Async examples#

The Python language provides keywords for asynchronous operations, i.e., “async” to define coroutines or “await” to schedule asynchronous calls in the event loop (see PEP 492). It is up to packages to implement an event loop, support for these keywords, and more.

The best-known package for this is asyncio. Since both an async package and Qt itself work with event loops, special care must be taken to ensure that both event loops work with each other. asyncio offers a function stop that allows stopping an event loop without closing it. If it is called while a loop is running through run_forever, the loop will run the current batch of callbacks and then exit. New callbacks wil be scheduled the next time run_forever is called.

This approach is highly experimental and does not represent the state of the art of integrating Qt with asyncio. Instead it should rather be regarded more as a proof of concept to contrast asyncio with other async packages such as trio, which offers a dedicated low-level API for more complicated use cases such as this. Specifically, there exists a function start_guest_run that enables running the Trio event loop as a “guest” inside another event loop - Qt’s in our case.

Based on this functionality, two examples for async usage with Qt have been implemented: eratosthenes and minimal:

Async example: Eratosthenes
  • eratosthenes is a more extensive example that visualizes the Sieve of Eratosthenes algorithm. This algorithm per se is not one that is particularly suitable for asynchronous operations as it’s not I/O-heavy, but synchronizing coroutines to a configurable tick allows for a good visualization.

  • minimal is a minimal example featuring a button that triggers an asynchronous coroutine with a sleep. It is designed to highlight which boilerplate code is essential for an async program with Qt and offers a starting point for more complex programs.

Both examples feature:

  1. A window class.

  2. An AsyncHelper class containing start_guest_run plus helpers and callbacks necessary for its invocation. The entry point for the Trio/asyncio guest run is provided as an argument from outside, which can be any async function.

While eratosthenes offloads the asynchronous logic that will run in trio’s/asyncio’s event loop into a separate class, minimal demonstrates that async functions can be integrated into any class, including subclasses of Qt classes.

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