defaultdict is a subclass of the built-in dict class from the collections module. It provides a convenient way to handle missing keys by automatically assigning a default value when accessing a key that doesn't exist in the dictionary.

To use defaultdict, you need to import it from the collections module. Here's an example:

from collections import defaultdict

fruit_prices = defaultdict(lambda: "Unknown")
fruit_prices["apple"] = 1.0
fruit_prices["banana"] = 0.5

print(fruit_prices["apple"])  # Output: 1.0
print(fruit_prices["banana"])  # Output: 0.5
print(fruit_prices["orange"])  # Output: Unknown

In this code, we create a defaultdict object called fruit_prices with a default value of "Unknown". We assign prices for "apple" and "banana", but when we access the price for "orange", which is not present in the dictionary, the default value "Unknown" is returned.

The default value is specified as a lambda function in this example, but you can use any callable object e.g. list, dict or a function that doesn't require any arguments.

The benefit of using defaultdict is that it eliminates the need for explicit checks and handling of missing keys. When accessing a missing key, it automatically creates the key with the default value assigned to it.