code: def test_add_constant_dataframe(self):
      df = pd.DataFrame(
          [[1.0, "a", 4], [2.0, "bc", 9], [3.0, "def", 16]]
      )
      output = tools.add_constant(df)
      expected = pd.Series([1.0, 1.0, 1.0], name="const")
      assert_series_equal(expected, output["const"])
      dfc = df.copy()
      dfc.insert(0, "const", np.ones(3))
      assert_frame_equal(dfc, output)
  
imports: from statsmodels.compat.pandas import assert_frame_equal, assert_series_equal
  from statsmodels.compat.python import lrange
  import string
  import numpy
  from numpy.random import standard_normal
  from numpy.testing import assert_almost_equal, assert_array_equal, assert_equal, assert_string_equal
  import pandas
  import pytest
  from statsmodels.datasets import longley
  from statsmodels.tools import tools
  from statsmodels.tools.tools import pinv_extended
label_desc: Use image node_id 6 for calling the TestTools obj's underlying member method code with example usage: obj.test_add_constant_dataframe() without return types