(year, month)
return _DAYS_IN_MONTH[month]
year, month -> number of days in that month in that year.
year, month -> number of days in that month in that year.
year month - > number of days in that month in that year .
def _days_in_month(year, month):
    "year, month -> number of days in that month in that year."
    assert 1 <= month <= 12, month
    if month == 2 and _is_leap(year):
        return 29
    return _DAYS_IN_MONTH[month]
def _days_in_month ( year , month ) : assert 1 <= month <= 12 , month if month == 2 and _is_leap ( year ) : return 29 return _DAYS_IN_MONTH [ month ]