def test_passThrough(self):
        """
        If an unknown errno is passed to L{ftp.errnoToFailure}, it should let
        the originating exception pass through.
        """
        try:
            raise RuntimeError("bar")
        except:
            d = ftp.errnoToFailure(-1, "foo")
            return self.assertFailure(d, RuntimeError)

        If an unknown errno is passed to L{ftp.errnoToFailure}, it should let
        the originating exception pass through.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/test/test_ftp.py