Custom 404 Errorpage with Python tornado

Question

I'm trying to implement a custom 404 page with python tornado. The docs say: "default_handler_class and default_handler_args: This handler will be used if no other match is found; use this to implement custom 404 pages (new in Tornado 3.2).". There is no good information about how the handler needs to work. I tried using this code:

    class defaultHandler(tornado.web.RequestHandler):
        def __init__(self, arg2, arg3):
            print("Called default handler")
            self.arg2 = arg2
            self.arg3 = arg3
    
        def get(self):
            self.write("404 - my thing")

Using this tornado config:

 return tornado.web.Application([
        (r"/main", MainHandler),
        (r"/update_kurse", updateCallback),
        (r"/", LoginPage),
    ], static_path=os.path.join(os.path.dirname(__file__), "static"),
    template_path=os.path.join(os.path.dirname(__file__), "templates"),
    default_handler_class=defaultHandler)

This results in an error:

ERROR:tornado.application:Uncaught exception
Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\http1connection.py", line 273, in _read_message
    delegate.finish()
  File "C:\Users\******\lib\site-packages\tornado\routing.py", line 268, in finish
    self.delegate.finish()
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 2297, in finish
    self.execute()
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 2337, in execute
    return self.handler._prepared_future
AttributeError: 'defaultHandler' object has no attribute '_prepared_future'
ERROR:tornado.application:Error in exception logger
Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1665, in _execute
    if self.request.method not in self.SUPPORTED_METHODS:
AttributeError: 'function' object has no attribute 'method'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1752, in _handle_request_exception
    self.log_exception(*sys.exc_info())
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1790, in log_exception
    self._request_summary(),
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1740, in _request_summary
    self.request.method,
AttributeError: 'function' object has no attribute 'method'
ERROR:tornado.application:Exception in exception handler
Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1665, in _execute
    if self.request.method not in self.SUPPORTED_METHODS:
AttributeError: 'function' object has no attribute 'method'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1708, in _execute
    self._handle_request_exception(e)
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1757, in _handle_request_exception
    if self._finished:
AttributeError: 'defaultHandler' object has no attribute '_finished'
ERROR:asyncio:Exception in callback _HandlerDelegate.execute.<locals>.<lambda>(<Task finishe...ed_future'",)>) at C:\Users\******\lib\site-packages\tornado\web.py:2333
handle: <Handle _HandlerDelegate.execute.<locals>.<lambda>(<Task finishe...ed_future'",)>) at C:\Users\******\lib\site-packages\tornado\web.py:2333>
Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1665, in _execute
    if self.request.method not in self.SUPPORTED_METHODS:
AttributeError: 'function' object has no attribute 'method'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\******\lib\asyncio\events.py", line 145, in _run
    self._callback(*self._args)
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 2333, in <lambda>
    fut.add_done_callback(lambda f: f.result())
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1714, in _execute
    if self._prepared_future is not None and not self._prepared_future.done():
AttributeError: 'defaultHandler' object has no attribute '_prepared_future'
Called default handler
ERROR:tornado.application:Uncaught exception
Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\http1connection.py", line 273, in _read_message
    delegate.finish()
  File "C:\Users\******\lib\site-packages\tornado\routing.py", line 268, in finish
    self.delegate.finish()
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 2297, in finish
    self.execute()
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 2337, in execute
    return self.handler._prepared_future
AttributeError: 'defaultHandler' object has no attribute '_prepared_future'
ERROR:tornado.application:Error in exception logger
Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1665, in _execute
    if self.request.method not in self.SUPPORTED_METHODS:
AttributeError: 'function' object has no attribute 'method'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1752, in _handle_request_exception
    self.log_exception(*sys.exc_info())
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1790, in log_exception
    self._request_summary(),
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1740, in _request_summary
    self.request.method,
AttributeError: 'function' object has no attribute 'method'
ERROR:tornado.application:Exception in exception handler
Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1665, in _execute
    if self.request.method not in self.SUPPORTED_METHODS:
AttributeError: 'function' object has no attribute 'method'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1708, in _execute
    self._handle_request_exception(e)
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1757, in _handle_request_exception
    if self._finished:
AttributeError: 'defaultHandler' object has no attribute '_finished'
ERROR:asyncio:Exception in callback _HandlerDelegate.execute.<locals>.<lambda>(<Task finishe...ed_future'",)>) at C:\Users\******\lib\site-packages\tornado\web.py:2333
handle: <Handle _HandlerDelegate.execute.<locals>.<lambda>(<Task finishe...ed_future'",)>) at C:\Users\******\lib\site-packages\tornado\web.py:2333>
Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1665, in _execute
    if self.request.method not in self.SUPPORTED_METHODS:
AttributeError: 'function' object has no attribute 'method'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\******\lib\asyncio\events.py", line 145, in _run
    self._callback(*self._args)
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 2333, in <lambda>
    fut.add_done_callback(lambda f: f.result())
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1714, in _execute
    if self._prepared_future is not None and not self._prepared_future.done():
AttributeError: 'defaultHandler' object has no attribute '_prepared_future'
Called default handler
ERROR:tornado.application:Uncaught exception
Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\http1connection.py", line 273, in _read_message
    delegate.finish()
  File "C:\Users\******\lib\site-packages\tornado\routing.py", line 268, in finish
    self.delegate.finish()
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 2297, in finish
    self.execute()
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 2337, in execute
    return self.handler._prepared_future
AttributeError: 'defaultHandler' object has no attribute '_prepared_future'
ERROR:tornado.application:Error in exception logger
Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1665, in _execute
    if self.request.method not in self.SUPPORTED_METHODS:
AttributeError: 'function' object has no attribute 'method'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1752, in _handle_request_exception
    self.log_exception(*sys.exc_info())
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1790, in log_exception
    self._request_summary(),
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1740, in _request_summary
    self.request.method,
AttributeError: 'function' object has no attribute 'method'
ERROR:tornado.application:Exception in exception handler
Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1665, in _execute
    if self.request.method not in self.SUPPORTED_METHODS:
AttributeError: 'function' object has no attribute 'method'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1708, in _execute
    self._handle_request_exception(e)
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1757, in _handle_request_exception
    if self._finished:
AttributeError: 'defaultHandler' object has no attribute '_finished'
ERROR:asyncio:Exception in callback _HandlerDelegate.execute.<locals>.<lambda>(<Task finishe...ed_future'",)>) at C:\Users\******\lib\site-packages\tornado\web.py:2333
handle: <Handle _HandlerDelegate.execute.<locals>.<lambda>(<Task finishe...ed_future'",)>) at C:\Users\******\lib\site-packages\tornado\web.py:2333>
Traceback (most recent call last):
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1665, in _execute
    if self.request.method not in self.SUPPORTED_METHODS:
AttributeError: 'function' object has no attribute 'method'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\******\lib\asyncio\events.py", line 145, in _run
    self._callback(*self._args)
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 2333, in <lambda>
    fut.add_done_callback(lambda f: f.result())
  File "C:\Users\******\lib\site-packages\tornado\web.py", line 1714, in _execute
    if self._prepared_future is not None and not self._prepared_future.done():
AttributeError: 'defaultHandler' object has no attribute '_prepared_future'

What am I doing wrong? I can't find any example of a working 404 page with python tornado. I looked into routers, but that seemed not to fit here.

Answer 1

You should never override __init__ without calling the superclass's __init__ method. RequestHandler.__init__ takes two arguments which you must pass along. For this reason, it is conventional to override the RequestHandler.initialize method instead of __init__. But it's not clear what arg2 and arg3 are in this example, so I'm not sure why you're overriding this method at all.

A conventional 404 handler would look like this:

class DefaultHandler(RequestHandler):
    def prepare(self):
        # Use prepare() to handle all the HTTP methods
        self.set_status(404)
        self.finish("404 - my thing")