code: def test_is_on_offset_weekday_none(self, n, date):
      # GH 18510 Week with weekday = None, normalize = False
      # should always be is_on_offset
      offset = Week(n=n, weekday=None)
      ts = Timestamp(date, tz="Africa/Lusaka")
      fast = offset.is_on_offset(ts)
      slow = (ts + offset) - offset == ts
      assert fast == slow
  
imports: from __future__ import annotations
  from datetime import datetime, timedelta
  import pytest
  from pandas._libs.tslibs import Timestamp
  from pandas._libs.tslibs.offsets import Day, LastWeekOfMonth, Week, WeekOfMonth
  import pandas._testing
  from pandas.tests.tseries.offsets.common import WeekDay, assert_is_on_offset, assert_offset_equal
label_desc: Use image node_id 6 for calling the TestWeek obj's underlying member method code with example usage: obj.test_is_on_offset_weekday_none(n, date) without return types