Virtual Environments (on india)¶

Often when you use shared computing resources, such as india.futuresystems.org you will not have permission to install applications in the default global location.

Let’s see where grep is located:

$ which grep
/bin/grep

It seems that there are many programs installed in /bin such as mkdir and pwd:

$ ls /bin
alsacard    dbus-cleanup-sockets  env             hostname         mailx          pwd
alsaunmute  dbus-daemon           ex              igawk            mkdir          raw
...

If we wished to add a new program it seems like putting it in /bin is the place to start. Let’s create an empty file /bin/hello-$PORTALNAME:

$ touch /bin/hello-$(whoami)
touch: cannot touch `/bin/hello-albert': Permission denied

We can still create our program in our home directory:

$ touch ~/hello-$(whoami)

but this becomes cumbersome very quickly if we have a large number of programs to install. Additionally, it is not a good idea to modify the global environment of one’s computing system as this can lead to instability and bizarre errors.

A virtual environment is a way of encapsulating and automating the creation and use of a computing environment that is consistent and self-contained.

The tool we use with Python to accomplish this is called virtualenv.

Let’s try it out. Start by cleaning up our test earlier and going into the home directory:

$ rm ~/hello-$(whoami)
$ cd ~

Now lets create a virtual env:

$ virtualenv ENV
PYTHONHOME is set.  You *must* activate the virtualenv before using it
New python executable in ENV/bin/python
Installing setuptools............done.
Installing pip...............done.

When using virtualenv you pass the directory where you which to create the virtual environment, in this case ENV in the current (home) directory. We are then told that we must activate the virtual environment before using it and that the python program, setuptools, and pip are installed.

Let’s see what we have:

$ ls ENV/bin
activate  activate.csh  activate.fish  activate_this.py  easy_install
easy_install-2.7  pip  pip-2.7  python  python2  python2.7

It seems that there are several programs installed. Let’s see where our current python is and what happens after activating this environment:

$ which python
/N/soft/python/2.7/bin/python
$ source ENV/bin/activate
(ENV) $ which python
~/ENV/bin/python