code: def test_grouper_period_index(self):
      # GH 32108
      periods = 2
      index = pd.period_range(
          start="2018-01", periods=periods, freq="M", name="Month"
      )
      period_series = Series(range(periods), index=index)
      result = period_series.groupby(period_series.index.month).sum()
  
      expected = Series(
          range(periods),
          index=Index(range(1, periods + 1), name=index.name),
      )
      tm.assert_series_equal(result, expected)
  
imports: from datetime import datetime, timedelta
  import numpy
  import pytest
  import pytz
  import pandas
  from pandas import DataFrame, DatetimeIndex, Index, MultiIndex, Series, Timestamp, date_range, offsets
  import pandas._testing
  from pandas.core.groupby.grouper import Grouper
  from pandas.core.groupby.ops import BinGrouper
label_desc: Use image node_id 23 for calling the TestGroupBy obj's underlying member method code with example usage: obj.test_grouper_period_index() without return types