Iterating over several layers and running process using PyQGIS?

Question

I am trying to find road intersections within each of my memory Polyline layers (which are named road_layer_i for i in (0-99)) so this is what I tried:

import processing
self=qgis.utils
layers = self.iface.mapCanvas().layers()
for layer in layers:
   for i in range(0,100):
      if layer.name()=='road_layer_%s' % (i) :
        processing.runalg('qgis:lineintersections', layer, layer, 'id', 'id', 'C:/Phoenix/intersection_%s.shp' % (i))
        IntersectionLayer=QgsVectorLayer('C:/Phoenix/intersection_%s.shp' % (i) , 'intersection_%s' % (i), "ogr")
        QgsMapLayerRegistry.instance().addMapLayers([IntersectionLayer])

I don't get any error message but each layer created contains points that are the road intersections for the layer 'road_layer_0'.

I really don't understand why it does that.

Also, whenever I simply try

import processing
self=qgis.utils
layers = self.iface.mapCanvas().layers()
for layer in layers:
  if layer.name()=='road_layer_33' :
    processing.runalg('qgis:lineintersections', layer, layer, 'id', 'id', 'C:/Phoenix/intersection_33.shp')
    IntersectionLayer=QgsVectorLayer('C:/Phoenix/intersection_33.shp' , 'intersection', "ogr")

    QgsMapLayerRegistry.instance().addMapLayers([IntersectionLayer])

I still get the intersections for the road_layer_0...

After answer by @gcarrillo it is still not working. I indeed obtain 100 layers labbeled interesection_i (i in 0-99) but they all contain only the intersections for road_layer_0. Here is the code I used to create my road layers :

from fTools.fTools import fToolsPlugin
import shapely
from qgis.core import QgsRasterLayer, QgsCoordinateReferenceSystem, QgsGeometry
RoadLayer=QgsVectorLayer("C:/Phoenix/Project/Phoenix/Phoenix_roads.shp" ,    "Phoenix_roads", "ogr")
RoadLayer.setCrs( QgsCoordinateReferenceSystem(3857, QgsCoordinateReferenceSystem.EpsgCrsId) )
#QgsMapLayerRegistry.instance().addMapLayers([RoadLayer])
#Get the data from this layer
provider = RoadLayer.dataProvider()
fields = RoadLayer.pendingFields()
field_names = [field.name() for field in fields]
create list of points used to create rectangles
partit=partition(10,c,d)
for i in range(0,100):
create new layer in which we will put the roads that are inside the rectangle
layer =  QgsVectorLayer('LineString', 'road_layer_%s' %(i) , "memory")
   pr = layer.dataProvider()
   attrib_layer=input_layer_attrib_names = RoadLayer.dataProvider().fields()
   oldattributeList = RoadLayer.dataProvider().fields().toList()
#Generate attributes in destination layer
   newattributeList=[]
     for attrib in oldattributeList:
        if layer.fieldNameIndex(attrib.name())==-1:
           newattributeList.append(QgsField(attrib.name(),attrib.type()))
     pr.addAttributes(newattributeList)
add empty attribute fields to new layer
 layer.updateFields()

#create points left-bottom and right-top of rectangle we want to select
     x=partit[2*i]
     pointA = QgsPoint(x[0],x[1])
     y=partit[2*i+1]
     pointB = QgsPoint(y[0],y[1])
create rectangle
 Rect=QgsRectangle(x[0],x[1],y[0],y[1])  

create request
 request = QgsFeatureRequest().setFilterRect(Rect)
 for f in RoadLayer.getFeatures(request):
#        add geometry from road to new road
     geom=f.geometry()
     coord=geom.asPolyline()
     pt = QgsFeature()
     pt.setGeometry(QgsGeometry.fromPolyline(coord))
#       add attributes
     atr = dict(zip(field_names, f.attributes()))
     pt.setAttributes(f.attributes())
 #        add new feature to layer
     pr.addFeatures([pt])
     layer.updateExtents()
#        add layer to map canva
  QgsMapLayerRegistry.instance().addMapLayers([layer])

Is there anything wrong with this method that makes the use of processing.runalg('qgis:lineintersections', ... ) impossible to iterate over the layers thus created ?

score 4 · Accepted Answer · answered Feb 10 '15 at 14:28

I assume your road_layer_i (for i in [0-99]) layers are not the same layer, that is, they do not have exactly the same lines.

Having said that, I've modified your code snippet a bit, making these changes:

Use QgsMapLayerRegistry.instance().mapLayers().values() instead of self.iface.mapCanvas().layers(). This ensures you'll iterate on all layers and not only on those that are visible.
Instead of checking matching names, extract the i from the layer name.
Load all resulting intersection layers at once instead of one by one.

This is the resulting code:

import processing
layers = QgsMapLayerRegistry.instance().mapLayers().values()
outputLayers=[]

for layer in layers:
    i = layer.name().split("_")[-1]
    processing.runalg('qgis:lineintersections', layer, layer, 'id', 'id', 'C:/Phoenix/intersection_%s.shp' % (i))
    outputLayers.append(QgsVectorLayer('C:/Phoenix/intersection_%s.shp' % (i) , 'intersection_%s' % (i), "ogr"))

QgsMapLayerRegistry.instance().addMapLayers( outputLayers )

I've checked it on GNU/Linux, QGIS 2.6.1.

Iterating over several layers and running process using PyQGIS?

create list of points used to create rectangles

create new layer in which we will put the roads that are inside the rectangle

add empty attribute fields to new layer

create rectangle

create request

1 Answers1