CODEWITHSUNDEEP
multithreadingqtmatplotlibpyqt
user3419537
user3419537

Reputation: 5000

How to keep PyQt GUI responsive when blocking tasks are GUI related?

I'm writing an app that embeds a bunch of matplotlib figures into a PyQt GUI. The updating of these figures can take up to a few seconds, so I would like to introduce a waiting indicator to display while the plots are being drawn. I've moved all the data processing code into its own thread, but it seems the actual plotting calls are often making up the majority of processing time.

I have written a waiting indicator that uses a QTimer instance to trigger paintEvent on the widget. This works just fine when all the intensive processing can be pushed into another thread. The problem is that these calls to construct the matplotlib plots cannot be moved outside of the main thread due to the way Qt is designed, and so block the updating of the waiting indicator, rendering it kind of useless.

I've introduced some calls to QCoreApplication.processEvents() after the updating of each figure, which improves the performance a little. I've also toyed with the idea of monkeypatching a bunch of methods of matplotlib.axes.Axes to include calls to QCoreApplication.processEvents(), but I can see that getting messy. Is this the best I can do? Is there any way to interrupt the main thread at regular intervals and force it to process new events?

Upvotes: 0

Views: 775

Answers (1)

Aaron
Aaron

Reputation: 1191

It should also help a big deal to do the actual drawing on a QPixmap in a thread. Drawing that pixmap with QPainters drawPixmap() method is very fast. And you need to recreate the pixmap only when really needed (e.g. after Zooming or so). In the meantime you just have to reuse that already drawn pixmap. The actual paintEvents using drawPixmap() will cost close to nothing and your GUI will be completely responsive.

Clobbering the code with processEvent() is not only ugly but can cause very nasty and very hard to debug malfunctions. E.g. it might cause premature deletion of objects which are still in use but were scheduled for deletion using deleteLater().

This Answer might be also of use: Python - matplotlib - PyQT: plot to QPixmap

I havn't used matplotlib, yet. But in case it uses directly QWidgets and can not be used without it won't be so easy as you mentioned above. But you could do the drawing in another process started by your GUI which uses matplotlib as in the link above and stores the pixmap to disk and your gui loads whenever a new pixmap is ready. QFileSystemWatcher might help here to avoid polling.

Upvotes: 1

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