[FEATURE][processing] New extract by expression algorithm

Filters an input layer by expression

python/plugins/processing/algs/help/qgis.yaml

@@ -159,10 +159,15 @@ qgis:exportaddgeometrycolumns: >
   Depending on the geometry type of the vector layer, the attributes added to the table will be different.
 
 qgis:extractbyattribute: >
-  This algorithms creates new vector layer that only contain certain features from an input layer. The criteria for adding features to the resulting layer is defined based on the values of an attribute from the input layer.
+  This algorithms creates a new vector layer that only contains matching features from an input layer. The criteria for adding features to the resulting layer is defined based on the values of an attribute from the input layer.
+
+qgis:extractbyexpression: >
+  This algorithms creates a new vector layer that only contains matching features from an input layer. The criteria for adding features to the resulting layer is based on a QGIS expression.
+
+  For more information about expressions see the <a href ="{qgisdocs}/user_manual/working_with_vector/expression.html">user manual</a>
 
 qgis:extractbylocation: >
-  This algorithms creates new vector layer that only contain certain features from an input layer. The criteria for adding features to the resulting layer is defined based on the spatial relationship between each feature and the features in an additional layer.
+  This algorithms creates a new vector layer that only contains matching features from an input layer. The criteria for adding features to the resulting layer is defined based on the spatial relationship between each feature and the features in an additional layer.
 
 qgis:extractnodes: >
   This algorithm takes a line or polygon layer and generates a point layer with points representing the nodes in the input lines or polygons. The attributes associated to each point are the same ones associated to the line or polygon that the point belongs to.
@@ -419,7 +424,7 @@ qgis:selectbyattributesum:
 qgis:selectbyexpression: >
   This algorithms creates a selection in a vector layer. The criteria for selecting features is based on a QGIS expression.
 
-  For more information about expressions see the<a href ="{qgisdocs}/user_manual/working_with_vector/expression.html">user manual</a>
+  For more information about expressions see the <a href ="{qgisdocs}/user_manual/working_with_vector/expression.html">user manual</a>
 
 
 qgis:selectbylocation: >

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

@@ -0,0 +1,67 @@
+# -*- coding: utf-8 -*-
+
+"""
+***************************************************************************
+    ExtractByExpression.py
+    ---------------------
+    Date                 : October 2016
+    Copyright            : (C) 2016 by Nyall Dawson
+***************************************************************************
+*                                                                         *
+*   This program is free software; you can redistribute it and/or modify  *
+*   it under the terms of the GNU General Public License as published by  *
+*   the Free Software Foundation; either version 2 of the License, or     *
+*   (at your option) any later version.                                   *
+*                                                                         *
+***************************************************************************
+"""
+
+__author__ = 'Nyall Dawson'
+__date__ = 'October 2016'
+__copyright__ = '(C) 2016, Nyall Dawson'
+
+# This will get replaced with a git SHA1 when you do a git archive
+
+__revision__ = '$Format:%H$'
+
+import processing
+from qgis.core import QgsExpression, QgsVectorLayer, QgsFeatureRequest
+from processing.core.GeoAlgorithmExecutionException import GeoAlgorithmExecutionException
+from processing.core.parameters import ParameterVector
+from processing.core.outputs import OutputVector
+from processing.core.GeoAlgorithm import GeoAlgorithm
+from processing.core.parameters import ParameterString
+from processing.tools import dataobjects
+
+
+class ExtractByExpression(GeoAlgorithm):
+
+    INPUT = 'INPUT'
+    EXPRESSION = 'EXPRESSION'
+    OUTPUT = 'OUTPUT'
+
+    def defineCharacteristics(self):
+        self.name, self.i18n_name = self.trAlgorithm('Extract by expression')
+        self.group, self.i18n_group = self.trAlgorithm('Vector selection tools')
+
+        self.addParameter(ParameterVector(self.INPUT,
+                                          self.tr('Input Layer')))
+        self.addParameter(ParameterString(self.EXPRESSION,
+                                          self.tr("Expression")))
+        self.addOutput(OutputVector(self.OUTPUT, self.tr('Extracted (expression)')))
+
+    def processAlgorithm(self, progress):
+        layer = dataobjects.getObjectFromUri(self.getParameterValue(self.INPUT))
+        expression_string = self.getParameterValue(self.EXPRESSION)
+        writer = self.getOutputFromName(self.OUTPUT).getVectorWriter(layer.fields(), layer.wkbType(), layer.crs())
+
+        expression = QgsExpression(expression_string)
+        if not expression.hasParserError():
+            req = QgsFeatureRequest().setFilterExpression(expression_string)
+        else:
+            raise GeoAlgorithmExecutionException(expression.parserErrorString())
+
+        for f in layer.getFeatures(req):
+            writer.addFeature(f)
+
+        del writer

