code: 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[monthf _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]
docs: year, month -> number of days in that month in that year.