class LiFailed():
	pass

class TrFailed():
	pass


try:
	# whole code of li structure
	if a == " ":
		raise LiFailed
except LiFailed:
	try: 
		# whole code of tr
		if b == " ":
			raise TrFailed
	except TrFailed:
		try:
			# your code to exhaustive search of word "Shipping Weight"