PyQGIS – Creating Memory Layer and Populating Fields

Question

Seems like a basic operation using PyQGIS but can't see what I am missing. I want to create a memory layer with the exact same features and attributes from a shapefile. I have looked several posts such as:

• Copying attributes from point layer to memory linestring layer PYQGIS)
• [URL IS BROKEN] PyQGIS manually copy all features with attributes from a layer into a memory layer

The following script creates a memory layer, gets the correct fields and correct number of features but it doesn't populate the fields with data:

input = "C:/Users/Me/Desktop//example.shp"
layer = QgsVectorLayer(example,"line","ogr")

temp = QgsVectorLayer("LineString?crs=epsg:4326", "result", "memory")
temp_data = temp.dataProvider()
temp.startEditing()

layer_fields = layer.dataProvider().fields().toList()
attr = layer.dataProvider().fields().toList()
temp_data.addAttributes(attr)
temp.updateFields()

feat = QgsFeature()

for elem in layer.getFeatures():
    feat.setGeometry(elem.geometry())
    feat.setAttributes(attr)
    temp.addFeatures([feat])
    temp.updateExtents()

temp.commitChanges()
QgsMapLayerRegistry.instance().addMapLayer(temp)

Answer 1

I modified your code to consider a polygon vector layer. However, if you get the features of the original layer, you only need a little portion of your original code for working adequately (without any editing session if you use the QgsDataProvider class).

This is my code:

input = "/home/zeito/pyqgis_data/polygon8.shp"

layer = QgsVectorLayer(input,"polygon","ogr")

feats = [ feat for feat in layer.getFeatures() ]

temp = QgsVectorLayer("Polygon?crs=epsg:32612", "result", "memory")

QgsProject.instance().addMapLayer(temp)

temp_data = temp.dataProvider()

attr = layer.dataProvider().fields().toList()
temp_data.addAttributes(attr)
temp.updateFields()
 
temp_data.addFeatures(feats)

When above code was running at the Python Console (Ctrl+Alt+P) of QGIS, fields of original layer were correctly copied at the memory layer named "result"; as can be observed at the next image. It worked for me.

On the other hand, your approach of set attributes to a list of fields instead a list of values is correct because if I comment these code lines:

attr = layer.dataProvider().fields().toList()
temp_data.addAttributes(attr)

it doesn't populate the fields with data; as it also observed at the next image: