Building Robust Object-Oriented Python Applications and Libraries/

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Using defaultdict

Using defaultdict

Understand how to use the defaultdict with the help of multiple examples.

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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:

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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:

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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, to ...