d9.py

from collections import defaultdict
#default_factory is mentioned.
def func():
    return NotAvailable

d=defaultdict(func)
d['a']=1
d['b']=2
#default_factory attribute is given as user_defined function.
print (d)#Output:defaultdict(<function func at 0x00827808>, {'a': 1, 'b': 2})


#Calling func raises NameError.
print (d.__missing__('a')) #Output:NameError: name 'NotAvailable' is not defined
print (d.__missing__('z'))#Output:NameError: name 'NotAvailable' is not defined