import strings

def could_be_year(str_): return str_.isdigit() and len(str_) == 4

def could_be_day(str_): return str_.isdigit() and len(str_) <= 2

def parse_date_range(str_, range_sep='-', part_sep=' '):

    str_ = strings.single_space(str_)

    # range_sep might be used as date part separator instead

    if str_.find(part_sep) < 0: return (str_, None)

    start, sep, end = str_.partition(range_sep)

    if sep == '': return (str_, None) # not a range

    start = start.split(part_sep)

    end = end.split(part_sep)

    # Has form M D1-D2 or M D1-D2 Y (not M1 Y1-M2 Y2 or M1 D1-M2 D2)

    if len(start) == 2 and (len(end) == 1 or (

            len(end) == 2 and could_be_day(start[-1]) and could_be_day(end[0])

            and could_be_year(end[-1])

        )):

        end.insert(0, start[0]) # make end fully specified

    ct_diff = len(end) - len(start)

    # Has form D1-D2 M Y, M1 D1-M2 D2 Y, M1-M2 Y, etc.

    if ct_diff > 0: start += end[-ct_diff:] # make start fully specified

    # Other forms are invalid and will be left as-is

    return (start, end)