def METHOD_NAME(self):
    class Test(MixedRTStruct):
        field1 = Field(str, default='42', frozen=True)
        field2 = Field(bool)
    t1 = Test(field1='field1', field2=True, spam='ham')
    with self.assertRaisesRegex(ValueError, 'cannot assign'):
        t1.update(field1='spam')
    with self.assertRaisesRegex(ValueError, 'cannot assign'):
        t1.field1 = 'aaa'
    self.assertEqual(t1.field1, 'field1')
    self.assertEqual(t1.field2, True)
    t1.field2 = False
    self.assertEqual(t1.field2, False)