d2.py

from collections import defaultdict
#default_factory is given as list class
d=defaultdict(list)
d['a']=1
d['b']=2
print (d)#Output:defaultdict(<class 'list'>, {'a': 1, 'b': 2})

print (d['a'])#Output:1
# key 'c' doesn't exists in the dictionary,it returns the default value of list which is [].
print (d['c'])#Output:[]