Return the path to the source file of the given class.

def class_path(cls):
    """Return the path to the source file of the given class."""
    if cls.__module__ == '__main__':
        path = None
    else:
        path = os.path.dirname(inspect.getfile(cls))

    if not path:
        path = os.getcwd()

    return os.path.realpath(path)