code: def test_mass_conversion(dataframe):
      df = dataframe.convert_units(
          "a",
          existing_units="g",
          to_units="kg",
          dest_column_name="a_kg",
      )
      assert df["a"].sum() == pytest.approx(df["a_kg"].sum() * 1000.0)
  
file_path: pyjanitor/tests/engineering/test_convert_units.py
imports: import pytest
  from unyt.exceptions import UnitConversionError
  from janitor.engineering import convert_units
label_desc: Use image node_id 6 for calling a global function with example usage: test_mass_conversion(dataframe) without return types