[Qgis-user] PyGQIS output a layer? v3.16

[Richard Duivenvoorde]

On 8/24/21 12:57 PM, Andrew Hughes wrote:
> Hi All,
> 
> I am trying to create a python script that takes a URL as input and creates/outputs an ArcGIS Feature Service Vector Layer (used for further processing downstream). However,  I believe I am doing something wrong with how I am trying to return the layer.
> 
> The script in question can be found here:
> 
> https://github.com/ahhughes/pyqgisorama/blob/main/CreateArcGISFeatureLayer.py#L132 <https://github.com/ahhughes/pyqgisorama/blob/main/CreateArcGISFeatureLayer.py#L132>

Hi Andrew,

I'm not so experienced either in writing algorithms, but if I am right, you are not supposed to return a layer, but you need to return a 'sink' as it is possible that it will be used as input for another algorithm.
So I used:
self.parameterAsSink

Note there is also:
self.parameterAsFileOutput

Did some googling with 'pyqgis layer sink output' and came up with this (and the rest of your GIST):


    def processAlgorithm(self, parameters, context, feedback):
        """
        Here is where the processing itself takes place.
        """

        url = parameters[self.INPUT].strip()
        
        #out = self.parameterAsFileOutput(parameters, self.OUTPUT, context)
        
        lyr = QgsVectorLayer("crs='EPSG:3857' url='"+url+"'", "RT",  "arcgisfeatureserver")
        
        feedback.pushInfo("Layer has been created: "+str(lyr))
        feedback.pushInfo("Is the layer valid?: "+str(lyr.isValid()))
        
        (sink, dest_id) = self.parameterAsSink(
            parameters,
            self.OUTPUT,
            context,
            lyr.fields(),
            lyr.wkbType(),
            lyr.sourceCrs()
        )
        
        for f in lyr.getFeatures():
            sink.addFeature(f)
        
        return {self.OUTPUT: sink}


Which at least for me loads some lines :-)

I hope that others can maybe more context about the differences between parameterAsSink and parameterAsFileOutput.
If not, this would be maybe more a question for the dev list?

Regards,

Richard Duivenvoorde