code: def test_nan_str_index(self):
      s = Series([0, 1, 2, np.nan], index=list("abcd"))
      result = s.interpolate()
      expected = Series([0.0, 1.0, 2.0, 2.0], index=list("abcd"))
      tm.assert_series_equal(result, expected)
  
imports: import numpy
  import pytest
  import pandas.util._test_decorators
  import pandas
  from pandas import Index, MultiIndex, Series, date_range, isna
  import pandas._testing
label_desc: Use image node_id 14 for calling the TestSeriesInterpolateData obj's underlying member method code with example usage: obj.test_nan_str_index() without return types