Plugin processAlgorithm. Same code, different results in script vs plugin code

Question

I'm adding functionality to a QGIS Plugin to create a print layout and add stuff to it like a map, legend, etc.

To test this code, I wrote a script. The script works and creates the print layout the way I want. Now I'm trying to add the code into the processAlgorithm() of my plugin's code. I'm getting different results with the same code from the script.

Here is what I have so far:

    def processAlgorithm(self, parameters, context, feedback):
        """This actually does the processing for creating the print layout and exporting as .pdf"""
        #needs all the arguments (self, parameters, context, feedback)

        log = feedback.setProgressText

        input_vector = self.parameterAsVectorLayer(parameters, self.INPUT_VECTOR, context)
        input_title = self.parameterAsString(parameters, self.INPUT_TITLE, context)

        log(f"Input vector name: {input_vector.name()}")            
        log(f"Title: {input_title}")                    


        """This creates a new print layout"""
        project = QgsProject.instance()             
        manager = project.layoutManager()           
        layout = QgsPrintLayout(project)            
        layoutName = "PrintLayout"

        layout = QgsPrintLayout(project)
        layout.initializeDefaults()                 
        layout.setName(layoutName)
        manager.addLayout(layout)



        """This adds a map item to the Print Layout"""
        map = QgsLayoutItemMap(layout)
        map.setRect(20, 20, 20, 20)  
        #Set Extent
        canvas = iface.mapCanvas()
        map.setExtent(canvas.extent())                  #sets map extent to current map canvas
        layout.addLayoutItem(map)
        #Move & Resize
        map.attemptMove(QgsLayoutPoint(5, 27, QgsUnitTypes.LayoutMillimeters))
        map.attemptResize(QgsLayoutSize(239, 178, QgsUnitTypes.LayoutMillimeters))

        results = {}
        return results

When I run this part of the code in the script, I get a nice map in my print layout. It looks like this:

When I run my plugin with the code above, I get this: This map is the correct size and in the correct position. But obviously there is no map and something is broken.

Here is the log after I run the plugin:

Is the problem in the way I return the "results" from the processAlgorithm? In the QGIS documentation it says this:

However, I don't want to return anything. I just want to create the print layout with items on it. There is no output from this part of the plugin.

I just create "results" variable as an empty dictionary and return it because I am supposed to return a dictionary. So obviously I am missing something.

Answer 1

I think the reason is that your algorithm is being executed in a background thread, which is not allowed for print layouts. You'll need to adapt your algorithm and ensure it returns the FlagNoThreading flag:

     def flags(self):
        return super().flags() | QgsProcessingAlgorithm.FlagNoThreading