The collection lib has many cool classes, like defaultdict. Basically it is a dict that you can initialize with some value, so you don’t need to check if the key exists before each assignment, such as the following annoying computation of histogram: d={}

if item not in d:    

  d[item]=0 d[item]+=1

can be easily replaced with: from collections import defaultdict d=defaultdict(int) d[item]+=1

The cool thing is that you can give arbitrary “default factories”, that are simply a function without parameters. So let’s say you want to have a dict, with each key holding a list of three elements, all initialized to zero: d[k]=[0,0,0] d=defaultdict(lambda: [0,0,0]) d["hello"][1]+=1

Or you can even have a nested defaultdict, if you want double indirection: d = defaultdict(lambda:defaultdict(int)) d["hello"]["world"]+=1