Sometimes you need to know what Python modules you already installed, the easiest way to get a list is to:

[code lang=python]
help('modules')
[/code]

This will give you a list of installed modules typically as a series of columns. All you have are names, not version numbers. If you need to know version numbers, then try:

[code lang=python]
import matplotlib
print(matplotlib.__version__)
1.5.1
[/code]