code: def test_expanding_cov_pairwise_diff_length():
      # GH 7512
      df1 = DataFrame(
          [[1, 5], [3, 2], [3, 9]],
          columns=Index(["A", "B"], name="foo"),
      )
      df1a = DataFrame(
          [[1, 5], [3, 9]],
          index=[0, 2],
          columns=Index(["A", "B"], name="foo"),
      )
      df2 = DataFrame(
          [[5, 6], [None, None], [2, 1]],
          columns=Index(["X", "Y"], name="foo"),
      )
      df2a = DataFrame(
          [[5, 6], [2, 1]],
          index=[0, 2],
          columns=Index(["X", "Y"], name="foo"),
      )
      # TODO: xref gh-15826
      # .loc is not preserving the names
      result1 = df1.expanding().cov(df2, pairwise=True).loc[2]
      result2 = df1.expanding().cov(df2a, pairwise=True).loc[2]
      result3 = df1a.expanding().cov(df2, pairwise=True).loc[2]
      result4 = df1a.expanding().cov(df2a, pairwise=True).loc[2]
      expected = DataFrame(
          [[-3.0, -6.0], [-5.0, -10.0]],
          columns=Index(["A", "B"], name="foo"),
          index=Index(["X", "Y"], name="foo"),
      )
      tm.assert_frame_equal(result1, expected)
      tm.assert_frame_equal(result2, expected)
      tm.assert_frame_equal(result3, expected)
      tm.assert_frame_equal(result4, expected)
  
imports: import numpy
  import pytest
  from pandas import DataFrame, DatetimeIndex, Index, MultiIndex, Series, isna, notna
  import pandas._testing
label_desc: Use image node_id 32 for calling a global function with example usage: test_expanding_cov_pairwise_diff_length() without return types