How to merge multiple dicts with same key?

Question

I have multiple dicts/key-value pairs like this:

d1 = {key1: x1, key2: y1}  
d2 = {key1: x2, key2: y2}  

I want the result to be a new di

Answer 1

Python 3.x Update

From Eli Bendersky answer:

Python 3 removed dict.iteritems use dict.items instead. See Python wiki: https://wiki.python.org/moin/Python3.0

from collections import defaultdict

dd = defaultdict(list)

for d in (d1, d2):
    for key, value in d.items():
        dd[key].append(value)