Wednesday, May 30, 2012

Run multiple python versions on your system

I'm a software development consultant.  I write python (as well as other languages) code for many clients and I don't get to dictate what their environment looks like.  I've got clients running python as old as 2.4 while others are on the bleeding edge.  Additionally, each client may have their own packages installed as well as differing lists of third party packages.

This post is a description of how I went about getting multiple versions of python installed in my ubuntu development machine and how I go about managing different package sets for different clients.

Get multiple pythons installed

Ubuntu typically only supports one python 2.x version and one 3.x version at a time.  There's a popular ppa (personal package archive) called deadsnakes that contains older versions of python.  You can find it:


To install it (per the instructions in the link above), you do the following:

steve@ubuntu64 ~ $ sudo add-apt-repository ppa:fkrull/deadsnakes

Then you need to update your cache:

steve@ubuntu64 ~ $ sudo apt-get update

Finally, simply install the other versions (I'm running on ubuntu 12.04 LTS, so I have python 2.7 already):

steve@ubuntu64 ~ $ sudo apt-get install python2.4 python2.5 python2.6

If you're following along, we now have python versions 2.4 through 2.7 installed the computer.  If you run 'python', you'll see the default version is still 2.7.


steve@ubuntu64 ~ $ python
Python 2.7.3 (default, Apr 20 2012, 22:39:59) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

Here's why:  Each python version is stored in /usr/bin as python2.X where X is the version.  There is a symbolic link named python that points to the version you want to be the default.  Instead of typing python from the bash prompt, you could just as easily type python2.5:


steve@ubuntu64 ~ $ ls -l /usr/bin/python*
lrwxrwxrwx 1 root root       9 Apr 17 13:20 /usr/bin/python -> python2.7
lrwxrwxrwx 1 root root       9 Apr 17 13:20 /usr/bin/python2 -> python2.7
-rwxr-xr-x 1 root root 1216520 May 21 12:13 /usr/bin/python2.4
-rwxr-xr-x 1 root root 1403624 May  3 00:17 /usr/bin/python2.5
-rwxr-xr-x 1 root root 2652056 May 12 08:43 /usr/bin/python2.6
-rwxr-xr-x 1 root root 2993560 Apr 20 19:37 /usr/bin/python2.7


steve@ubuntu64 ~ $ python2.5
Python 2.5.6 (r256:88840, May  3 2012, 04:16:14) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 


This is a point of interest.  I would not mess with the symbolic link.  Ubuntu runs python for many internal maintenance scripts and those scripts are expecting the python version that shipped with ubuntu.

Use virtualenv to manage your python installations and package sets

So now that you have multiple versions of python on your system, how to manage them?  How do you keep packages installed for one version separate from packages installed for another.  What if you want to run one version of django for client X and a different version for client Y?

That's where virtualenv comes in.  If your a ruby programmer, this is analogous to rvm.  Virtualenv lets you manage the python versions and package installations separately for different projects or clients.

Installing virtualenv is simple

As always, you're just a single apt-get command away from having virtualenv ready to go:

steve@ubuntu64 ~ $ sudo apt-get install python-virtualenv

That it.  Virtualenv is ready to go now.

Quick example

Say your starting a new project for a client.  They are running python2.5 and want to use the mocks, nose and coverage packages for testing.  Here a walkthrough of how to use virtualenv to manage the project.

First, let's create a directory for the project:

steve@ubuntu64 ~ $ mkdir -p ~/dev/project1
steve@ubuntu64 ~ $ cd ~/dev/project1

Next, run virtualenv to create the environment for the project:


steve@ubuntu64 ~/dev/project1 $ virtualenv -p /usr/bin/python2.5 .env
Running virtualenv with interpreter /usr/bin/python2.5
New python executable in .env/bin/python2.5
Also creating executable in .env/bin/python
Installing distribute.............................................................................................................................................................................................done.
Installing pip...............done.
steve@ubuntu64 ~/dev/project1 $ 

This command tells virtualenv to create a .env directory and to place a copy of the 2.5 version of python in it.  This copy of the 2.5 python is brand-spankin' new.  It doesn't have any packages (beyond the standard library) installed.  You will need to install them yourself.  Any packages that you install in this instance of python will not be available to main python installation or other virtualenv instances.

Before your can use this new copy, you need to activate it:


steve@ubuntu64 ~/dev/project1 $ source .env/bin/activate
(.env)steve@ubuntu64 ~/dev/project1 $

The activate script manipulates your path environment variable, placing the new python instance first in your path.  This makes is so that when you run python, it will use the version from your instance:


(.env)steve@ubuntu64 ~/dev/project1 $ which python
/home/steve/dev/project1/.env/bin/python
(.env)steve@ubuntu64 ~/dev/project1 $ python
Python 2.5.6 (r256:88840, May  3 2012, 04:16:14) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>

Also, notice that your prompt starts with (.env).  This tells you that you're running with the virtualenv instance activated.  To install packages in your instance, use the pip command:


