A defaultdict is a subclass of dict that calls a factory function to supply missing values for any requested key.

… In today’s call, I was looking at a simple python solution that will try to count the number of fruits based on types.

Lets take this example. I want to know how many numbers apple in the list.

The typical way:

fruit_list = ["apple", "banana", "apple", "orange", "banana", "apple"]
fruit_counts = {}

for fruit in fruit_list:
    if fruit in fruit_counts:
        fruit_counts[fruit] += 1
    else:
        fruit_counts[fruit] = 1

print(fruit_counts)
# Output: {'apple': 3, 'banana': 2, 'orange': 1}

We will check the fruit_counts dict. If it’s there, add more. If not, set it it to 1.

Looks simple, but apparently… There’s a way to do this in more pythonic way..

from collections import defaultdict

fruit_list = ["apple", "banana", "apple", "orange", "banana", "apple"]
fruit_counts = defaultdict(int) # defaultdict with a default factory of int (which returns 0)

for fruit in fruit_list:
    fruit_counts[fruit] += 1 # If 'fruit' is not in fruit_counts, it defaults to 0, then 1 is added.

print(fruit_counts)
# Output: defaultdict(<class 'int'>, {'apple': 3, 'banana': 2, 'orange': 1})

If fruit is already a key in fruit_counts, its current value is incremented by 1. If fruit is not yet a key in fruit_counts, defaultdict(int) automatically creates fruit_counts[fruit] and initializes its value to 0. Then, 1 is added to it, making its value 1.

From GeeksforGeeks:

  • Using int: If you use int as the factory function, the default value will be 0 (since int() returns 0).
  • Using list: If you use list as the factory function, the default value will be an empty list ([]).
  • Using str: If you use str, the default value will be an empty string (’’).

Now… What if I want to start from 100? Apparently… we can! via lambda…

from collections import defaultdict

fruit_list = ["apple", "banana", "apple", "orange", "banana", "apple"]

fruit_counts = defaultdict(lambda: 50)

for fruit in fruit_list:
    # If 'fruit' is not in fruit_counts, it defaults to 50, then 1 is added.
    fruit_counts[fruit] += 1

print(fruit_counts)
# defaultdict(<function <lambda> at 0x7f73165e7d30>, {'apple': 53, 'banana': 52, 'orange': 51})