def assertWrapperExceptionTypes(self, deferred, mainType, reasonTypes):
    """
    Assert that the given L{Deferred} fails with the exception given by
    C{mainType} and that the exceptions wrapped by the instance of C{mainType}
    it fails with match the list of exception types given by C{reasonTypes}.

    This is a helper for testing failures of exceptions which subclass
    L{_newclient._WrapperException}.

    @param self: A L{TestCase} instance which will be used to make the
        assertions.

    @param deferred: The L{Deferred} which is expected to fail with
        C{mainType}.

    @param mainType: A L{_newclient._WrapperException} subclass which will be
        trapped on C{deferred}.

    @param reasonTypes: A sequence of exception types which will be trapped on
        the resulting C{mainType} exception instance's C{reasons} sequence.

    @return: A L{Deferred} which fires with the C{mainType} instance
        C{deferred} fails with, or which fails somehow.
    """
    def cbFailed(err):
        for reason, type in zip(err.reasons, reasonTypes):
            reason.trap(type)
        self.assertEqual(len(err.reasons), len(reasonTypes),
                         "len(%s) != len(%s)" % (err.reasons, reasonTypes))
        return err
    d = self.assertFailure(deferred, mainType)
    d.addCallback(cbFailed)
    return d

    Assert that the given L{Deferred} fails with the exception given by
    C{mainType} and that the exceptions wrapped by the instance of C{mainType}
    it fails with match the list of exception types given by C{reasonTypes}.

    This is a helper for testing failures of exceptions which subclass
    L{_newclient._WrapperException}.

    @param self: A L{TestCase} instance which will be used to make the
        assertions.

    @param deferred: The L{Deferred} which is expected to fail with
        C{mainType}.

    @param mainType: A L{_newclient._WrapperException} subclass which will be
        trapped on C{deferred}.

    @param reasonTypes: A sequence of exception types which will be trapped on
        the resulting C{mainType} exception instance's C{reasons} sequence.

    @return: A L{Deferred} which fires with the C{mainType} instance
        C{deferred} fails with, or which fails somehow.
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/test/test_newclient.py