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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.

We'll cover the following...

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 ...