What is defaultdict in Python?

In Python, defaultdict is a subclass of the built-in dictionary class dict. It is a dictionary-like object that provides a default value for a nonexistent key. This means that when you try to access a key that does not exist in the dictionary, defaultdict will automatically create a new entry for that key and initialize it with a default value, which you specify when you create the defaultdict object.

Here's the general syntax for creating a defaultdict object:

from collections import defaultdict

my_dict = defaultdict(default_value)

In the above syntax, default_value is the default value that you want to assign to a nonexistent key, and my_dict is the defaultdict object that you create.

Here's an example:

from collections import defaultdict

my_dict = defaultdict(int)
my_dict['a'] = 10
my_dict['b'] = 20

print(my_dict['a']) # Output: 10
print(my_dict['b']) # Output: 20
print(my_dict['c']) # Output: 0

In the above example, we create a defaultdict object with a default value of int, which is 0. We add two entries to the dictionary, my_dict['a'] and my_dict['b'], and then try to access a nonexistent key my_dict['c']. Since the key does not exist in the dictionary, defaultdict creates a new entry for it and initializes it with the default value, which is 0. When we print my_dict['c'], we get the output 0, which is the default value.

The Python defaultdict type behaves almost exactly like a regular Python dictionary, but if you try to access or modify a missing key, then defaultdict will automatically create the key and generate a default value for it. This makes defaultdict a valuable option for handling missing keys in dictionaries.