How to use the fastparquet.writer function in fastparquet

"""
    import fastparquet

    fmd = copy.copy(fmd)
    for fn, rg in zip(filenames, writes):
        if rg is not None:
            if isinstance(rg, list):
                for r in rg:
                    fmd.row_groups.append(r)
            else:
                for chunk in rg.columns:
                    chunk.file_path = fn
                fmd.row_groups.append(rg)

    fn = sep.join([path, "_metadata"])
    fastparquet.writer.write_common_metadata(
        fn, fmd, open_with=fs.open, no_row_groups=False
    )

    fn = sep.join([path, "_common_metadata"])
    fastparquet.writer.write_common_metadata(fn, fmd, open_with=fs.open)

if rg is not None:
            if isinstance(rg, list):
                for r in rg:
                    fmd.row_groups.append(r)
            else:
                for chunk in rg.columns:
                    chunk.file_path = fn
                fmd.row_groups.append(rg)

    fn = sep.join([path, "_metadata"])
    fastparquet.writer.write_common_metadata(
        fn, fmd, open_with=fs.open, no_row_groups=False
    )

    fn = sep.join([path, "_common_metadata"])
    fastparquet.writer.write_common_metadata(fn, fmd, open_with=fs.open)

):
                raise ValueError(
                    "Appended columns not the same.\n"
                    "Previous: {} | New: {}".format(pf.columns, list(df.columns))
                )
            elif (pd.Series(pf.dtypes).loc[pf.columns] != df[pf.columns].dtypes).any():
                raise ValueError(
                    "Appended dtypes differ.\n{}".format(
                        set(pf.dtypes.items()) ^ set(df.dtypes.iteritems())
                    )
                )
            else:
                df = df[pf.columns + partition_on]

            fmd = pf.fmd
            i_offset = fastparquet.writer.find_max_part(fmd.row_groups)
            if not ignore_divisions:
                if not set(index_cols).intersection([division_info["name"]]):
                    ignore_divisions = True
            if not ignore_divisions:
                minmax = fastparquet.api.sorted_partitioned_columns(pf)
                old_end = minmax[index_cols[0]]["max"][-1]
                divisions = division_info["divisions"]
                if divisions[0] < old_end:
                    raise ValueError(
                        "Appended divisions overlapping with previous ones."
                        "\n"
                        "Previous: {} | New: {}".format(old_end, divisions[0])
                    )
        else:
            fmd = fastparquet.writer.make_metadata(
                df._meta,