Python 3 Object-Oriented Programming - Third Edition

Using defaultdict

We've seen how to use setdefault to set a default value if a key doesn't exist, but this can get a bit monotonous if we need to set a default value every time we look up a value. For example, if we're writing code that counts the number of times a letter occurs in a given sentence, we could do the following:

def letter_frequency(sentence): 
    frequencies = {} 
    for letter in sentence: 
        frequency = frequencies.setdefault(letter, 0) 
        frequencies[letter] = frequency + 1 
    return frequencies 

Every time we access the dictionary, we need to check that it has a value already, and if not, set it to zero. When something like this needs to be done every time an empty key is requested, we can use a different version of the dictionary, called ...