code: def test_errors(self):
      y = self.rng.randn(100, 2)
      with pytest.raises(ValueError):
          cov_nw(y, 200)
      with pytest.raises(ValueError):
          cov_nw(y, axis=3)
      with pytest.raises(ValueError):
          cov_nw(y, ddof=200)
  
file_path: arch/arch/tests/utility/test_cov.py
imports: from __future__ import annotations
  from arch.compat.statsmodels import dataset_loader
  from numpy import diff, log
  from numpy.random import RandomState
  from numpy.testing import assert_almost_equal
  import pytest
  from statsmodels.datasets import macrodata
  from arch.utility import cov_nw
label_desc: Use image node_id 8 for calling the TestVarNW obj's underlying member method code with example usage: obj.test_errors() without return types