What does my PYTHONPATH contain?

I work on some personal python libraries and I need to display what PYTHONPATH contains and then manipulate it.

Typically, the environment variable $PYTHONPATH is empty (try echo $PYTHONPATH). The actual list of folders python searches for libraries can be found with (in python):

import sys
print(sys.path)

This will consist (in search order) of the current directory, any directories in your $PYTHONPATH, and finally the default library directories, set by site.py. The main default locations are (where X.Y is the python version, eg 2.7):

/usr/lib/pythonX.Y (python system libraries, eg re, urllib)
/usr/lib/pythonX.Y/dist-packages (python libraries installed with deb packages)

If you use pip install --user or similar to install libraries as yourself, it will also contain the user library directory:

/home/USERNAME/.local/lib/pythonX.Y/site-packages

You can manipulate the PYTHONPATH by either setting the environment variable before you launch python (PYTHONPATH=$PYTHONPATH:/foo/bar), or by editing sys.path once you've started python (import sys; sys.path = ["/foo/bar"] + sys.path).

However, if you want to play with your own python libraries, a good idea is to create a virtualenv. This is a directory in which you can play around with your own versions of python libraries without any risk of messing up the python libraries used by the system. See How to set up and use a virtual python environment in Ubuntu? for information about creating and using a virtualenv.