[GIS] How to copy all feature attributes to a feature in a new layer

pyqgispythonqgis

I'm trying to create a new layer in QGIS based on an existing layer. The new features should have all the attributes of the corresponding features in the original layer. I've been trying to use outFeat.setAttributeMap(inFeat.attributeMap() ) similar to what Victor Olaya is doing at http://qgissextante.blogspot.de/2012/11/spliting-vector-layer.html, but all the attributes come out as NULL.

Could you tell me where the error is in my code?

def test(outputFilename):
 layer = qgis.utils.iface.activeLayer()
 provider = layer.dataProvider()
 fields = provider.fields()
 writer = QgsVectorFileWriter(outputFilename, "CP1250", fields, provider.geometryType(), provider.crs(), "ESRI Shapefile")
 inFeat = QgsFeature()
 outFeat = QgsFeature()
 inGeom = QgsGeometry()
 while provider.nextFeature(inFeat):
  point = inFeat.geometry().asPoint()
  inGeom = inFeat.geometry()
  outFeat.setGeometry(inFeat.geometry() )
  outFeat.setAttributeMap(inFeat.attributeMap() )
  writer.addFeature( outFeat )
 del writer
 newlayer = QgsVectorLayer(outputFilename, "Output", "ogr")
 QgsMapLayerRegistry.instance().addMapLayer(newlayer)

Best Answer

You are missing a provider.select() method call.

provider.select(provider.attributeIndexes() )
while provider.nextFeature(inFeat):
    ...

Related Solutions

[GIS] How to add new columns with attributes in existing Qgis layer

To add info to an existing layer instead of in memory layer you need to point to a layer on disk with QgsVectorLayer:

vl = QgsVectorLayer("C:\Temp\pt.shp", "pt", "ogr")

Then you may continue to reference the code snippet from this Q/A:

How to create a new empty vector layer programmatically?

[GIS] Creating new layer in PyQGIS with same attributes of another preexisting and copy some elements in it

I have a layer containing lots of elements of type Point, Line and Polygon

It is impossible in QGIS, look at how to open layers with multiple geometry types in qgis? ( only points layers, polylines layers or polygons layers and multi-parts geometries layers, MultiPoints, etc.).

So, no need of if geom.type()==QGis.Polygon, unless the geometry type is QGis.MultiPolygon.

The correct syntax is (Geometry Handling):

a) Before iterating over the layer elements

layer = qgis.utils.iface.activeLayer()
if layer.wkbType()==QGis.WKBPolygon:
    print 'Layer is a polygon layer'
if layer.wkbType()==QGis.WKBMultiPolygon:
    print 'Layer is a multi-polygon layer'

b) during the iteration

print geom.type() # and the result is a value from QGis.GeometryType enumeration
print geom.wkbType() # same

Example in the Python console (different in a Plugin) with a simple polygon layer :

enter image description here

1) create the two empty shapefiles, layerAbove15 and layerBelow15

layer = qgis.utils.iface.activeLayer()
# source of the layer
provider = layer.dataProvider()
# creation of the shapefiles:
writer1 = QgsVectorFileWriter("layerAbove15Name.shp", "CP1250", provider.fields(), provider.geometryType(), provider.crs(), "ESRI Shapefile")
writer2 = QgsVectorFileWriter("layerBelow15Name.shp", "CP1250", provider.fields(), provider.geometryType(), provider.crs(), "ESRI Shapefile")

2) populate the shapefiles depending on the condition:

outelem = QgsFeature()
# iterating over the input layer
for elem in layer.getFeatures():
    if len(elem.geometry().asPolygon()[0])-1 >=15:
             outelem.setGeometry(elem.geometry() )
             outelem.setAttributes(elem.attributes())
             writer1.addFeature(outelem)
    else:
             ....
             writer2.addFeature(outelem)

3) with a simple function to avoid repetition:

def write(out,inp):
    out.setGeometry(inp.geometry() )
    out.setAttributes(inp.attributes())
    return out

for elem in layer.getFeatures():
    if len(elem.geometry().asPolygon()[0])-1 >=15:
        writer1.addFeature(write(outelem,elem))
    else:
        writer2.addFeature(write(outelem,elem))