What is the difference between dict and collections.defaultdict?

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.

Use a defaultdict if you have some meaningful default value for missing keys and don't want to deal with them explicitly.

The defaultdict constructor takes a function as a parameter and constructs a value using that function.

lambda: 1

is the same as the parameterless function f that does this

def f():
 return 1

I forgot the reason the API was designed this way instead of taking a value as a parameter. If I designed the defaultdict interface, it would be slightly more complicated, the missing value creation function would take the missing key as a parameter.

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/