What is the difference between dict and collections.defaultdict?

Question

I was checking out Peter Norvig's code on how to write simple spell checkers. At the beginning, he uses this code to insert words into a dictionary.

def train(features):     model = collections.defaultdict(lambda: 1)     for f in features:         model[f] += 1     return model 

What is the difference between a Python dict and the one that was used here? In addition, what is the lambda for? I checked the API documentation here and it says that defaultdict is actually derived from dict but how does one decide which one to use?

Answer 1

The difference is that a defaultdict will "default" a value if that key has not been set yet. If you didn't use a defaultdict you'd have to check to see if that key exists, and if it doesn't, set it to what you want.

The lambda is defining a factory for the default value. That function gets called whenever it needs a default value. You could hypothetically have a more complicated default function.

Help on class defaultdict in module collections:  class defaultdict(__builtin__.dict)  |  defaultdict(default_factory) --> dict with default factory  |    |  The default factory is called without arguments to produce  |  a new value when a key is not present, in __getitem__ only.  |  A defaultdict compares equal to a dict with the same items.  |   

(from help(type(collections.defaultdict())))

{}.setdefault is similar in nature, but takes in a value instead of a factory function. It's used to set the value if it doesn't already exist... which is a bit different, though.

Answer 2

Courtesy :- https://shirishweb.wordpress.com/2017/05/06/python-defaultdict-versus-dict-get/

Using Normal dict

d={} d['Apple']=50 d['Orange']=20 print(d['Apple']) print(d['Grapes'])# This gives Key Error 

We can avoid this KeyError by using defaulting in normal dict as well, let see how we can do it

d={} d['Apple']=50 d['Orange']=20 print(d['Apple']) print(d.get('Apple')) print(d.get('Grapes',0)) # DEFAULTING 

Using default dict

from collections import defaultdict d = defaultdict(int) ## inside parenthesis we say what should be the default value. d['Apple']=50 d['Orange']=20 print(d['Apple']) print(d['Grapes']) ##→ This gives Will not give error 

Using an user defined function to default the value

from collections import defaultdict def mydefault():         return 0  d = defaultdict(mydefault) d['Apple']=50 d['Orange']=20 print(d['Apple']) print(d['Grapes']) 

Summary

1. Defaulting in normal dict is on case to case basis and in defaultdict we can provide default in general manner

2. Efficiency of using defaulting by defaultdict is two time greater than defaulting with normal dict. You can refer below link to know better on this performance testing https://shirishweb.wordpress.com/2017/05/06/python-defaultdict-versus-dict-get/