What is defaultdict in Python and how is it different from a Dictionary

In Python, a defaultdict is a subclass of the built-in dict class that provides a default value for keys that do not exist. It's part of the collections module. The primary difference between a defaultdict and a regular dictionary is how it handles missing keys.

In a regular dictionary (i.e., a standard Python dictionary), if you try to access a key that doesn't exist, you will get a KeyError. For example:

python
my_dict = {}
value = my_dict['nonexistent_key']  # Raises KeyError

On the other hand, in a defaultdict, you can specify a default value that will be returned when you access a non-existent key. You provide the default value as a callable, typically using a function or a lambda expression.

Here's how you can use a defaultdict:

python
from collections import defaultdict

# Create a defaultdict with an int as the default factory
my_defaultdict = defaultdict(int)

# Accessing a non-existent key returns the default value (0 for int)
value = my_defaultdict['nonexistent_key']  # Returns 0

# You can also specify a different default value using a lambda
my_defaultdict = defaultdict(lambda: 'Default Value')

value = my_defaultdict['nonexistent_key']  # Returns 'Default Value'

The main benefit of defaultdict is that it allows you to avoid explicitly checking whether a key exists before accessing it. It can simplify code in situations where you want to ensure that keys have default values, and you don't want to handle missing keys with if statements.

In summary, a defaultdict is a subclass of a dictionary that provides default values for missing keys, making it convenient to work with and eliminating the need for explicit checks for key existence.