def test_Diff_MissingExternalDiff(self):
    os.environ['TEST_DIFF'] = self.data1_file
    if os.path.exists(self.data1_file):
      os.unlink(self.data1_file)  # Be 100% sure this does not exist
    with self.assertRaises(basetest.DiffFailureError) as error_context:
      basetest._Diff(self.data2_file, self.data2_file)
Test the internal _Diff when TEST_DIFF program is non-existant.
dataset/ETHPy150Open google/google-apputils/tests/basetest_test.py/GoogleTestBaseUnitTest.test_Diff_MissingExternalDiff