I have a snippet that intersects two map layers (blockLayer and ogLayer) in QGIS; and prints out when a bad geometry prevents the intersection from occurring. It builds a subclass from QgsProcessingFeedback, based on the answer posted here by @ndawson.

Here's the code:

class MyFeedBack(QgsProcessingFeedback): # create a subclass from the QgsProcessingFeedback class
    def reportError(self, error, fatalError=False): # QgsProcessingFeedback.reportError() is a method 
        if "invalid geometry" in error:
            featNum = error.split("(")[1].split(")")[0]
            print("{} has bad geometry".format(featNum))
blockLayer = QgsProject.instance().mapLayersByName("harvestApprovals")[0]
ogLayer = QgsProject.instance().mapLayersByName('Boundary old growth .gdb layer')[0]

#feedback = QgsProcessingFeedback.reportError(self, error, fatalError=False)
#feedback = QgsProcessingFeedback.reportError(error, fatalError=False)

feedback = QgsProcessingFeedback() # instantitate the QgsProcessingFeedback class

params = {'INPUT':blockLayer,'OVERLAY':ogLayer,'OUTPUT':'TEMPORARY_OUTPUT'}
rslt = processing.run("native:intersection", params, feedback=MyFeedBack())


I'd prefer to create a list to add those numbers to, so as to know which geometries need repair. I'm just not sure how (or if it's possible) to build a list within an instance of a class, and somehow return that list from the class.

Is there some way I could myList.append(featNum) as the errors are encountered, then get myList back after the process has run?

1 Answer 1


When I run your script, print statement never run although I get an error. Thus, I couldn't test it. But the following way should work for you.

  • Add a variable (myList = []) to root scope.
  • Use it in reportError method with global keyword.
myList = []

class MyFeedBack(QgsProcessingFeedback): 
    def reportError(self, error, fatalError=False): 
        if "invalid geometry" in error:
            featNum = error.split("(")[1].split(")")[0]

            global myList

# other lines


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