Playing with bimapped dictionaries in Python

bimap.py

class Bimap(dict):
#	def alias(self,obj):
		

	def lkey(self, obj):
		return self[obj]
	
	def rkey(self, obj):
		for key,val in self.iteritems():
			if val is obj:
				return key
		raise ValueError("object not in dictionary %s" % obj)

if __name__ == '__main__':
	dic1 = {'ach01':'pc01', 'ach02':'pc02'}

	bm = Bimap(dic1)
	print bm.rkey('pc01')
	print bm.lkey('ach01')

	try:
		print bm.rkey('hello')
	except ValueError as inst:
		print "KEY ERROR: %s" % inst
		
	try:
		print bm.lkey('world')
	except KeyError as inst:
		print "KEY ERROR: %s" % inst