#!/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:/Users/Prachi/Desktop/Qgis   sample dataWard_Saphale_updated.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})

layer = QgsVectorLayer("C:/Users/Prachi/Desktop/Qgis   sample dataWard_Saphale_updated.shp", "WardBoundary", "ogr")