[processing] Allow empty expressions in refactor fields alg

Fixes #15640

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

@@ -32,7 +32,8 @@ from qgis.core import (
     QgsFields,
     QgsProcessing,
     QgsProcessingException,
-    QgsProcessingParameterDefinition)
+    QgsProcessingParameterDefinition,
+    NULL)
 
 from processing.algs.qgis.QgisAlgorithm import QgisFeatureBasedAlgorithm
 
@@ -133,6 +134,7 @@ class FieldsMapper(QgisFeatureBasedAlgorithm):
                                         typeName="",
                                         len=field_def.get('length', 0),
                                         prec=field_def.get('precision', 0)))
+            if field_def['expression']:
                 expression = QgsExpression(field_def['expression'])
                 expression.setGeomCalculator(da)
                 expression.setDistanceUnits(context.project().distanceUnits())
@@ -144,6 +146,8 @@ class FieldsMapper(QgisFeatureBasedAlgorithm):
                                 expression.parserErrorString()))
                     return False
                 self.expressions.append(expression)
+            else:
+                self.expressions.append(None)
         return True
 
     def outputFields(self, inputFields):
@@ -151,6 +155,7 @@ class FieldsMapper(QgisFeatureBasedAlgorithm):
 
     def processAlgorithm(self, parameters, context, feeback):
         for expression in self.expressions:
+            if expression is not None:
                 expression.prepare(self.expr_context)
         self._row_number = 0
         return super().processAlgorithm(parameters, context, feeback)
@@ -158,6 +163,7 @@ class FieldsMapper(QgisFeatureBasedAlgorithm):
     def processFeature(self, feature, context, feedback):
         attributes = []
         for expression in self.expressions:
+            if expression is not None:
                 self.expr_context.setFeature(feature)
                 self.expr_context.lastScope().setVariable("row_number", self._row_number)
                 value = expression.evaluate(self.expr_context)
@@ -167,6 +173,8 @@ class FieldsMapper(QgisFeatureBasedAlgorithm):
                             .format(expression.expression(),
                                     expression.parserErrorString()))
                 attributes.append(value)
+            else:
+                attributes.append(NULL)
         feature.setAttributes(attributes)
         self._row_number += 1
         return [feature]

python/plugins/processing/tests/testdata/expected/refactor_fields_null.gml

@@ -0,0 +1,77 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ogr:FeatureCollection
+     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+     xsi:schemaLocation="http://ogr.maptools.org/ refactor_fields_null.xsd"
+     xmlns:ogr="http://ogr.maptools.org/"
+     xmlns:gml="http://www.opengis.net/gml">
+  <gml:boundedBy>
+    <gml:Box>
+      <gml:coord><gml:X>0</gml:X><gml:Y>-5</gml:Y></gml:coord>
+      <gml:coord><gml:X>8</gml:X><gml:Y>3</gml:Y></gml:coord>
+    </gml:Box>
+  </gml:boundedBy>
+                                                                                                                                                               
+  <gml:featureMember>
+    <ogr:refactor_fields_null fid="points.0">
+      <ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>1,1</gml:coordinates></gml:Point></ogr:geometryProperty>
+      <ogr:id>1</ogr:id>
+      <ogr:id2>2</ogr:id2>
+    </ogr:refactor_fields_null>
+  </gml:featureMember>
+  <gml:featureMember>
+    <ogr:refactor_fields_null fid="points.1">
+      <ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>3,3</gml:coordinates></gml:Point></ogr:geometryProperty>
+      <ogr:id>2</ogr:id>
+      <ogr:id2>1</ogr:id2>
+    </ogr:refactor_fields_null>
+  </gml:featureMember>
+  <gml:featureMember>
+    <ogr:refactor_fields_null fid="points.2">
+      <ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>2,2</gml:coordinates></gml:Point></ogr:geometryProperty>
+      <ogr:id>3</ogr:id>
+      <ogr:id2>0</ogr:id2>
+    </ogr:refactor_fields_null>
+  </gml:featureMember>
+  <gml:featureMember>
+    <ogr:refactor_fields_null fid="points.3">
+      <ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>5,2</gml:coordinates></gml:Point></ogr:geometryProperty>
+      <ogr:id>4</ogr:id>
+      <ogr:id2>2</ogr:id2>
+    </ogr:refactor_fields_null>
+  </gml:featureMember>
+  <gml:featureMember>
+    <ogr:refactor_fields_null fid="points.4">
+      <ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>4,1</gml:coordinates></gml:Point></ogr:geometryProperty>
+      <ogr:id>5</ogr:id>
+      <ogr:id2>1</ogr:id2>
+    </ogr:refactor_fields_null>
+  </gml:featureMember>
+  <gml:featureMember>
+    <ogr:refactor_fields_null fid="points.5">
+      <ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>0,-5</gml:coordinates></gml:Point></ogr:geometryProperty>
+      <ogr:id>6</ogr:id>
+      <ogr:id2>0</ogr:id2>
+    </ogr:refactor_fields_null>
+  </gml:featureMember>
+  <gml:featureMember>
+    <ogr:refactor_fields_null fid="points.6">
+      <ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>8,-1</gml:coordinates></gml:Point></ogr:geometryProperty>
+      <ogr:id>7</ogr:id>
+      <ogr:id2>0</ogr:id2>
+    </ogr:refactor_fields_null>
+  </gml:featureMember>
+  <gml:featureMember>
+    <ogr:refactor_fields_null fid="points.7">
+      <ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>7,-1</gml:coordinates></gml:Point></ogr:geometryProperty>
+      <ogr:id>8</ogr:id>
+      <ogr:id2>0</ogr:id2>
+    </ogr:refactor_fields_null>
+  </gml:featureMember>
+  <gml:featureMember>
+    <ogr:refactor_fields_null fid="points.8">
+      <ogr:geometryProperty><gml:Point srsName="EPSG:4326"><gml:coordinates>0,-1</gml:coordinates></gml:Point></ogr:geometryProperty>
+      <ogr:id>9</ogr:id>
+      <ogr:id2>0</ogr:id2>
+    </ogr:refactor_fields_null>
+  </gml:featureMember>
+</ogr:FeatureCollection>

