How are callbacks scheduled within HexChat's Python scripting interface?

Question

In the HexChat Python scripting interface, are event callbacks guaranteed to run to completion before servicing another callback? My specific worry has to do with timer callbacks. If I write a callback to handle a server JOIN message, for instance, and, while that callback is executing, a previously-registered timer expires, will the callback associated with the timer expiry run before the JOIN callback finishes, or can I count on the JOIN callback running to completion before executing the timer callback? I'm assuming callbacks are queued and executed in their entirety in queue order, but the documentation is silent on this point.