docstr: The volume roll finder can look for data up to a week before the given
  date. This test asserts that we not only return the correct active
  contract during that previous week (grace period), but also that we do
  not go into exception if one of the contracts does not exist.
code: def test_roll_in_grace_period(self):
      """
      The volume roll finder can look for data up to a week before the given
      date. This test asserts that we not only return the correct active
      contract during that previous week (grace period), but also that we do
      not go into exception if one of the contracts does not exist.
      """
      rolls = self.volume_roll_finder.get_rolls(
          root_symbol="CL",
          start=self.second_end_date,
          end=self.third_end_date,
          offset=0,
      )
      self.assertEqual(
          rolls,
          [
              (1002, pd.Timestamp("2017-03-16", tz="UTC")),
              (1003, None),
          ],
      )
  
imports: from collections import deque
  from functools import partial
  from textwrap import dedent
  from numpy import arange, array, int64, full, repeat, tile
  from numpy.testing import assert_almost_equal
  import pandas
  from pandas import Timestamp, DataFrame
  from zipline.assets.continuous_futures import OrderedContracts, delivery_predicate
  from zipline.assets.roll_finder import ROLL_DAYS_FOR_CURRENT_CONTRACT, VolumeRollFinder
  from zipline.data.minute_bars import FUTURES_MINUTES_PER_DAY
  from zipline.errors import SymbolNotFound
  import zipline.testing.fixtures
label_desc: Use image node_id 6 for calling the RollFinderTestCase obj's underlying member method code with example usage: obj.test_roll_in_grace_period() without return types