images/venv/bin/python Reason: image not found Trace/BPT trap: 5
There is an easy fix: manually remove the links back to the old Python, and rebuild the virtual environment.
NOTE: the following is
rm -rf; we only want to remove files and links, not directories!
$ cd /path/to/the/venv $ rm .Python bin/python* lib/python2.7/* include/python2.7 rm: lib/python2.7/distutils: is a directory rm: lib/python2.7/site-packages: is a directory
This will have cleared out the
python executables, links back to the standard library, headers, and the primary shared library. Now you can recreate a virtual environment on top:
$ virtualenv . New python executable in ./bin/python2.7 Also creating executable in ./bin/python Installing setuptools, pip...done. $ python -c 'print "Hello, blog!"' Hello, blog!