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Editorial Team
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Asked: 2026-05-30T23:44:18+00:00

I’ve been playing around with the Tornado chat demo . At a casual glance

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I’ve been playing around with the Tornado chat demo. At a casual glance it seems like the new_messages method is not thread safe – it seems like items might get added to the waiters array while that same array is being iterated in the for loop.

Is this demo not thread safe? Or, is it thread safe simply because the Python set object is itself thread-safe? Are Python set objects thread safe? I seem to find conflicting opinions on this question (and the word set is demonically difficult to search for effectively in Google!)

Bonus points – why is the waiters array set to a new set at the conclusion of the iteration instead of emptying the set?

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1 Answer

  1. Editorial Team

    There are no threads involved in Tornado applications by default. Tornado is an event based system, so there is only one execution path. The thing that you will need to figure out about tornado is at what moments you yield execution back to the IOLoop.

    While the GIL does protect against a class of thread errors, you still can write applications that access and modify the data outside the access path of the program.

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