def testHiddenException(self):
        """
        What happens if an error is raised in a DelayedCall and an error is
        also raised in the test?

        L{test_reporter.ErrorReportingTests.testHiddenException} checks that
        both errors get reported.

        Note that this behaviour is deprecated. A B{real} test would return a
        Deferred that got triggered by the callLater. This would guarantee the
        delayed call error gets reported.
        """
        reactor.callLater(0, self.go)
        reactor.iterate(0.01)
        self.fail("Deliberate failure to mask the hidden exception")

        What happens if an error is raised in a DelayedCall and an error is
        also raised in the test?

        L{test_reporter.ErrorReportingTests.testHiddenException} checks that
        both errors get reported.

        Note that this behaviour is deprecated. A B{real} test would return a
        Deferred that got triggered by the callLater. This would guarantee the
        delayed call error gets reported.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/trial/test/erroneous.py