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Building Robust Object-Oriented Python Applications and Libraries

Using defaultdict

Explore how to use Python's defaultdict to simplify handling missing dictionary keys and create default values. Understand how to use it for counting, storing stock prices, and managing nested dictionaries with custom default objects for cleaner, more maintainable code.

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We’ve seen how to use the setdefault method 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. Let’s have a look at an example to better understand this concept.

Example: Count letters

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:

Python 3.10.4
from __future__ import annotations
def letter_frequency(sentence: str) -> dict[str, int]:
  frequencies: dict[str, int] = {}
  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 already has a value; if not, set it to zero. When something like this needs to be done every time an empty key is requested, we can create a different version of a dictionary. The defaultdict, defined in the collections module, handles missing keys elegantly:

Python 3.10.4
from collections import defaultdict
def letter_frequency_2(sentence: str) -> defaultdict[str, int]:
    frequencies: defaultdict[str, int] = defaultdict(int)
    for letter in sentence:
      frequencies[letter] += 1
    return frequencies

This code looks odd: the defaultdict() evaluation accepts a function, int, in its constructor. We’re not evaluating the int() function; we’re providing a reference to this function to defaultdict(). Whenever a key that is not already in the dictionary is accessed, it calls that function, with no parameters, ...