Without more information, it is hard to know exactly what problem you are seeing but it appears you are trying to run pdb from within the tornado web server.  That may be difficult to do; I'm guessing that the pdb debugger main loop does not play well with an event loop in tornado.  You might want to ask first on the tornado mailing list: http://www.tornadoweb.org/en/stable/index.html.  Otherwise, please show exactly how to reproduce the behavior in pdb that you believe to be in error.