How to set Debug mode in Tornado?

Question

I googled and found a hint that I should set debug mode in Tornado so that it could reload files automatically on update. But I didn't find an exact string to insert. I tried different combinations similar to this application.settings = {"Debug": True} but it doesn't help.

i think it should be {"debug":True} – jondinham Mar 02 '12 at 05:52 — jondinham, Mar 02 '12 at 05:52

Jon Haddad · Answer 1 · 2012-05-01T20:00:21.633

15

Here's a tweaked example from the tornado site:

if __name__ == "__main__":
    application = tornado.web.Application([
        (r"/", MainHandler) 
    ], debug=True)
    application.listen(8888)
    tornado.ioloop.IOLoop.instance().start()

edited May 01 '12 at 20:00

answered Apr 04 '12 at 22:06

Jon Haddad

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"debug=True" has to be outside of the list of handlers for it to work---like this: "...],debug=True)" – Hemm Apr 17 '12 at 04:18
Good catch, I had written it without checking it. Updated my answer accordingly. – Jon Haddad May 01 '12 at 20:01

score 0 · Answer 2 · answered Jun 03 '19 at 09:29

For those who like the debug=True option and using Tornado-multiprocess the following error will be raised:

Traceback (most recent call last):
  File "./main.py", line 54, in <module>
    server.start(0)  # forks one process per cpu
  File "/Users/me/Library/Python/2.7/lib/python/site-packages/tornado/tcpserver.py", line 221, in start
    process.fork_processes(num_processes)
  File "/Users/me/Library/Python/2.7/lib/python/site-packages/tornado/process.py", line 130, in fork_processes
    raise RuntimeError("Cannot run in multiple processes: IOLoop instance "
RuntimeError: Cannot run in multiple processes: IOLoop instance has already been initialized. You cannot call IOLoop.instance() before calling start_processes()

This is because debug=True load IOLoop and it cannot be loaded twice.

According to the documentation

Setting debug=True is equivalent to autoreload=True, compiled_template_cache=False, static_hash_cache=False, serve_traceback=True.

So when using debug=True Tornado also sets ( for convenience ) another flag: autoreload=True which "watch" for changes and reload the server.

The autoreload option is the one that cannot be turned on when using multi-process. So you need to configure it like so:

if __name__ == "__main__":
    application = tornado.web.Application([
        (r"/", MainHandler) 
    ], debug=True, autoreload=False)
    application.listen(8888)
    tornado.ioloop.IOLoop.instance().start()

How to set Debug mode in Tornado?

2 Answers2