Setting up Python virtualenv
With many Python packages available, which are often in conflict or requiring different versions depending on application, installing and controlling packages and versions is not always easy. In addition, so many packages are often used only occasionally, that it is questionable whether a system administrator of a centralized server system or a High Performance Compute (HPC) infrastructure can be expected to resolve all issues posed by users of the infrastructure. Even on a local system with full administrative rights managing versions, dependencies, and package collisions is often very difficult. The solution is to use a virtual environment, in which a specific set of packages can then be installed. As many different virtual environments can be created, and used side-by-side, as is necessary.
NOTE: as of Python 3.3 virtual environment support is built-in. See this page for an alternative set-up of your virtual environment if using Python 3.4 or higher.
creating a new virtual environment
It is assumed that the appropriate virtualenv executable for the Python version of choice is installed. A new virtual environment, in this case called newenv is created like so:
<source lang='bash'>
virtualenv newenv
</source>
Should virtualenv not be installed, the virtualenv script can be downloaded and accessed directly: <source lang='bash'> curl -O https://pypi.python.org/packages/source/v/virtualenv/virtualenv-1.9.tar.gz tar -xzvf virtualenv-1.9.tar.gz python3 virtualenv-1.9/virtualenv.py testenv </source>
When the new environment is created, one will see a message similar to this:
New python executable in newenv/bin/python3
Also creating executable in newenv/bin/python
Installing Setuptools.........................................................................done.
Installing Pip................................................................................done.
activating a virtual environment
Once the environment is created, each time the environment needs to be activated, the following command needs to be issued:
<source lang='bash'>
source newenv/bin/activate
</source>
This assumes that the folder that contains the virtual environment documents (in this case called newenv), is in the present working directory.
When working on the virtual environment, the virtual environment name will be between brackets in front of the user-host-prompt string.
(newenv)user@host:~$
installing modules on the virtual environment
Installing modules is the same as usual. The difference is that modules are in /path/to/virtenv/lib, which may be living somewhere on your home directory. When working from the virtual environment, the default easy_install will belong to the python version that is currently active. This means that the executable in /path/to/virtenv/bin are in fact the first in the $PATH.
<source lang='bash'>
easy_install numpy
</source>
Similarly, installing packages from source works exactly the same as usual.
<source lang='bash'>
python setup.py install
</source>
deactivating a virtual environment
Quitting a virtual environment can be done by using the command deactivate, which was loaded using the source command upon activating the virtual environment.
<source lang='bash'>
deactivate
</source>
Make IPython work under virtualenv
IPython may not work initially under a virtual environment. It may produce an error message like below:
File "/usr/bin/ipython", line 11
print "Could not start qtconsole. Please install ipython-qtconsole"
^
This can be resolved by adding a soft link with the name ipython to the bin directory in the virtual environment folder.
<source lang='bash'>
ln -s /path/to/virtenv/bin/ipython3 /path/to/virtenv/bin/ipython
</source>