Python – Using Different Versions on Linux

linuxpythonsoftware installation

Background:

Since I develop python programs that must run on different python versions, I have installed different versions of python on my computer.

I am using FC 13 so it came with python 2.6 pre-installed in /usr/bin/python2.6 and /usr/lib/python2.6.

I installed python 2.5 from source, and to keep things neat, I used the --prefix=/usr option, which installed python in /usr/bin/python2.5 and /usr/lib/python2.5.

Now, when I run python my prompt shows I am using version 2.5. However, I am having some issues with the install.

Package management:

Using easy_install, packages are always installed in /usr/lib/python2.6/site-packages/. I downloaded setuptools .egg for python 2.5 and tried to install it, but it gives me an error:

/usr/lib/python2.5/site-packages does NOT support .pth files

It seems that python2.5 is not in my PYTHONPATH. I thought the default install would add itself to the PYTHONPATH, but when I write echo $PYTHONPATH at promt, I just receive an empty line.

Best Answer

The recommended way of having multiple Python versions installed is to install each from source - they will happily coexist together. You can then use virtualenv with the appropriate interpreter to install the required dependencies (using pip or easy_install). The trick to easier installation of multiple interpreters from source is to use:

sudo make altinstall

instead of the more usual "sudo make install". This will add the version number to the executable (so you'd have python-2.5, python-2.6, python-3.2 etc) thus preventing any conflicts with the system version of Python.

Related Solutions

bash scripting python – How to Have Multiple Possibilities in a Script’s Shebang Line

I'm not expert, but I believe that you shouldn't specify exact python version to use and leave that choice to system/user.

Also you should use that instead of hardcoding path to python in script:

#!/usr/bin/env python

#!/usr/bin/env python3 (or python2)

It is recommended by Python doc in all versions:

A good choice is usually
#!/usr/bin/env python
which searches for the Python interpreter in the whole PATH. However, some Unices may not have the env command, so you may need to hardcode /usr/bin/python as the interpreter path.

In various distributions Python may be installed in different places, so env will search for it in PATH. It should be available in all major Linux distributions and from what I see in FreeBSD.

Script should be executed with that version of Python which is in your PATH and which is chosen by your distribution*.

If your script is compatible with all version of Python except 2.4 you should just check inside of it if it is run in Python 2.4 and print some info and exit.

Footnote

*In Gentoo there is tool called eselect. Using it you may set default versions of different applications (including Python) as default:

$ eselect python list
Available Python interpreters:
  [1]   python2.6
  [2]   python2.7 *
  [3]   python3.2
$ sudo eselect python set 1
$ eselect python list
Available Python interpreters:
  [1]   python2.6 *
  [2]   python2.7
  [3]   python3.2

Debian – Install newer & older versions of python on debian

Using the PPA

You can use the PPA on Debian. Pick an Ubuntu version that's from slightly before your Debian version, and it should have all the necessary libraries. For wheezy, the oneiric PPA seems ok (but it lacks more recent Python versions). For jessie, the trusty PPA should work.

To add a PPA on Debian, create a file /etc/apt/sources.list.d/deadsnakes.list containing

deb http://ppa.launchpad.net/fkrull/deadsnakes/ubuntu trusty main 
deb-src http://ppa.launchpad.net/fkrull/deadsnakes/ubuntu trusty main

Download the PPA signing key with

gpg --keyserver keyserver.ubuntu.com --recv-keys DB82666C

then add the key to the APT key database with

gpg --export DB82666C | sudo apt-key add -

Finally run apt-get update and install the desired packages.

If you can't get the PPA working for some reasons (maybe you can't find a version that works with the libraries you have), you can download the source and recompile them for your distribution.

Using a chrooted system

What I usually do to test compatibility with other versions is to run older or newer distributions in a chrooted system. For example, you could install various versions of Ubuntu with the Python versions you're interested in, or you could install trusty in a chroot and install the PPA there. For more information, see my schroot guide.