defaultdict in python

Topics

• idiomatic python

The defaultdict class from the collections built-in module automatically stores a default value when a key doesn’t exist. All you have to do is provide a function that will return the default value to use each time a key is missing:

from collections import defaultdict
 
class Visits:
    def __init__(self):
        self.data = defaultdict(set)
 
    def add(self, country, city):
        self.data[country].add(city)
 
visits = Visits()
visits.add("England", "Bath")
visits.add("England", "London")

Using defaultdict is much better than using setdefault for this type of situation

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