def add_abs_path_directories(opts_dict):
    """
    Adds 'abs_input' and 'abs_output' to opts_dict

    :param opts_dict: dictionary that will be modified
    """
    assert_has_input_output(opts_dict)
    opts_dict['abs_input'] = path.abspath(opts_dict['input'])
    if opts_dict['is_dir']:
        # Only add path separator to input if it is a directory
        opts_dict['abs_input'] += get_path_separator()
    opts_dict['abs_output'] = path.abspath(opts_dict['output']) + \
        get_path_separator()