This snippet shows how to use https://github.com/halst/schema to validate command line arguments in a python script. You should use https://github.com/docopt/docopt to handle command line arguments

using schema to validate command line arguments.py

    schema = Schema(
        {
            '--schema_dir':
                And(
                    And(
                        os.path.exists,
                        os.path.isdir,
                        lambda s: os.access(s, os.R_OK),
                        error="The path '%s' should be an existing directory and be readable. Please check" % arguments['--schema_dir']
                    ),
                    And(
                        lambda s: os.path.exists(os.path.join(os.path.realpath(s), 'Rovi 2.0 Bound US')),
                        error="Expected the directory 'Rovi 2.0 Bound US' to exist under the schema directory: '%s'. Please check!" % arguments['--schema_dir'],
                    ),
                    And(
                        lambda s: os.path.exists(os.path.join(os.path.realpath(s), 'Rovi 2.0 Unbound')),
                        error="Expected the directory 'Rovi 2.0 Unbound' to exist under the schema directory : '%s'. Please check!" % arguments['--schema_dir'],
                    )
                ),
            '--output':
                Or(
                    #Either the directory exists and is writeable..
                    And(os.path.exists,
                        os.path.isdir,
                        lambda s: os.access(s, os.W_OK),
                        error="The path to the database output, '%s', doesn't exist, or doesn't point to a writeable directory. Please check! " % arguments['--output']
                    ),
                    #Or the directory should be creatable...(the existing root of the specified directory tree should be writeable)
                    And(lambda s: os.access(find_existing_root_dir(s), os.W_OK),
                        error="The path to the database output, '%s', has to be created, but the current process doesn't have permissions to create the directory tree. Please check!" % arguments['--output']
                    ),
                )
        })