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

From: sven (salvadorindali_at_gmx.de)
Date: 11/29/03 

Date: Sat, 29 Nov 2003 21:51:51 +0000 (UTC)

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