test.py

def parse_range(years):
    years = years.split(', ')
    years = [split_years(year.strip()) for year in years if split_years(year)]
    years = flatten(years)
    return [x for x in sorted(set(years)) if in_valid_years(x)]


def in_valid_years(year):
    return 1999 <= year <= 2018


def split_years(year):
    if '-' not in year:
        return int(year) if year else None

    beginning, ending = [year for year in year.split('-')]
    return range(int(beginning), int(ending)+1)

def flatten(array):
    arr = []
    for item in array:
        if isinstance(item, list):
            arr.extend(instance for instance in item if instance)
        else:
            arr.append(item)
    return arr

assert parse_range('1999, 2002-2005, 2007') == [1999, 2002, 2003, 2004, 2005, 2007]
assert parse_range('2002-2005, 2002-2005, 2002 - 2005') == [2002, 2003, 2004, 2005]
assert parse_range('2017') == [2017]
assert parse_range('2017 - 2015') == []
assert parse_range('2015 - 2015') == [2015]
assert parse_range('1999 , 2000,    , 2008') == [1999, 2000, 2008]
assert parse_range('2015, 2014, 2010') == [2010, 2014, 2015]
assert parse_range('1999, 3000') == [1999]
assert parse_range('1998, 2020') == []