def diff_files(from_file, to_file, index_columns, sep=',', ignored_columns=None):
    """
    Diff two CSV files, returning the patch which transforms one into the
    other.
    """
    with open(from_file) as from_stream:
        with open(to_file) as to_stream:
            from_records = records.load(from_stream, sep=sep)
            to_records = records.load(to_stream, sep=sep)
            return patch.create(from_records, to_records, index_columns,
                                ignore_columns=ignored_columns)
Diff two CSV files, returning the patch which transforms one into the
    other.