d4.py

from collections import defaultdict
#default_factory is given as user defined function
def func():
    return "None"
d=defaultdict(func)
d['a']=1
d['b']=2
print (d)#Output:defaultdict(<function func at 0x015A7808>, {'a': 1, 'b': 2})

print (d['a'])#Output:1
# key 'c' doesn't exists in the dictionary,it returns the default value which is "None"
print (d['c'])#Output:None