Adding new field in PyQGIS

Question

I am trying to add new field to my table based on two other existing fields. In the field Calculator it's quite simple:

When I am trying to run as a Python code I got into trouble, Here's my code:

vLayer = iface.activeLayer()    
vLayer.dataProvider().addAttributes([QgsField('1m_Cases', QVariant.Int)])
vLayer.updateFields()
for f in vLayer.getFeatures(): 
        f.setAttribute(f.fieldNameIndex('1m_Cases'), f(['J_TotalCases'] *1000000)/ f['Pop'])
vLayer.commitChanges()    

Answer 1

you can change the code like below, which should work:

vLayer = iface.activeLayer()
vLayer.dataProvider().addAttributes([QgsField('1m_Cases', QVariant.Int)])
vLayer.updateFields()
vLayer.startEditing()
for f in vLayer.getFeatures():
    f['1m_Cases'] = f['J_TotalCases'] *1000000/ f['Pop']
    vLayer.updateFeature(f)
vLayer.commitChanges()   

Answer 2

Based on the answer from @eurojam but using only the editing buffer, like in QGIS GUI.

from qgis.core import edit

vLayer = iface.activeLayer()
with edit(vLayer)
    vLayer.addAttribute(QgsField('1m_Cases', QVariant.Int))
    for f in vLayer.getFeatures():
        f['1m_Cases'] = f['J_TotalCases'] *1000000/ f['Pop']
        vLayer.updateFeature(f)