Add a directory to Python path

In order to use your own python modules ubiquitously you need to add the containing directory MODULES_DIR to the python path.

There are two ways to do it:

Append at run-time

Use sys.path.append($MODULES_DIR).
You should note that:

1. the path will dissapear as soon as the execution of your new module finish, and
2. if you refactor your library you will have two change $MODULES_DIR in every file containing it.

Append to the PYTHONPATH environment variable

Add export PYTHONPATH=$PYTHONPATH:$MODULES_DIR to your ~/.bashrc file.
Run source ~/.bashrc to reload the file without re-logging.