code: def df():
      return pd.DataFrame(
          {
              "name": ("black", "black", "black", "red", "red"),
              "type": ("chair", "chair", "sofa", "sofa", "plate"),
              "num": (4, 5, 12, 4, 3),
              "nulls": (1, 1, np.nan, np.nan, 3),
          }
      )
  
file_path: pyjanitor/tests/functions/test_groupby_agg.py
imports: import numpy
  import pandas
  import pytest
  from pandas.testing import assert_frame_equal
label_desc: Use image node_id 4 for calling a global function with example usage: df() and returns: pd