Running Processing Script crashes QGIS3

Question

Trying to create a Python script that processes some data. I'm running this on QGIS 3.01 on MacOS 10.12.6. Having trouble with it as every time I run the below code, it completely crashes QGIS.

All I'm trying to do is run a series of processing algorithms. If I comment out the processing.run line, QGIS doesn't crash but also I can't do anything.

import processing
from qgis.PyQt.QtCore import QCoreApplication
from qgis.core import QgsProcessingAlgorithm, QgsProcessing, QgsProcessingParameterFeatureSink
class testAlg(QgsProcessingAlgorithm):
 OUTPUT = 'OUTPUT'
 def tr(self, text):
 return QCoreApplication.translate('testalg', text)
 def createInstance(self):
 return type(self)()
 def group(self):
 return self.tr('Test')
 def groupId(self):
 return 'test'
 def __init__(self):
 super().__init__()
 def initAlgorithm(self, config=None):
 self.addParameter(QgsProcessingParameterFeatureSink(self.OUTPUT, self.tr('Output'), QgsProcessing.TypeVectorPolygon))
 def name(self):
 return 'testalg'
 def displayName(self):
 return self.tr('Test Algorithm')
 def processAlgorithm(self, parameters, context, feedback):
 # Commenting out the next line stops QGIS from crashing, but it means I can't actually do anything
 processing.run('qgis:fieldcalculator', {'INPUT':'/Users/myusername/Documents/temp/test.geojson','FIELD_NAME':'test','FIELD_TYPE':0,'FIELD_LENGTH':10,'FIELD_PRECISION':6,'NEW_FIELD':True,'FORMULA':'( "count" / sum( "count" )) * 100','OUTPUT':'memory:'})
 return {}

Am I doing something horribly wrong in my script or is this a bug in QGIS 3?

--

Edit: By crash, I mean that QGIS freezes and becomes unresponsive. I have to Force Quit it and reopen it before I can do anything else.

Edit 2: This also freeze/crashes Windows 10.

• lcoandrade: I'm facing the same problem. I've made a processing scrip to perform K-Means classification with sklearn. The script works perfectly on my Linux installation, but crashes my QGIS on my Mac.

Answer 1

Use processing.run in this way:

# it'll be sensible to assing the result of processing to a variable
output = processing.run(
 'qgis:fieldcalculator',
 {'INPUT':'/Users/myusername/Documents/temp/test.geojson',
 'FIELD_NAME':'test',
 'FIELD_TYPE':0,
 'FIELD_LENGTH':10,
 'FIELD_PRECISION':6,
 'NEW_FIELD':True,
 'FORMULA':'( "count" / sum( "count" )) * 100',
 'OUTPUT':'memory:'},
 feedback=feedback, # add this line
 context=context)['OUTPUT'] # and this line # ['OUTPUT'] is optional

When I use in that way, it doesn't crash QGIS. Otherwise, QGIS freezes. (I tried in Windows 10)