turn snapping on/off in QGIS

snapping.py

from qgis.core import *
from qgis.utils import iface
from PyQt4.QtCore import * 
from PyQt4.QtGui import * 

def snapping_toggle():
    layer = QgsMapLayerRegistry.instance().mapLayersByName("topographicline")[0]
    #layer = iface.activeLayer()
    _, enabled, _, _, _, _ = QgsProject.instance().snapSettingsForLayer(layer.id())
    QgsProject.instance().setSnapSettingsForLayer(layer.id(), not enabled, 0, 1, 10, True)
    print "set {} snapping {}".format(layer.name(), "Off" if not enabled else "On")

shortcut = QShortcut(QKeySequence(Qt.SHIFT + Qt.Key_S), iface.mainWindow())
shortcut.setContext(Qt.ApplicationShortcut)
shortcut.activated.connect(snapping_toggle)