How do I use a relative path in a Python module when the CWD has changed?

Store the absolute path to the module directory at the very beginning of the module:

package_directory = os.path.dirname(os.path.abspath(__file__))

Afterwards, load your resources based on this package_directory:

font_file = os.path.join(package_directory, 'fonts', 'myfont.ttf')

And after all, do not modify of process-wide resources like the current working directory. There is never a real need to change the working directory in a well-written program, consequently avoid os.chdir().