code: def test_diff(self):
      pdf = pd.DataFrame(
          {
              "a": [1, 2, 3, 4, 5, 6],
              "b": [1, 1, 2, 3, 5, 8],
              "c": [1, 4, 9, 16, 25, 36],
          },
          index=np.random.rand(6),
      )
      psdf = ps.from_pandas(pdf)
  
      self.assert_eq(pdf.diff(), psdf.diff())
      self.assert_eq(pdf.diff().diff(-1), psdf.diff().diff(-1))
      self.assert_eq(pdf.diff().sum().astype(int), psdf.diff().sum())
  
      msg = "should be an int"
      with self.assertRaisesRegex(TypeError, msg):
          psdf.diff(1.5)
      msg = 'axis should be either 0 or "index" currently.'
      with self.assertRaisesRegex(NotImplementedError, msg):
          psdf.diff(axis=1)
  
      # multi-index columns
      columns = pd.MultiIndex.from_tuples(
          [("x", "Col1"), ("x", "Col2"), ("y", "Col3")]
      )
      pdf.columns = columns
      psdf.columns = columns
  
      self.assert_eq(pdf.diff(), psdf.diff())
  
imports: import unittest
  import numpy
  import pandas
  from pyspark.sql import functions
  from pyspark import pandas
  from pyspark.testing.pandasutils import ComparisonTestBase
  from pyspark.testing.sqlutils import SQLTestUtils
label_desc: Use image node_id 7 for calling the FrameComputeMixin obj's underlying member method code with example usage: obj.test_diff() without return types