list: from 2 to 3 dimensions..looking for a nice way

[sven]

I've got a nested list-> 
a = [[1,'house'],[2,'house'],[3,'garden']] 


and I want to get one level deeper  with the lists having the same value in 
index value 1

b =[[[1, 'house'], [2, 'house']], [[3, 'garten']]]


I' achieving this with an ugly syntax:

a = [[1,'house'],[2,'house'],[3,'garden']]
b = [[]]
b[0].append(a.pop(0))
 
for id in range(len(a)):
	if(b[-1][0][1]==a[id][1]):
		b[-1].append(a[id])
	else:
		b.append([a[id]])

What is the pythonic way to do this?
Thanks for any insight