python/plugins/processing/tests/testdata/expected/refactor_fields_null.xsd

@@ -0,0 +1,44 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<xs:schema targetNamespace="http://ogr.maptools.org/" xmlns:ogr="http://ogr.maptools.org/" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:gml="http://www.opengis.net/gml" elementFormDefault="qualified" version="1.0">
+<xs:import namespace="http://www.opengis.net/gml" schemaLocation="http://schemas.opengis.net/gml/2.1.2/feature.xsd"/>
+<xs:element name="FeatureCollection" type="ogr:FeatureCollectionType" substitutionGroup="gml:_FeatureCollection"/>
+<xs:complexType name="FeatureCollectionType">
+  <xs:complexContent>
+    <xs:extension base="gml:AbstractFeatureCollectionType">
+      <xs:attribute name="lockId" type="xs:string" use="optional"/>
+      <xs:attribute name="scope" type="xs:string" use="optional"/>
+    </xs:extension>
+  </xs:complexContent>
+</xs:complexType>
+<xs:element name="refactor_fields_null" type="ogr:refactor_fields_null_Type" substitutionGroup="gml:_Feature"/>
+<xs:complexType name="refactor_fields_null_Type">
+  <xs:complexContent>
+    <xs:extension base="gml:AbstractFeatureType">
+      <xs:sequence>
+        <xs:element name="geometryProperty" type="gml:PointPropertyType" nillable="true" minOccurs="0" maxOccurs="1"/>
+        <xs:element name="id" nillable="true" minOccurs="0" maxOccurs="1">
+          <xs:simpleType>
+            <xs:restriction base="xs:integer">
+              <xs:totalDigits value="10"/>
+            </xs:restriction>
+          </xs:simpleType>
+        </xs:element>
+        <xs:element name="id2" nillable="true" minOccurs="0" maxOccurs="1">
+          <xs:simpleType>
+            <xs:restriction base="xs:integer">
+              <xs:totalDigits value="10"/>
+            </xs:restriction>
+          </xs:simpleType>
+        </xs:element>
+        <xs:element name="no_exp" nillable="true" minOccurs="0" maxOccurs="1">
+          <xs:simpleType>
+            <xs:restriction base="xs:string">
+              <xs:maxLength value="255"/>
+            </xs:restriction>
+          </xs:simpleType>
+        </xs:element>
+      </xs:sequence>
+    </xs:extension>
+  </xs:complexContent>
+</xs:complexType>
+</xs:schema>

python/plugins/processing/tests/testdata/qgis_algorithm_tests.yaml

@@ -2482,6 +2482,38 @@ tests:
         name: expected/refactorfields.gml
         type: vector
 
+  - algorithm: qgis:refactorfields
+    name: Refactor fields, empty expression
+    params:
+      FIELDS_MAPPING:
+      - expression: '"fid"'
+        length: 0
+        name: fid
+        precision: 0
+        type: 10
+      - expression: '"id"'
+        length: 0
+        name: id
+        precision: 0
+        type: 2
+      - expression: '"id2"'
+        length: 0
+        name: id2
+        precision: 0
+        type: 2
+      - expression: ''
+        length: 0
+        name: no_exp
+        precision: 0
+        type: 10
+      INPUT:
+        name: points.gml
+        type: vector
+    results:
+      OUTPUT:
+        name: expected/refactor_fields_null.gml
+        type: vector
+
   - algorithm: native:reprojectlayer
     name: reproject vector layer
     params: