switch versions of python

Question

Story: One of the app that i have works on python 2.4 and other on 2.6. I tried to do a sym link of python2.4 to python and things started to break loose on ubuntu jaunty. Now i am downloading every dependency of 2.4 and installing it using python2.4 setup.py install. The dependencies seem to be endless.

Question1: How will i tell any framework that go and use version so and so pf python like day django to use 2.6 and say mjango to use 2.4? Something like we say use database databasename kinda syntax.

Question2: Is there more elegant way to switch between version as my hack of symlinking was a virtual disaster?

Question3: Can I download a deb for say hardy and make jaunty believe its for her?

Answer 1

Use Virtualenv.

There is more information here: Working with virtualenv.

Using virtualenv you can create a new virtual python environment with whatever version of Python you want for each project or application. You can then activate the appropriate environment when you need it.

To expand on my answer:

You can install multiple versions of Python on your computer (I have 2.4, 2.5, 2.6 and 3.1 on my machine - I install each from source). I use a Mac, and keep my system Python as whatever OS X sets as the default.

I use *easy_install* to install packages. On ubuntu you can get easy_install like this:

sudo apt-get install python-setuptools

To install virtualenv then do:

easy_install virtualenv

I tend to create a new virtualenv for each project I'm working on and don't give it access to the global site-packages. This keeps all the packages tight together and allows me to have the specific versions of everything I need.

virtualenv -p python2.6 --no-site-packages ~/env/NEW_DJANGO_PROJECT

And then whenever I am doing anything related to this project I activate it:

source ~/env/NEW_DJANGO_PROJECT/bin activate

If I run python now it uses this new python. If I use easy_install it installs things into my new virtual environment.

So, virtualenv should be able to solve all of your problems.

Answer 2

A more grassroot approach than Virtualenv is the side-by-side installation of two Python versions.

If there is an existing installation, and you want a second installation into the same root path (e.g. /usr/local), use this target when making install:

make altinstall

When your second installation is Python 2.6, this will leave you with a /usr/local/bin/python2.6 alongside the old /usr/local/bin/python.

A simple way to switch between these two versions is using a shell alias (alias python=/usr/local/bin/python2.6) on the shell where you invoke the interpreter. But this won't work across sub-shells and she-bang invocations.

Answer 3

"Question1: How will i tell any framework that go and use version so and so pf python like day django to use 2.6 and say mjango to use 2.4?"

You simply run them with the specific python version they need. Run mjango with /usr/bin/python2.4 and django with /usr/bin/python2.6. As easy as that.

"Question2: Is there more elegant way to switch between version as my hack of symlinking was a virtual disaster?"

Yes, see above. Have two separate installs of Python, and run explicitly with the different versions.

"Question3: Can I download a deb for say hardy and make jaunty believe its for her?"

That generally works. If it doesn't, it's because it has dependencies that exist in Hardy, and does not exist in Jaunty, and then you can't.

And here is a Question 4 you didn't ask, but should have. ;)

"Is there an easier way to download all those Python modules?"

Yes, there is. Install setuptools, and use easy_install. It will not help you with library dependecies for those Python modules that have C code and need to be compiled. But it will help with all others. easy_install will download and install all the Python dependencies of the module in question in one go. That makes it a lot quicker to install Python modules.