Question

I write a python class which makes asynchronous method calls using D-Bus. When my reply_handler is called, it stores data in list. This list can be used by another class methods at the same time. Is it safe or I can use only synchronized data structures like Queue class?

Answer 1

If you do not modify the list outside of the callback context, then you do not necessarily need synchronization – you will just need to be aware that the list object’s state is volatile.

If the list must be modified both in the callback handler as well as, say, the main execution context (or other threads, etc.), then yes you will need synchronization.

The Python synchronized Queue works naturally for message pumps – allowing you to perform actions sequentially in the order that the events come in one of your own contexts. This benefits code simplicity and readability as well since major state changes are easier to track. Callbacks generally shouldn’t be too complicated anyway as the outside context in which the callbacks are called shouldn’t (and probably doesn’t) have to deal with exceptions raised from your code. There are also potential timing considerations as well – the callback will block the async emitter’s context – so keeping the handler short and sweet is also good.