d7.py

from collections import defaultdict


# default_factory is specified.
def func():
    return "Not available"


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 0x009B7808>, {'a': 1, 'b': 2})

# Returns the default value of key which is "Not available"
print(d.__missing__('a'))  # Output:Not available
print(d.__missing__('z'))  # Output:Not available

#'z' is inserted to defaultdict
print (d)#Output:defaultdict(<function func at 0x01537808>, {'a': 'Not available', 'b': 2, 'z': 'Not available'})