def _get_filenames(year, industry_scope=None):
    """Return filenames for oes files based on pattern.

    Parameters
    ----------
    year : string
        4 digit year in string format 'YYYY'.

    Returns
    -------
    dict.

    """
    if industry_scope == None:
        prefix = 'natsector'
    elif industry_scope == 5 or industry_scope == 6:
        prefix = 'nat5d_6d'
    else:
        prefix = 'nat{}d'.format(industry_scope)
    return {'national': f'oesm{year[2:]}nat/national_M{year}_dl.xlsx',
        'state': f'oesm{year[2:]}st/state_M{year}_dl.xlsx', 'metros':
        f'oesm{year[2:]}ma/MSA_M{year}_dl.xlsx', 'metros-divisions':
        f'oesm{year[2:]}ma/aMSA_M{year}_dl.xlsx', 'non-metros':
        f'oesm{year[2:]}ma/BOS_M{year}_dl.xlsx', 'industry':
        f'oesm{year[2:]}in4/{prefix}_M{year}_dl.xlsx'}
