[processing] Don't throw an exception if an expression cannot

nyalldawson · nyalldawson · commit a8110369a80a · 2018-02-15T15:43:26.000+11:00
be prepared There's cases where this happens without reflecting an invalid expression. So we can try to prepare, but not abort if the preparation fails. Fixes #18103
diff --git a/python/plugins/processing/algs/qgis/FieldsCalculator.py b/python/plugins/processing/algs/qgis/FieldsCalculator.py
@@ -123,9 +123,7 @@ def processAlgorithm(self, parameters, context, feedback):
         if layer is not None:
             exp_context.appendScope(QgsExpressionContextUtils.layerScope(layer))
 
-        if not expression.prepare(exp_context):
-            raise QgsProcessingException(
-                self.tr('Evaluation error: {0}').format(expression.parserErrorString()))
+        expression.prepare(exp_context)
 
         features = source.getFeatures()
         total = 100.0 / source.featureCount() if source.featureCount() else 0
diff --git a/python/plugins/processing/algs/qgis/RandomPointsPolygons.py b/python/plugins/processing/algs/qgis/RandomPointsPolygons.py
@@ -116,9 +116,7 @@ def processAlgorithm(self, parameters, context, feedback):
             raise QgsProcessingException(expression.parserErrorString())
 
         expressionContext = self.createExpressionContext(parameters, context, source)
-        if not expression.prepare(expressionContext):
-            raise QgsProcessingException(
-                self.tr('Evaluation error: {0}').format(expression.evalErrorString()))
+        expression.prepare(expressionContext)
 
         fields = QgsFields()
         fields.append(QgsField('id', QVariant.Int, '', 10, 0))
