warning as the 'warning' attribute; similarly, the 'filename'
           and 'lineno' attributes give you information about the line
           of Python code from which the warning was triggered.
           This allows you to inspect the warning after the assertion::

               with self.assertWarns(SomeWarning) as cm:
                   do_something()
               the_warning = cm.warning
               self.assertEqual(the_warning.some_attribute, 147)
        