Use Python defaultdict: student_scores example

Imagine you have a magic notebook that magically gives you an empty page whenever you ask for one that doesn’t exist. That’s what Python’s defaultdict does for missing keys—no errors, just auto-magic!

This snippet:

from collections import defaultdict

student_scores = defaultdict(list)
student_scores['Azeem Teli'].append(95)

is part of my article Stop Abusing Python Dictionaries — Use Them Like a Pro!

Why Use defaultdict Instead of a Dict?

Python Collections — DefaultDict : Dictionary with Default values and Automatic Keys

With a normal dict, asking for a missing key causes a KeyError crash and tears. 😭 You’d need to do this:

scores = {}
if name not in scores:
    scores[name] = []
scores[name].append(score)

defaultdict(list) does all that in one line. It auto-creates a new list for any new key.

How It Works: The default factory

default_factory is a function (e.g. list, int) that runs only when a missing key is accessed.
For list, it returns []; for int, it returns 0.
Only getitem triggers it, not .get()

Real-Life Example:

Think of a Gachapon machine:

Empty cup? It gives you a bubblegum (magic default).
Cup already filled? It just drops in more treats.

Perfect for building student_scores lists — one line, zero fuss.

Student Score Jar 🍭

You have a jar for each student’s candies (scores).
If a student walks in and you forgot to give them a jar, defaultdict gives them a new one.
Now you can drop candies (scores) in any student jar, even if it’s their first candy.

Like this:

student_scores = defaultdict(list)  # Magic candy jar machine
student_scores['Azeem Teli'].append(95)  # Drop 95 candy in Azeem’s jar

Zoom! No complaints, no crying—just candy.

Joke Break and Meme Moment 😂

(Insert a meme image of a kid saying “Defaultdict? More like Default-GIF!”) Defaultdict is like the friend who brings extra pizza whenever you forget to order more. You never run out!

More Fun Examples with defaultdict

1. Count scores like Pokémon caught:

counts = defaultdict(int)
for score in [95, 80, 95]:
    counts[score] += 1
# counts = {95: 2, 80: 1}

Because int() gives you 0

2. Nested defaultdicts — A jar of jars:

scores = defaultdict(lambda: defaultdict(list))
scores['Class A']['Azeem Teli'].append(95)

Magic, times two!

When Not to Use It

If you don't want hidden keys created (like accidentally checking .get()), use a normal dict.
Remember: defaultdict does insert the missing key—even if you never append ([Stack Overflow][5], [Real Python][3]).

For super-pro dictionary skills, read my main article: "Stop Abusing Python Dictionaries — Use Them Like a Pro!" 🔗
And here’s a detailed To-Do List app in Python tutorial that uses dictionaries beautifully: check it out! Build a To-Do List App – it's naturally linked and explained step-by-step.

Summary: Why `defaultdict(list)` Rocks

Handles missing keys gracefully
Saves you from writing extra if or try/except clauses
Great for grouping, counting, managing data
Teaches best practices—use dicts like pros!

Final Meme Fiesta 🎉

(Imagine a comic: a regular dict crying “KeyError!”, while defaultdict strolls by with sunglasses and a thumbs-up)