qgis 3 - Adding Area column in Polygon layer using PyQGIS?

I have a WardBoundary.shp file with Polygon Features in it. I have to calculate the area of each Polygon and write into attribute Table.

I can do manually by using '$area` but I need to automate this process using Pyqgis3.

I have seen Add area column using PyQGIS but I am unable to get area as well as code.

Answer

Here I put some lines that must be execute from the python console of Qgis. I use the Developer Cookbook of Qgis 3.

#!/usr/bin/python3

from qgis.core import QgsVectorLayer, QgsVectorDataProvider
from qgis.PyQt.QtCore import QVariant

# ****Choose**** where you want to calculate your area

# from a layer stored somewhere in your computer

layer = QgsVectorLayer('c:\temp\ward_boundary.shp', 'WardBoundary', 'ogr')

# in the active layer in the TreeLayer (the undelying layer)
#layer = iface.activeLayer()

# Here we get the capabilities of your layer (Add attribute layer, edit feature ect ..
caps = layer.dataProvider().capabilities()

# We make a list of fields from their name
fields_name = [f.name() for f in layer.fields()]


# We check if we can add an attribute to the layer.
if caps & QgsVectorDataProvider.AddAttributes:
    # We check if the attribute field is not exist
    if "Area" not in fields_name:
        # We add the field name Area and with the double type (it can be integer or text
        layer.dataProvider().addAttributes([QgsField("Area", QVariant.Double)])
        # We update layer's field otherwise we'll not have the field
        layer.updateFields()
        # Recreate the list field by the name to have index of the field

        fields_name = [f.name() for f in layer.fields()]
        # we get the index of the Area field
        fareaidx = fields_name.index('Area')
    else:
        # We are here because there is a field name Area
        print("The Area field is already added")
        # Recreate the list field by the name to have index of the field
        fields_name = [f.name() for f in layer.fields()]
        # we get the index of the Area field
        fareaidx = fields_name.index('Area')


# Here we check if we can change attribute of the layer
if caps & QgsVectorDataProvider.ChangeAttributeValues:
    # we loop^on every feature
    for feature in layer.getFeatures():
        # For each feature :
        # We calculate the area and put the index of the field Area
        # We round the area value by 2 digit
        attrs = {fareaidx : round(feature.geometry().area(), 2)}
        # We change the the value of Area Field for this feature.

        layer.dataProvider().changeAttributeValues({feature.id() : attrs})