Duplicating layer in memory using PyQGIS

Question

I have a layer in QGIS, and I want to duplicate it through a plugin so I can use the copy of it as I want, without modifying the original.

Of course layer2 = layer1 will not work, because everything that happens to layer2 will also happen to layer1, as it is the same object behind all this.

The only way I found to do it is as such:

QgsVectorFileWriter.writeAsVectorFormat(layer1, r"C:\Users\ABC\AppData\Local\Temp\NewLayer.shp", "utf-8", None, "ESRI Shapefile")
layer2 = QgsVectorLayer("C:\Users\ABC\AppData\Local\Temp\NewLayer.shp", "New vector", "ogr")
#do something with layer2

Is there a simple way to duplicate the layer in memory, without having to write a new file?

Answer 1

In QGIS 3 you can make a copy of a layer without saving any reference to the parent layer in this way:

layer.selectAll()
clone_layer = processing.run("native:saveselectedfeatures", {'INPUT': layer, 'OUTPUT': 'memory:'})['OUTPUT']
layer.removeSelection()
QgsProject.instance().addMapLayer(clone_layer)

The QgsVectorLayer class has a clone() method that allows you to clone the layer in a new layer, the problem is that if you modify the geometry in the cloned layer the original layer will be affected: the reason for this is that the data source is the same for the original layer and the cloned layer.

Answer 2

The following code works for me from both the Python Console and plugin. It takes the features from the source input layer and copies the attributes to a memory layer (in this case, a polygon layer but you can change it to LineString or Point depending on layer type):

layer = QgsVectorLayer("path/to/layer", "polygon", "ogr")
feats = [feat for feat in layer.getFeatures()]

mem_layer = QgsVectorLayer("Polygon?crs=epsg:4326", "duplicated_layer", "memory")

mem_layer_data = mem_layer.dataProvider()
attr = layer.dataProvider().fields().toList()
mem_layer_data.addAttributes(attr)
mem_layer.updateFields()
mem_layer_data.addFeatures(feats)

QgsMapLayerRegistry.instance().addMapLayer(mem_layer)

Answer 3

Another approach is to use the materialize() method from the QgsFeatureSource class. It is available since QGIS 3.0.

Over the entire layer with the allFeatureIds() method:

#choose a layer
layer = iface.activeLayer()
#or with: layer = QgsMapLayerRegistry.instance().mapLayersByName('name_of_the_layer')[0]
#create a new layer from all features   
new_layer = layer.materialize(QgsFeatureRequest().setFilterFids(layer.allFeatureIds()))
#add a new layer to the map
QgsProject.instance().addMapLayer(new_layer)

Over the selection of all features with the selectedFeatureIds() method from the QgsVectorLayer class:

#choose a layer
layer = iface.activeLayer()
#or with: layer = QgsMapLayerRegistry.instance().mapLayersByName('name_of_the_layer')[0]
#select all features in the original layer
layer.selectAll()
#create a new layer from selected features
new_layer = layer.materialize(QgsFeatureRequest().setFilterFids(layer.selectedFeatureIds()))
#remove selection in the original layer
layer.removeSelection()
#add a new layer to the map  
QgsProject.instance().addMapLayer(new_layer)

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References:

• Creating Vector layer from selected features with PyQGIS