In Dictionary, the key must be unique and immutable. This means that a Python Tuple can be a key whereas a Python List can not. A Dictionary can be created by placing a sequence of element within curly {} braces, separated by ‘comma’.

Defaultdict is a container like dictionaries present in the module collections. Defaultdict is a sub-call of the dictionary class that returns a dictionary-like object. The functionality of both dictionaries and defaultdict are almost same except for the fact that defaultdict never raises a KeyError. It provides a default value for the key that does not exists.

Syntax: defaultdict(default_factory)
Parameters:
   default_factory: A function returning the default value for the dictio nary defined. If this argument is absent then the dictionary raises a KeyError

from collections import defaultdict

d = defaultdict(lambda: "Not Present")
d["a"] = 1
d["b"] = 2

print(d["a"])  # 1
print(d["b"])  # 2
print(d["c"])  # Not Present