Using Threads in PyQGIS3

While porting a plugin to QGIS3 I decided to also move all it’s threading infrastructure to QgsTasks. Here three possible variants to implement this.
the first uses the static method QgsTask.fromFunction and is simpler to use. A great quick solution. If you want need control you can look at the second solution that subclasses QgsTask. In this solution I also show how to create subtasks with interdependencies. The third variant, illustrates how to run a processing algorithm in a separate thread.

One thing to be very careful about is never to create widgets or alter gui in a task. This is a strict Qt guideline – gui must never be altered outside of the main thread. So your progress dialog must operate on the main thread, connecting to the progress report signals from the task which operates in the background thread. This also applies to “print” statements — these aren’t safe to use from a background thread in QGIS and can cause random crashes. Use the thread safe QgsMessageLog.logMessage() approach instead. Actually you should forget print and always use QgsMessageLog.

using QgsTask.fromFunction

this is a quick and simple way of running a function in a separate thread. When calling QgsTask.fromFunction() you can pass an on_finished argument with a callback to be executed at the end of run.

Subclassing QgsTask

this solution gives you the full control over the task behaviour. In this example I also illustrate how to create subtasks dependencies.

NEVER, EVER, EVER use print in the QgsTask outside from finished(). finished() is called on the main event loop

Call a Processing algorithm in a separate thread

You can simply execute a processing algorithm in a separate thread thanks to QgsProcessingAlgRunnerTask. This class takes a processing algorithm, its parameters, a context and a feedback objects and execute the algorithm. QgsProcessingAlgRunnerTask offers an executed signal to which you can connect and execute further code. executed sends two arguments bool successful and dict results. If you want to retrieve a memory layer you can pass the context as well by using partial or lambda.

If you’re wondering what parameter values you need to specify for an algorithm, and what values are acceptable, try running processing.algorithmHelp('qgis:randompointsinextent') in the python console. In QGIS 3.2 you’ll get a detailed list of all the parameter options for the algorithm and a summary of acceptable value types and formats for each. Another nice possibility is to run the algorithm from the gui and check the history after.

I hope this post can help you porting your plugins to QGIS3 and again if you need professional help for your plugins, don’t hesitate to contact us.

Posted in Non-commercial, PyQt, Python, QGIS

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