(.env)steve@ubuntu64 ~/dev/project1 $ pip install mock nose coverage
Downloading/unpacking mock
  Downloading mock-0.8.0.tar.gz (749Kb): 749Kb downloaded
  Running setup.py egg_info for package mock
    warning: no files found matching '*.png' under directory 'docs'
    warning: no files found matching '*.css' under directory 'docs'
    warning: no files found matching '*.html' under directory 'docs'
    warning: no files found matching '*.js' under directory 'docs'
Downloading/unpacking nose
  Downloading nose-1.1.2.tar.gz (729Kb): 729Kb downloaded
  In the tar file /tmp/pip-dH_WYa-unpack/nose-1.1.2.tar.gz the member nose-1.1.2/doc/doc_tests/test_selector_plugin/support/tests/mymodule/my_function$py.class is invalid: 'filename None not found'
  In the tar file /tmp/pip-dH_WYa-unpack/nose-1.1.2.tar.gz the member nose-1.1.2/doc/doc_tests/test_restricted_plugin_options/restricted_plugin_options.rst.py3.patch is invalid: 'filename None not found'
  Running setup.py egg_info for package nose
Downloading/unpacking coverage  Downloading coverage-3.5.2.tar.gz (115Kb): 115Kb downloaded
  Running setup.py egg_info for package coverage
    no previously-included directories found matching 'test'Installing collected packages: mock, nose, coverage
  Running setup.py install for mock
    warning: no files found matching '*.png' under directory 'docs'
    warning: no files found matching '*.css' under directory 'docs'
    warning: no files found matching '*.html' under directory 'docs'
    warning: no files found matching '*.js' under directory 'docs'
  Running setup.py install for nose
    Installing nosetests script to /home/steve/dev/project1/.env/bin
    Installing nosetests-2.5 script to /home/steve/dev/project1/.env/bin
  Running setup.py install for coverage
    building 'coverage.tracer' extension
    gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/python2.5 -c coverage/tracer.c -o build/temp.linux-x86_64-2.5/coverage/tracer.o
    coverage/tracer.c:3:20: fatal error: Python.h: No such file or directory
    compilation terminated.
    **
    ** Couldn't install with extension module, trying without it...
    ** SystemExit: error: command 'gcc' failed with exit status 1
    **
    no previously-included directories found matching 'test'
    Installing coverage script to /home/steve/dev/project1/.env/bin
Successfully installed mock nose coverage
Cleaning up...
(.env)steve@ubuntu64 ~/dev/project1 $

To see that the packages have been installed, simply use them:




(.env)steve@ubuntu64 ~/dev/project1 $ python
Python 2.5.6 (r256:88840, May  3 2012, 04:16:14) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import mock
>>> import nose
>>> import coverage
>>>

When you're done working on the project, deactivate it.  You can always come back later and activate it again.


(.env)steve@ubuntu64 ~/dev/project1 $ deactivate
steve@ubuntu64 ~/dev/project1 $


Notice that when you deactivate the environment, your packages are no longer available:


steve@ubuntu64 ~/dev/project1 $ python
Python 2.7.3 (default, Apr 20 2012, 22:39:59) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import mock
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named mock
>>> import nose
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named nose
>>> import coverage
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named coverage
>>>

You can create as many virtualenv environments as you like.  I create one for each project that I work on.


Bonus material


To make life even easier, here's a couple additional things I do that you might find helpful!

First, once you've started to use virtualenv with some frequency, you start to get tired of downloading and installing the same packages over and over.  Pip has the ability to cache your downloaded packages for reuse.  To do that, you'll need to create a directory to store the download packages in:

steve@ubuntu64 ~ $ mkdir ~/.pip_download_cache

Then you'll need to set a variable to inform pip of the new directory.  Add the following to your .bashrc file:

export PIP_DOWNLOAD_CACHE=/home/steve/.pip_download_cache

Now when you do a pip install, it will keep the downloaded files in the ~/.pip_download_cache directory.  The next time you do a pip install of the same package, it will just use the copy from the directory instead of downloading it again.


Second, it can be tedious to always have to type 'source .env/bin/activate' every time you want to activate an environment.  Since I always put my virtual environments in a .env directory I can count on the command to activate always being the same.  So I create an alias for it.  I added the the following to my ~/.bash_aliases file:

alias activate='source .env/bin/activate'

Now once I cd into the projects directory, I simply type activate to activate my virtual environment.






8 comments:

  1. unfortunately, if you try this with python2.4 you will get error from virtual env:

    ERROR: None
    ERROR: this script requires Python 2.5 or greater.

    ReplyDelete
    Replies
    1. I've got this error and though I linked python to python2.6 it still rises up this error. Any ideas? Server seems to be RedHat

      Delete
  2. Thanks for posting. I'm the computer volunteer at my local library and a Python-based program that we were running and needed desperately just wouldn't work after we upgraded to Ubuntu 12.04 LTS from 8.04 recently. Your guide was a big help in replicating the python 2.5 environment.

    ReplyDelete
  3. Amazing tutorial, helps so much.
    Thank you!

    ReplyDelete
  4. Awesome article! I'm new to Ubuntu and command line in general and you couldn't have explained this any better! Everything worked the first time through. Loving Ubuntu thus far too.

    ReplyDelete
    Replies
    1. Here is a link for newbies like me that shows you how to add lines to the files Steve mentions in the 'Bonus Material' section of this post.

      http://askubuntu.com/questions/211716/add-environment-variable-to-bashrc-through-script

      Delete
  5. I got such an error:
    ImportError: libpython2.6.so.1.0: cannot open shared object file: No such file or directory
    So obviously I've got the binary (python2.6), but not the libs. What can I do?

    ReplyDelete
  6. This was very useful. Thank You!

    ReplyDelete