[QGIS-Developer] Access QgsVectorLayer from child algorithm

[Simon Gröchenig]

Hi developers,

I am developing a Python processing algorithm with some child 
algorithms. I am struggling to find a way to access a child algorithm's 
output vector layer. I want to use isChildAlgorithm=True in order to 
correctly set the feedback progress.

* If I use processing.run(...) with setting the parameter 
isChildAlgorithm to False (= default), the vector layer (via sink) is 
returned as a QgsVectorLayer.
* If I set isChildAlgorithm to True, only the dest_id is returned as a 
string. I did not find a solution on how I retrieve the corresponding 
QgsVectorLayer.
* Is it intentional that the vector layer is returned as QgsVectorLayer 
or str (dest_id), depending on the isChildAlgorithm parameter?
* How can I access output QgsVectorLayer from a child algorithm?

This is a minimum example where you can modify the is_child_algorithm 
parameter:

parameters = {
     'INPUT': QgsVectorLayer('Point?crs=epsg:31258', 'locations', "memory"),
     'DISTANCE': 10,
     'OUTPUT': 'TEMPORARY_OUTPUT'
}
result = processing.run("native:buffer", parameters, 
is_child_algorithm=False )
vector_layer = result['OUTPUT']
print(str(type(vector_layer)))

Best regards
Simon

-- 
Simon Gröchenig MSc
MTA - Mobility & Transport Analytics

Salzburg Research Forschungsgesellschaft mbH
Jakob-Haringer-Straße 5/3 | 5020 Salzburg, Austria
T: +43.662.2288-314
simon.groechenig at salzburgresearch.at
http://www.salzburgresearch.at
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