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

def merge(d1, d2, merge):
    result = dict(d1)
    for k,v in d2.iteritems():
        if k in result:
            result[k] = merge(result[k], v)
        else:
            result[k] = v
    return result

d1 = {'a': 1, 'b': 2}
d2 = {'a': 1, 'b': 3, 'c': 2}
print merge(d1, d2, lambda x, y:(x,y))

{'a': (1, 1), 'c': 2, 'b': (2, 3)}