code: def test_interp_combo(self):
      df = DataFrame(
          {
              "A": [1.0, 2.0, np.nan, 4.0],
              "B": [1, 4, 9, np.nan],
              "C": [1, 2, 3, 5],
              "D": list("abcd"),
          }
      )
  
      result = df["A"].interpolate()
      expected = Series([1.0, 2.0, 3.0, 4.0], name="A")
      tm.assert_series_equal(result, expected)
  
      msg = "The 'downcast' keyword in Series.interpolate is deprecated"
      with tm.assert_produces_warning(FutureWarning, match=msg):
          result = df["A"].interpolate(downcast="infer")
      expected = Series([1, 2, 3, 4], name="A")
      tm.assert_series_equal(result, expected)
  
imports: import numpy
  import pytest
  from pandas._config import using_pyarrow_string_dtype
  from pandas.errors import ChainedAssignmentError
  import pandas.util._test_decorators
  from pandas import DataFrame, NaT, Series, date_range
  import pandas._testing
label_desc: Use image node_id 8 for calling the TestDataFrameInterpolate obj's underlying member method code with example usage: obj.test_interp_combo() without return types