Multi-field labeling of shapefile with PyQGIS?

I wrote the following code in order to draw the line labels with multiple fields. However, the label do not appear automatically in QGIS and I have to switch the “Layer Properties/Labels”, and toggle OK.

def labelLine(lyr, champNom1, champNom2):
    label = QgsPalLayerSettings()
    label.readFromLayer(lyr)
    label.enabled = True
    label.fieldName = '''concat('BASE: ' + to_string("''' + champNom1 + '''"),'nL = ' + to_string("''' + champNom2 + '''") + 'm')'''
    label.placement= QgsPalLayerSettings.Line
    label.bufferDraw= True
    label.bufferSize= 1
    label.setDataDefinedProperty(QgsPalLayerSettings.Size,True,True,'8','')
    label.writeToLayer(lyr)


def saveCoucheFRAC():
    layer=iface.activeLayer()
    nom=nom2+"_FRAC"
    lien=path2+"_FRAC"+".shp"
    QgsVectorFileWriter.writeAsVectorFormat(layer,lien,"utf-8",QgsCoordinateReferenceSystem(22332, QgsCoordinateReferenceSystem.EpsgCrsId),"ESRI Shapefile")
    #Charger cette couche dans QGis
    vlayer=QgsVectorLayer(lien, nom, "ogr")
    if not vlayer.isValid():
        print " Verifier la validite du fichier"
    else:
        # appliquer les couleurs selon le niveau de tension avec la fonction appCouleur ci-dessous
        appCouleur(vlayer)
        # changer l'epaisseur de la couche ligne
        epaiLyr(vlayer, "0.7")
        # ajouter l'etiquette de longueur sur chaque segment
        labelLine(vlayer,"BASE", "Longueur_m")
        # Definir la visibilite de la couche entre ech min 0 et max 100 000
        visLyr(vlayer, 0, 100000)
        #Charger dans le Canvas
        QgsMapLayerRegistry.instance().addMapLayer(vlayer)
saveCoucheFRAC()

I try to add the following to enable the label draw but it does not work:

layer.triggerRepaint()

or

layer.commitChanges()
layer.updateExtents()

A screenshot of the map (After toggling OK, line labels appears in QGIS)

label.placement = QgsPalLayerSettings.Line