code: def test_groupby_function_tuple_1677(self):
      df = DataFrame(
          np.random.default_rng(2).random(100),
          index=date_range("1/1/2000", periods=100),
      )
      monthly_group = df.groupby(lambda x: (x.year, x.month))
  
      result = monthly_group.mean()
      assert isinstance(result.index[0], tuple)
  
imports: import datetime
  from datetime import date
  import re
  import numpy
  import pytest
  from pandas.compat.numpy import np_long
  import pandas
  from pandas import DataFrame, DatetimeIndex, Index, Timestamp, date_range, offsets
  import pandas._testing
label_desc: Use image node_id 7 for calling the TestDatetimeIndex obj's underlying member method code with example usage: obj.test_groupby_function_tuple_1677() without return types