Install and manage multiple versions of Python on Mountain Lion

When I was doing python development on snow leopard, I was installing python modules up the wazoo on the default python which came with the laptop. I later realized that was not the right way of doing it. To keep things organized will help you when you are developing projects which have different requirements and using different modules and different versions. So when I installed mountain lion on my laptop, I thought this time around I will be more organised about this.

I didn’t want to mess with the system python, so I installed python2.7.3 and python 3.2.3 from python.org. After each install, the installation process is going to try to add that version of python in your path on the command line. I recommend commenting the lines which were added to your .bash_profile

Now one would wonder how would you switch between the different versions of python? This is where I found this cool script called update_python_switchers.py from http://sevas.github.com/2011/06/14/multiple-python-osx.html

When you execute this script, it creates a file called .python_switchers.sh in your home directory. This script detects all the different versions of python and then creates bash functions based on what were found. To make these functions available, just source this file. As recommended on the site just add the following to your .bash_profile

export OLD_PATH=$PATH

. $HOME/.python_switchers.sh

When I ran the script update_python_switchers.py, I saw the following error in the execution.

Using generic prompt  
Adding System Python 2.3  
Adding System Python 2.5  
Adding System Python 2.6  
Adding System Python 2.7  
Adding MacPython 2.7  
Adding MacPython 3.2  
Adding EPD 32 7.3  
Traceback (most recent call last):  
File "./update_python_switchers.py", line 152, in <module>  
epd64_versions = detect_epd64_installs()  
File "./update_python_switchers.py", line 48, in
detect_epd64_installs  
directories = os.listdir(os.path.join(EPD64_ROOT, "Versions"))  
OSError: [Errno 2] No such file or directory:
'/Library/Frameworks/EPD64.framework/Versions'

Looks like the script is failing because the script is trying to look for the 64 bit version of Enthought Python. As I don’t have it, I just commented out the call to detect_epd64_installs() in the script. Now running the script gives me the following output

Using generic prompt  
Adding System Python 2.3  
Adding System Python 2.5  
Adding System Python 2.6  
Adding System Python 2.7  
Adding MacPython 2.7  
Adding MacPython 3.2  
Adding EPD 32 7.3  
Saved python switcher bash functions to
/Users/username/.python_switchers.sh

where I replaced my real username with username. As you can see I have a lot of versions of python installed.
Just source the .python_switchers.sh in your home directory and now you should have all the versions of python available to you on a press of a bash function really quickly. All that function is doing is adding the version of python to the beginning of your path.
Type ‘typeset -F’ on the command line to see all bash functions available. In my case I see

declare -f select_epd_32_73  
declare -f select_macpython_27  
declare -f select_macpython_32  
declare -f select_system_python_23  
declare -f select_system_python_25  
declare -f select_system_python_26  
declare -f select_system_python_27

I can now use any of these functions to switch to any of the versions of python installed on my laptop. Typing the function select_macpython_27 will make python 2.7.3 be my default version.

One other change I did to the script update_python_switchers.py was to comment out code which was modifying my command prompt. I like my default prompt. I can easily tell what is my version of python by running the command ‘python —version’.

In part 2 I will go through setting up virtualenv and virtualenvwrapper..