See the problem described here. I want to use collections.Counter, but any key whose sum-of-value amounts to 0 - in this case d1 - gets omitted:
In [128]: d1 = {'a':1,'b':2,'c':3, 'd':0}
In [129]: d2 = {'b':76}
In [130]: d3 = {'a': 45, 'c':0}
In [131]: from collections import Counter
In [132]: Counter(d1) + Counter(d2) + Counter(d3)
Out[132]: Counter({'b': 78, 'a': 46, 'c': 3})