For my recent projects itertoolz and functoolz I’ve decided to simultaneously support Python 2.x and 3.x with a single codebase. This is after a number of other projects (including SymPy) have demonstrated the feasibility of this approach.

I thought I’d share how I now set up my testing environment for these projects as they include a few of my favorite utilities.


First, nosetests. I think most readers of my blog (both of you) are familiar with nosetests so I’ll be brief here. Nosetests reads through my directory, finds all files named, runs all functions they contain named test_X(), and reports all encountered assertion errors.

It’s a simple solution that does exactly one thing and does it well.


Lesser known is the conttest tool, written by @eigenhombre. conttest allows me to run my tests every time I save a file in my workspace. Actually it allows me to run any command on the command line each time my directory changes. It can be composed with nosetests like so

conttest nosetests

I run this in a separate window and keep an eye on it to make sure I don’t introduce errors each time I save. This only works with files for which the tests are cheap (as most good tests are). Actually I usually use it as follows

conttest "clear && nosetests --with-doctest"

I really respect conttest because it actually has nothing to do with testing. It, like nosetests, does one and only one thing well without reaching for more. It composes well with other tools, like nosetests, to great effect.

Conttest is available on the PyPI and so is pip/easy_installable

pip install conttest


The combined Python 2.x 3.x (I’m calling this Python “twenty-three”) source code means that I need to simultaneously run two testing systems, one running 2.7 and one running 3.3. I manage this with conda. First, I create two environments; this only needs to be done once on each system

conda create -n py33 python=3.3 anaconda
conda create -n py27 python=2.7 anaconda

This takes a while to set up but does a wonderful job creating two completely independent, self-consistent, and fully dependable virtual environments. I can switch to either with

source activate py33  # Use Python 3.3
source activate py27  # Use Python 2.7

All Together

So when I start work I set up something like this

# Terminal 1
source activate py33
conttest "clear && nosetests --with-doctest"

# Terminal 2
source activate py27
conttest "clear && nosetests --with-doctest"

# Terminal 3
# Do actual work

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