Assuming that both your directories are real Python packages (do have the __init__.py file inside them), here is a safe solution for the inclusion of modules relative to the location of the script.
I assume that you want to do this because you need to include a set of modules with your script. I use this in production in several products and works in many special scenarios like scripts called from another directory or executed with python execute instead of opening a new interpreter.
import os, sys, inspect
# realpath() will make your script run, even if you symlink it :)
cmd_folder = os.path.realpath(os.path.abspath(os.path.split(inspect.getfile( inspect.currentframe() ))))
if cmd_folder not in sys.path:
# Use this if you want to include modules from a subfolder
cmd_subfolder = os.path.realpath(os.path.abspath(os.path.join(os.path.split(inspect.getfile( inspect.currentframe() )),"subfolder")))
if cmd_subfolder not in sys.path:
# cmd_folder = os.path.dirname(os.path.abspath(__file__)) # DO NOT USE __file__ !!!
# __file__ fails if the script is called in different ways on Windows.
# __file__ fails if someone does os.chdir() before.
# sys.argv also fails, because it doesn't not always contains the path.
As a bonus, this approach does let you force Python to use your module instead of the ones installed on the system.
Warning! I don't really know what is happening when the current module is inside an egg file. It probably fails too.