@Trüff said

but I used the class Image from the PIL module instead of the one from Pythonista‘s ui module and that worked

👍

It's possible that the issue you're experiencing with the canvas not displaying in Pythonista 3 could be related to changes or updates in the Pythonista environment. However, without more specific information about your code and environment, it's challenging to pinpoint the exact cause.

Here are a few troubleshooting steps you can take:

Check the Code: Make sure the code you're running is correct and compatible with Pythonista 3. Some differences in the Python 2 and Python 3 syntax can affect graphics rendering.

Update Pythonista: Ensure that you are using the latest version of Pythonista, as updates may include bug fixes and improvements. Check the App Store for any available updates.

Check for Python 3 Compatibility: If the code is written for Python 2, it may need some modifications to work correctly in Pythonista 3, which uses Python 3. Make sure any print statements, division, and other syntax differences are addressed.

Canvas Settings: Review any canvas settings and ensure that you're specifying the canvas size and parameters correctly.

Community Forums: Check the Pythonista community forums or contact their support for specific help related to Pythonista 3, as other users may have encountered and resolved similar issues.

If you've verified these points and are still experiencing issues, it might be worthwhile to reach out to the Pythonista support community or directly to the Pythonista developers for assistance with this specific problem, as they can provide more in-depth troubleshooting and guidance.

Note to self (and others so afflicted)—
Starting with this seemed to fix it:
set_fill_color(1.0,1.0,1.0)
fill_rect(0,0,scrSize[0],scrSize[1])

where i've done this at global scope:
scrSize=(1136,640) # iPhone 5
set_size(*scrSize) # min 16, max 2048

(my canvas is meant for landscape viewing)

https://gist.github.com/389a67c5aacb097b87fd

Simply import this function, and run it after drawing to the canvas. see example.
It is not necessary to fill the canvas white- it is actually transparent, unless you have drawn to it (as should be obvious when you save an image then quickview it)