Python Dictionaries: When Should You Use `collections.defaultdict`?

Delving into the Distinction: Collections.defaultdict vs. Ordinary Dict

In Python, the default dictionary (collections.defaultdict) differs from a regular dictionary in a crucial way. While a standard dict raises a KeyError when accessing a non-existent key, a defaultdict automatically creates the missing item by invoking a specified function.

Understanding the Examples

Let's examine the provided examples:

d = defaultdict(int)

Here, int() is the default function, which initializes the missing key with an integer value (defaulting to 0).

for k in s:
    d[k] += 1

This loop iterates over each character (k) in the string s and increments its corresponding count stored in the defaultdict.

d.items()
dict_items([('m', 1), ('i', 4), ('s', 4), ('p', 2)])

As a result, we obtain a dictionary with the character frequencies.

In the second example:

d = defaultdict(list)

list() is the default function, creating empty lists as the default for missing keys.

for k, v in s:
    d[k].append(v)

This loop pairs keys and values from the list s and appends values to the corresponding key's list.

d.items()
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

The outcome is a dictionary where keys are colors, and values are lists of their corresponding values from the original list.

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