'reportError' in subclass of QgsProcessingFeedback never running

Question

I have two layers, layer_a and layer_b. Both have invalid geometries. When I run a processing tool, it throws an error as expected.

Based on the answer posted here by @ndawson, if you subclass QgsProcessingFeedback, you can implement your own custom logic dealing with the error.

class MyFeedBack(QgsProcessingFeedback):

    def reportError(self, error, fatalError=False):
        print(error)

...

res = processing.run("TOOL_NAME", params, feedback=MyFeedBack())

When I run the following script in QGIS Python Editor

class MyFeedBack(QgsProcessingFeedback): 
    def reportError(self, error, fatalError=False): 
        print("ERROR! ERROR!")
            
layer_a = QgsProject.instance().mapLayersByName("a")[0]
layer_b = QgsProject.instance().mapLayersByName('b')[0]

feedback = MyFeedBack()

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

print("\nDone!")

I get this error:

Traceback (most recent call last):
  File "/usr/lib/python3.8/code.py", line 90, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
  File "<string>", line 9, in <module>
  File "/usr/share/qgis/python/plugins/processing/tools/general.py", line 108, in run
    return Processing.runAlgorithm(algOrName, parameters, onFinish, feedback, context)
  File "/usr/share/qgis/python/plugins/processing/core/Processing.py", line 159, in runAlgorithm
    ret, results = execute(alg, parameters, context, feedback, catch_exceptions=False)
  File "/usr/share/qgis/python/plugins/processing/gui/AlgorithmExecutor.py", line 71, in execute
    results, ok = alg.run(parameters, context, feedback, {}, False)
_core.QgsProcessingException: Feature (0) from “b” has invalid geometry. Please fix the geometry or change the Processing setting to the “Ignore invalid input features” option.

According to documentation, reportError method should run in case of any error. But the method never runs although the processing tool throws an error.

What am I doing wrong?

I use
QGIS 3.16.2-Hannover, Ubuntu 20.04.1 LTS
QGIS 3.16.0-Hannover, Windows 10 x64

Answer 1

If you check the lines between 61 and 71 of the algorithmExecutor, you can see that there is a condition with the catch_exceptions test. For some reasons, in the runAlgorithm of Processing.py the catch_exceptions is set to False. So the error is never transmitted by the reportError method (line 68)

So you could submit an issue to ask if it's normal. To me, it seems like a bug.

EDIT : According to core dev, it's not. See his response : https://github.com/qgis/QGIS/issues/40978