What is the proper way to manage multiple python versions?

Question

I have a machine with Python 2.6 installed as the default Python. Then, I installed Python 2.7, and manually created /usr/bin/python as a symlink to the new installation.

Then, I was running into problems with command-not-found. I'm trying to reinstall it:

sudo apt-get remove command-not-found

and I get this error:

/usr/bin/python does not match the
python default version. It must be
reset to point to python2.6

But I really want Python 2.7 to be the default. How do I fix this mess?

Answer 1

Changing the default Python (or Perl, etc) on an OS is really bad idea. This interpreter is actually part of the OS and there may well be other OS components that are written specifically to work with that version of the interpreter.

For example on Redhat the yum tool that performs system software updates is a python application. You really don't want to break this. Such applications may depend on specific, perhaps non standard, python modules being installed which the version you installed may not have. For example on Ubuntu I believe some of the built-in OS tools written in Python use an ORM called Storm that isn't part of the Python standard library. Does your clean Python 2.7 install have the specific expected version of the Storm module installed? Does it have any version of Storm? No? Then you've just broken a chunk of your OS.

The right way to do this is install your preferred version of python and set up your user account to use it by setting up your .bash_profile, path and such. You might also want to look into the virtualenv module for Python.

Answer 2

How do I fix this mess?

Nothing more than reinstalling python. It will undo your change (the symlink).

Why do you want it as default? Each time you need it, just use python2.7 or include #!/usr/bin/python2.7 (the shebang) at the beginning of your (executable) scripts.

If you insist on having python2.7 as system-wide default, use a later release of Ubuntu (currently it's Ubuntu 11.04, codenamed Natty). It uses that version as default.

In future, avoid doing manual interventions like what you did with the symlink thing. This is especially true for distro-managed files, and most especially for complex beasts like Python installations.