from qgis.core import *
from qgis.utils import *
from qgis.gui import *
from PyQt4.QtCore import QVariant
import processing
import os

layer_P = QgsProject.instance().mapLayersByName("BXO")[0]

SUM = QgsField('t_branch', QVariant.Int)
layer_P.addAttribute (SUM)
idx = layer_P.fieldNameIndex('t_branch')

layer_P.startEditing()

e = QgsExpression ('CB_Num_Ent' + 'CB_Num_Ext')
e.prepare( 't_branch'() )

for f in layer_P.getFeatures():
    f[idx] = e.evaluate( f )
    layer_P.updateFeature( f )

layer_P.updateField()
layer_P.commitChanges()

layer_P = QgsProject.instance().mapLayersByName("BXO")[0]

SUM = QgsField('t_branch', QVariant.Int)
layer_P.addAttribute (SUM)

idx = layer_P.lookupField('t_branch')

layer_P.startEditing()

e = QgsExpression ('CB_Num_Ent' + 'CB_Num_Ext')
e.prepare( layer_P.fields() )

for f in layer_P.getFeatures():
    f[idx] = e.evaluate( f )
    layer_P.updateFeature( f )

layer_P.updateField()
layer_P.commitChanges()

> e = QgsExpression ('CB_Num_Ent' + 'CB_Num_Ext')