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

@@ -56,6 +56,7 @@
 from .RandomExtract import RandomExtract
 from .RandomExtractWithinSubsets import RandomExtractWithinSubsets
 from .ExtractByLocation import ExtractByLocation
+from .ExtractByExpression import ExtractByExpression
 from .PointsInPolygon import PointsInPolygon
 from .PointsInPolygonUnique import PointsInPolygonUnique
 from .PointsInPolygonWeighted import PointsInPolygonWeighted
@@ -231,7 +232,7 @@ def __init__(self):
                         ReliefAuto(), ZonalStatisticsQgis(),
                         IdwInterpolationZValue(), IdwInterpolationAttribute(),
                         TinInterpolationZValue(), TinInterpolationAttribute(),
-                        RemoveNullGeometry()
+                        RemoveNullGeometry(), ExtractByExpression()
                         ]
 
         if hasMatplotlib:

python/plugins/processing/tests/testdata/expected/extract_expression.gfs

@@ -0,0 +1,32 @@
+<GMLFeatureClassList>
+  <GMLFeatureClass>
+    <Name>extract_expression</Name>
+    <ElementPath>extract_expression</ElementPath>
+    <!--POLYGON-->
+    <GeometryType>3</GeometryType>
+    <SRSName>EPSG:4326</SRSName>
+    <DatasetSpecificInfo>
+      <FeatureCount>2</FeatureCount>
+      <ExtentXMin>4.00000</ExtentXMin>
+      <ExtentXMax>10.00000</ExtentXMax>
+      <ExtentYMin>-3.00000</ExtentYMin>
+      <ExtentYMax>5.00000</ExtentYMax>
+    </DatasetSpecificInfo>
+    <PropertyDefn>
+      <Name>name</Name>
+      <ElementPath>name</ElementPath>
+      <Type>String</Type>
+      <Width>5</Width>
+    </PropertyDefn>
+    <PropertyDefn>
+      <Name>intval</Name>
+      <ElementPath>intval</ElementPath>
+      <Type>Integer</Type>
+    </PropertyDefn>
+    <PropertyDefn>
+      <Name>floatval</Name>
+      <ElementPath>floatval</ElementPath>
+      <Type>Integer</Type>
+    </PropertyDefn>
+  </GMLFeatureClass>
+</GMLFeatureClassList>

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

@@ -0,0 +1,29 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ogr:FeatureCollection
+     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+     xsi:schemaLocation=""
+     xmlns:ogr="http://ogr.maptools.org/"
+     xmlns:gml="http://www.opengis.net/gml">
+  <gml:boundedBy>
+    <gml:Box>
+      <gml:coord><gml:X>4</gml:X><gml:Y>-3</gml:Y></gml:coord>
+      <gml:coord><gml:X>10</gml:X><gml:Y>5</gml:Y></gml:coord>
+    </gml:Box>
+  </gml:boundedBy>
+
+  <gml:featureMember>
+    <ogr:extract_expression fid="polys.1">
+      <ogr:geometryProperty><gml:Polygon srsName="EPSG:4326"><gml:outerBoundaryIs><gml:LinearRing><gml:coordinates>5,5 6,4 4,4 5,5</gml:coordinates></gml:LinearRing></gml:outerBoundaryIs></gml:Polygon></ogr:geometryProperty>
+      <ogr:name>Aaaaa</ogr:name>
+      <ogr:intval>-33</ogr:intval>
+      <ogr:floatval>0</ogr:floatval>
+    </ogr:extract_expression>
+  </gml:featureMember>
+  <gml:featureMember>
+    <ogr:extract_expression fid="polys.3">
+      <ogr:geometryProperty><gml:Polygon srsName="EPSG:4326"><gml:outerBoundaryIs><gml:LinearRing><gml:coordinates>6,1 10,1 10,-3 6,-3 6,1</gml:coordinates></gml:LinearRing></gml:outerBoundaryIs><gml:innerBoundaryIs><gml:LinearRing><gml:coordinates>7,0 7,-2 9,-2 9,0 7,0</gml:coordinates></gml:LinearRing></gml:innerBoundaryIs></gml:Polygon></ogr:geometryProperty>
+      <ogr:name>ASDF</ogr:name>
+      <ogr:intval>0</ogr:intval>
+    </ogr:extract_expression>
+  </gml:featureMember>
+</ogr:FeatureCollection>

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

@@ -1212,4 +1212,16 @@ tests:
     results:
       OUTPUT_LAYER:
         name: expected/remove_null_polys.gml
-        type: vector
+        type: vector
+
+  - algorithm: qgis:extractbyexpression
+    name: Extract by Expression
+    params:
+      EXPRESSION: left( "Name",1)='A'
+      INPUT:
+        name: polys.gml
+        type: vector
+    results:
+      OUTPUT:
+        name: expected/extract_expression.gml
+        type: vector