code: def test_cov(self):
      # SPARK-36396: Implement DataFrame.cov
  
      # int
      pdf = pd.DataFrame(
          [(1, 2), (0, 3), (2, 0), (1, 1)], columns=["a", "b"]
      )
      psdf = ps.from_pandas(pdf)
      self.assert_eq(pdf.cov(), psdf.cov(), almost=True)
      self.assert_eq(
          pdf.cov(min_periods=4), psdf.cov(min_periods=4), almost=True
      )
      self.assert_eq(pdf.cov(min_periods=5), psdf.cov(min_periods=5))
  
      # ddof
      with self.assertRaisesRegex(TypeError, "ddof must be integer"):
          psdf.cov(ddof="ddof")
      for ddof in [-1, 0, 2]:
          self.assert_eq(
              pdf.cov(ddof=ddof), psdf.cov(ddof=ddof), almost=True
          )
          self.assert_eq(
              pdf.cov(min_periods=4, ddof=ddof),
              psdf.cov(min_periods=4, ddof=ddof),
              almost=True,
          )
          self.assert_eq(
              pdf.cov(min_periods=5, ddof=ddof),
              psdf.cov(min_periods=5, ddof=ddof),
          )
  
      # bool
      pdf = pd.DataFrame(
          {
              "a": [1, np.nan, 3, 4],
              "b": [True, False, False, True],
              "c": [True, True, False, True],
          }
      )
      psdf = ps.from_pandas(pdf)
      self.assert_eq(pdf.cov(), psdf.cov(), almost=True)
      self.assert_eq(
          pdf.cov(min_periods=4), psdf.cov(min_periods=4), almost=True
      )
      self.assert_eq(pdf.cov(min_periods=5), psdf.cov(min_periods=5))
  
      # extension dtype
      numeric_dtypes = [
          "Int8",
          "Int16",
          "Int32",
          "Int64",
          "Float32",
          "Float64",
          "float",
      ]
      boolean_dtypes = ["boolean", "bool"]
  
      sers = [
          pd.Series([1, 2, 3, None], dtype=dtype)
          for dtype in numeric_dtypes
      ]
      sers += [
          pd.Series([True, False, True, None], dtype=dtype)
          for dtype in boolean_dtypes
      ]
      sers.append(
          pd.Series(
              [
                  decimal.Decimal(1),
                  decimal.Decimal(2),
                  decimal.Decimal(3),
                  None,
              ]
          )
      )
  
      pdf = pd.concat(sers, axis=1)
      pdf.columns = [
          dtype for dtype in numeric_dtypes + boolean_dtypes
      ] + ["decimal"]
      psdf = ps.from_pandas(pdf)
  
      self.assert_eq(
          pdf.cov(numeric_only=True), psdf.cov(), almost=True
      )
  
      # string column
      pdf = pd.DataFrame(
          [
              (1, 2, "a", 1),
              (0, 3, "b", 1),
              (2, 0, "c", 9),
              (1, 1, "d", 1),
          ],
          columns=["a", "b", "c", "d"],
      )
      psdf = ps.from_pandas(pdf)
      self.assert_eq(
          pdf.cov(numeric_only=True), psdf.cov(), almost=True
      )
      self.assert_eq(
          pdf.cov(numeric_only=True, min_periods=4),
          psdf.cov(min_periods=4),
          almost=True,
      )
      self.assert_eq(
          pdf.cov(numeric_only=True, min_periods=5),
          psdf.cov(min_periods=5),
      )
  
      # nan
      np.random.seed(42)
      pdf = pd.DataFrame(
          np.random.randn(20, 3), columns=["a", "b", "c"]
      )
      pdf.loc[pdf.index[:5], "a"] = np.nan
      pdf.loc[pdf.index[5:10], "b"] = np.nan
      psdf = ps.from_pandas(pdf)
      self.assert_eq(
          pdf.cov(min_periods=11), psdf.cov(min_periods=11), almost=True
      )
      self.assert_eq(
          pdf.cov(min_periods=10), psdf.cov(min_periods=10), almost=True
      )
  
      # return empty DataFrame
      pdf = pd.DataFrame(
          [("1", "2"), ("0", "3"), ("2", "0"), ("1", "1")],
          columns=["a", "b"],
      )
      psdf = ps.from_pandas(pdf)
      self.assert_eq(pdf.cov(numeric_only=True), psdf.cov())
  
file_path: spark/python/pyspark/pandas/tests/computation/test_cov.py
imports: import unittest
  import decimal
  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 1 for calling the FrameCovMixin obj's underlying member method code with example usage: obj.test_cov() without return types