code: def test_groupby_example(self):
      # groupby example
      NUM_ROWS = 100
      NUM_COLS = 10
      col_names = [
          "A" + num for num in map(str, np.arange(NUM_COLS).tolist())
      ]
      index_cols = col_names[:5]
  
      df = DataFrame(
          np.random.default_rng(2).integers(
              5, size=(NUM_ROWS, NUM_COLS)
          ),
          dtype=np.int64,
          columns=col_names,
      )
      df = df.set_index(index_cols).sort_index()
      grp = df.groupby(level=index_cols[:4])
      df["new_col"] = np.nan
  
      # we are actually operating on a copy here
      # but in this case, that's ok
      for name, df2 in grp:
          new_vals = np.arange(df2.shape[0])
          df.loc[name, "new_col"] = new_vals
  
imports: import numpy
  import pytest
  from pandas.errors import SettingWithCopyError
  import pandas
  from pandas import DataFrame, MultiIndex, Series, date_range, isna, notna
  import pandas._testing
label_desc: Use image node_id 9 for calling the TestMultiIndexSetItem obj's underlying member method code with example usage: obj.test_groupby_example() without return types