How to add a rownumber in QGIS 3 using Python

Question

I want to add a row number in QGIS 3 using Python.

First of all I add a new field with the code below and it works.

Then I want to fill the table with the row number. In QGIS Field Calculator there is the function @row_number. That is exactly what I want. But I want to do it automatically with Python. The code below doesn't work.

## join ID field
vlayer=##
myField1 = QgsField('ID', QVariant.Double)
vlayer.startEditing()
vlayer.dataProvider().addAttributes([myField1])
vlayer.updateFields()
idx=vlayer.dataProvider().fieldNameIndex('ID')
e = QgsExpression('$rownum')

##fill the attributes with ID-Number
for f in vlayer.getFeatures():
    f[idx]=e
    vlayer.updateFeature(f)

vlayer.commitChanges()

Is there an easy way to add a row number or an ID with Python?

Answer 1

Create a count variable:

field_to_update = 'int_id' #Change
layer = iface.activeLayer()
field_index = layer.dataProvider().fieldNameIndex(field_to_update)
layer.startEditing()
count=1
for feature in layer.getFeatures():
    _=layer.changeAttributeValue(feature.id(),field_index,count)
    count+=1
layer.commitChanges()

Answer 2

I used the input I got and modified it. The new code is:

## vlayer=iface.activeLayer()    

## join ID field
myField1 = QgsField('ID', QVariant.Double)
vlayer.startEditing()
vlayer.dataProvider().addAttributes([myField1])
vlayer.updateFields()
ID=vlayer.dataProvider().fieldNameIndex('ID')
vlayer.commitChanges()

count=1
vlayer.startEditing()
# fill the field ID with rownumber
for f in vlayer.getFeatures():
    attr_ID=f.attributes()[ID]
    rownum = count
    count+=1
    f[ID]=rownum
    vlayer.updateFeature(f)

vlayer.commitChanges()