This problem has haunted me many times when I want to work on two frameworks (Turbogears and Pylons) and there are some packages which interfere with each other. Having both installed in /usr/lib/python-2.5/site-packages is a nightmare. You won’t know what causes your nosetests to fail, you will have to comment out the other dirty package and try again. Also some packages may be upgraded which will cause some problem in some place in your application.
All this can be easily solved by using the virtualenv package in python. What it does is – create isolated python working environments. So that packages you install in one environment doesn’t interfere with the other. Also if you can’t install packages in the global site-packages, this will be really helpful.
You just have to easy_install virtualenv and it gets installed as a executable. If you can’t easy_install, there is a single python script (virtualenv.py) which can be used instead.
You can create your sandbox with the command $ virtualenv --no-site-packages env. The –no-site-packages option make sure that the global site-packages doesn’t inherit any packages from the global site-packages.
It will create a directory ‘env’ in your current directory. The bin folder inside this has its own python executable and a seperate site-packages in the lib directory. It also has easy_install installed in the bin.
Now you can install whatever package you want by typing $ env/bin/easy_install packagename
All your commands need to be prefixed with the env/bin/ path else it will use the wrong python version. But you can avoid typing the long commands by using the activate script bundled with the bin folder.
On *nix systems, you have to $ source bin/activate and on a windows machine > \path\to\env\bin\activate.bat. After you are finished with it, you just have to type deactivate and you get back to the old shell.
Most of the times, it would work perfectly, but sometimes the shell will cache the path of the commands. So, it is always advisable to check whether the path is set correctly by typing which python.
Now this solves my problem of mismatched and wrong packages and can start working in Pylons.
Tags: Python, virtualenv
