Using tox to test code under multiple python versions

Tox is a great project that makes testing against different python versions simple, standing on the shoulders of pip and virtualenv. For our project the tox specification is incredibly simple:

$ cat tox.ini 
[tox]
envlist = py27, py34

[testenv]
commands = nosetests -v
deps =
    nose
    -rrequirements.txt

This tells tox to create two virtualenv's, one for python 2.7 and one for python 3.4, install dependencies listed in requirements.txt, and then run tests with "nosetests -v". You can run the test against all of the configured environments with just

tox

Or a particular subset like this:

tox -e py34