Setting all the values in attribute field to the same value with PyQGIS

Question

When I use "Update existing fields" in the Field Calculator in QGIS 3.16.0-Hannover, it takes only a second to change all the values in a attribute field to the same value. However when I try to do that with PyQGIS, QGIS crashes.

I have a layer with line geometry with 82800 features inside. I want to change all feature's value inside "visited" column to '0'. Here is how I try to do that:

layer = iface.activeLayer()
lines = [feat for feat in layer.getFeatures()]

layer.startEditing()

for line in lines:
    line["visited"] = 0
    layer.updateFeature(line)

layer.commitChanges()

Can I do that like I do that with field calculator via using PyQGIS (with expressions or something)?

Answer 1

Use layer.dataProvider().changeAttributeValues() method instead of layer.updateFeature(). In this case you don't need to start an edit mode.

import time
    
layer = iface.activeLayer()    
visited_index = layer.fields().indexFromName("visited")
attr_map = {}
new_value = 5

tic = time.perf_counter() ###
for line in layer.getFeatures():
    attr_map[line.id()] = {visited_index: new_value}

layer.dataProvider().changeAttributeValues(attr_map)
toc = time.perf_counter() ###

print(f"{toc - tic:0.4f} seconds")

Using dictionary comprehension instead of for loop is more convenient to minimize the script.

layer = iface.activeLayer()    
i = layer.fields().indexFromName("visited") # index of the field

attr_map = {line.id(): {i: 5} for line in layer.getFeatures()} # comprehension    
layer.dataProvider().changeAttributeValues(attr_map)