code: def test_last_valid_index(self):
      pdf = pd.DataFrame(
          {
              "a": [1, 2, 3, None],
              "b": [1.0, 2.0, 3.0, None],
              "c": [100, 200, 400, None],
          },
          index=["Q", "W", "E", "R"],
      )
      psdf = ps.from_pandas(pdf)
      self.assert_eq(pdf.last_valid_index(), psdf.last_valid_index())
      self.assert_eq(
          pdf[[]].last_valid_index(), psdf[[]].last_valid_index()
      )
  
      # MultiIndex columns
      pdf.columns = pd.MultiIndex.from_tuples(
          [("a", "x"), ("b", "y"), ("c", "z")]
      )
      psdf = ps.from_pandas(pdf)
      self.assert_eq(pdf.last_valid_index(), psdf.last_valid_index())
  
      # Empty DataFrame
      pdf = pd.Series([]).to_frame()
      psdf = ps.Series([]).to_frame()
      self.assert_eq(pdf.last_valid_index(), psdf.last_valid_index())
  
file_path: spark/python/pyspark/pandas/tests/frame/test_time_series.py
imports: from datetime import datetime
  import unittest
  import numpy
  import pandas
  from pyspark import pandas
  from pyspark.testing.pandasutils import ComparisonTestBase
  from pyspark.testing.sqlutils import SQLTestUtils
label_desc: Use image node_id 3 for calling the FrameTimeSeriesMixin obj's underlying member method code with example usage: obj.test_last_valid_index() without return types