Gracefully handling application exceptions in a Tornado application

tornado

Solution

First of all, the exception that you are raising in `prepare` has code `200`, therefore it's not caught in the `get_error_html` function.

Secondly, `get_error_html` is deprecated: use `write_error`, instead (write_error).

Finally, you don't need to call `__init__` on `ErrorHandler`: to initialize a handler use `initialize` (initialize), but in this case you don't need it.

Here is a working example:

import tornado
import tornado.web


class ErrorHandler(tornado.web.RequestHandler):
    """Generates an error response with status_code for all requests."""

    def write_error(self, status_code, **kwargs):
        print 'In get_error_html. status_code: ', status_code
        if status_code in [403, 404, 500, 503]:
            self.write('Error %s' % status_code)
        else:
            self.write('BOOM!')

    def prepare(self):
        print 'In prepare...'
        raise Exception('Error!')


application = tornado.web.Application([
        (r"/", ErrorHandler),
        ])

if __name__ == "__main__":
    application.listen(8899)
    tornado.ioloop.IOLoop.instance().start()

Problem

Based on some googling I installed the following error handler. However the python exceptions which appear to return a http 500 are not trapped by this stuff, although 404's are. With the print statements I have left in the code below, I can see that it does not hit any of these routines. What should I really be doing? ``` class ErrorHandler(tornado.web.RequestHandler): """Generates an error response with status_code for all requests.""" def __init__ (self, application, request, status_code): print 'In ErrorHandler init' tornado.web.RequestHandler.__init__(self, application, request) self.set_status(status_code) def get_error_html (self, status_code, **kwargs): print 'In get_error_html. status_code: ', status_code if status_code in [403, 404, 500, 503]: filename = '%d.html' % status_code print 'rendering filename: ', filename return self.render_string(filename, title=config.get_title()) return "<html><title>%(code)d: %(message)s</title>" \ "<body class='bodyErrorPage'>%(code)d: %(message)s</body>"\ "</html>" % { "code": status_code, "message": httplib.responses[status_code], } def prepare (self): print 'In prepare...' raise tornado.web.HTTPError(self._status_code) ```

Original source