import glob, os, processing

path_in = "/home/giulio/Progetti/Qgis/Rilievo tablet/"
path_out = "/home/giulio/Progetti/Qgis/output/"
print ("inzio conversione")
for file in glob.glob(path_in + "*.shp"):
    layer = QgsVectorLayer(file, "layer", "ogr")
    if layer.isValid():
        QgsVectorFileWriter.writeAsVectorFormat( layer, 
                                         path_out + os.path.basename(file) + ".csv", 
                                         "utf-8", 
                                         layer.crs(), 
                                         "CSV",  
                                         layerOptions=['GEOMETRY=AS_XYZ'] )
        print("File " + os.path.basename(file) + " convertito.")
    else:
        print("File " + os.path.basename(file) + " non valido.")
print ("fine conversione")