Use contextGenerator in refactor fields algorithm

python/plugins/processing/algs/qgis/FieldsMapper.py

@@ -78,10 +78,7 @@ class FieldsMapper(GeoAlgorithm):
         da.setEllipsoid(QgsProject.instance().readEntry(
             'Measure', '/Ellipsoid', GEO_NONE)[0])
 
-        exp_context = QgsExpressionContext()
-        exp_context.appendScope(QgsExpressionContextUtils.globalScope())
-        exp_context.appendScope(QgsExpressionContextUtils.projectScope())
-        exp_context.appendScope(QgsExpressionContextUtils.layerScope(layer))
+        exp_context = layer.createExpressionContext()
 
         for field_def in mapping:
             fields.append(QgsField(name=field_def['name'],
@@ -93,18 +90,12 @@ class FieldsMapper(GeoAlgorithm):
             expression.setGeomCalculator(da)
             expression.setDistanceUnits(QgsProject.instance().distanceUnits())
             expression.setAreaUnits(QgsProject.instance().areaUnits())
-
+            expression.prepare(exp_context)
             if expression.hasParserError():
                 raise GeoAlgorithmExecutionException(
                     self.tr(u'Parser error in expression "{}": {}')
-                    .format(str(field_def['expression']),
-                            str(expression.parserErrorString())))
-            expression.prepare(exp_context)
-            if expression.hasEvalError():
-                raise GeoAlgorithmExecutionException(
-                    self.tr(u'Evaluation error in expression "{}": {}')
-                    .format(str(field_def['expression']),
-                            str(expression.evalErrorString())))
+                    .format(unicode(expression.expression()),
+                            unicode(expression.parserErrorString())))
             expressions.append(expression)
 
         writer = output.getVectorWriter(fields,
@@ -112,8 +103,7 @@ class FieldsMapper(GeoAlgorithm):
                                         layer.crs())
 
         # Create output vector layer with new attributes
-        error = ''
-        calculationSuccess = True
+        error_exp = None
         inFeat = QgsFeature()
         outFeat = QgsFeature()
         features = vector.features(layer)
@@ -132,8 +122,7 @@ class FieldsMapper(GeoAlgorithm):
                 exp_context.lastScope().setVariable("row_number", rownum)
                 value = expression.evaluate(exp_context)
                 if expression.hasEvalError():
-                    calculationSuccess = False
-                    error = expression.evalErrorString()
+                    error_exp = expression
                     break
 
                 attrs.append(value)
@@ -145,7 +134,8 @@ class FieldsMapper(GeoAlgorithm):
 
         del writer
 
-        if not calculationSuccess:
+        if error_exp is not None:
             raise GeoAlgorithmExecutionException(
-                self.tr('An error occurred while evaluating the calculation'
-                        ' string:\n') + error)
+                self.tr(u'Evaluation error in expression "{}": {}')
+                    .format(unicode(error_exp.expression()),
+                            unicode(error_exp.parserErrorString())))

python/plugins/processing/algs/qgis/ui/FieldsMappingPanel.py

@@ -35,7 +35,7 @@ from qgis.PyQt.QtGui import QBrush, QIcon, QSpacerItem
 from qgis.PyQt.QtWidgets import QComboBox, QHeaderView, QLineEdit, QMessageBox, QSpinBox, QStyledItemDelegate
 from qgis.PyQt.QtCore import QItemSelectionModel, QAbstractTableModel, QModelIndex, QVariant, Qt, pyqtSlot
 
-from qgis.core import QgsExpression, QgsExpressionContextUtils, QgsApplication, QgsFeature
+from qgis.core import QgsExpression, QgsProject, QgsApplication
 from qgis.gui import QgsFieldExpressionWidget
 
 from processing.gui.wrappers import WidgetWrapper, DIALOG_STANDARD, DIALOG_MODELER
@@ -87,21 +87,30 @@ class FieldsMappingModel(QAbstractTableModel):
     def testExpression(self, row):
         self._errors[row] = None
         field = self._mapping[row]
+        exp_context = self.contextGenerator().createExpressionContext()
+
         expression = QgsExpression(field['expression'])
+        expression.prepare(exp_context)
         if expression.hasParserError():
             self._errors[row] = expression.parserErrorString()
             return
+
+        # test evaluation on the first feature
         if self._layer is None:
             return
-        context = QgsExpressionContextUtils.createFeatureBasedContext(QgsFeature(), self._layer.fields())
         for feature in self._layer.getFeatures():
-            context.setFeature(feature)
-            expression.evaluate(context)
+            exp_context.setFeature(feature)
+            exp_context.lastScope().setVariable("row_number", 1)
+            expression.evaluate(exp_context)
             if expression.hasEvalError():
                 self._errors[row] = expression.evalErrorString()
-                return
             break
 
+    def contextGenerator(self):
+        if self._layer:
+            return self._layer
+        return QgsProject.instance()
+
     def layer(self):
         return self._layer
 
@@ -254,6 +263,7 @@ class FieldDelegate(QStyledItemDelegate):
         elif fieldType == QgsExpression:
             editor = QgsFieldExpressionWidget(parent)
             editor.setLayer(index.model().layer())
+            editor.registerExpressionContextGenerator(index.model().contextGenerator())
             editor.fieldChanged.connect(self.on_expression_fieldChange)
 
         else: