Lesson · ⏱ 7 min read

🧺 Lists and dictionaries: storing many values

Q: You want to look up a student's marks by their roll number. Which is best?

A dictionary. A dictionary maps a key (roll number) straight to a value (marks), with no searching through positions.

🏠 Real-life example

A shopping list is ordered: first milk, then bread, then eggs. That's a Python list. Your phone's contact book links a name to a number: 'Amma → 98xxxx'. You look things up by name, not by position. That's a dictionary.

💡 In simple words

Real data comes in groups, so Python has collections. A list [ ] keeps items in order and you access them by position, starting from 0. A dictionary { } stores key → value pairs and you look up values by key, which is very fast. A tuple ( ) is like a list that can't be changed, and a set keeps only unique items. Most data you'll see in AI work, such as JSON from APIs, is lists of dictionaries.

🪜 Step by step

List: an ordered collection you can change. scores = [72, 88, 95]; scores.append(60); scores[0] is the first item.
Dictionary: look things up by a key. emp = {"name": "Ravi", "city": "Pune"}; emp["city"] gives "Pune". Use emp.get("age", 0) when a key may be missing.
Set: unique values with fast membership checks. seen = set(); if user_id in seen: ... Great for removing duplicates.
Loop over them: for k, v in emp.items(): print(k, v). Most real data is a list of dictionaries, like rows from an API.
Build new lists quickly with comprehensions: high = [s for s in scores if s > 80].

🔑 Key terms

List: An ordered collection: ['milk', 'bread'].
Dictionary: Key → value pairs: {'name': 'Asha', 'city': 'Pune'}.
Index: The position of an item in a list. The first item is index 0.

👀 See it in action

python

cart = ["milk", "bread", "eggs"]
print(cart[0])            # milk
cart.append("tea")

prices = {"milk": 30, "bread": 45, "eggs": 72, "tea": 120}
total = sum(prices[item] for item in cart)
print(total)              # 267

A list for what's in the cart, and a dictionary to look up each price.

⚠️ Common mistakes to avoid

Searching a big list with 'in' inside a loop. It gets very slow; convert to a set first.
KeyError from emp["age"] when the key doesn't exist. Use .get() or check 'if "age" in emp'.
Changing a list while looping over it. Build a new list instead.

✅ Quick check

You want to look up a student's marks by their roll number. Which is best?

A list
A single string
A dictionary

Show answer

C. A dictionary: A dictionary maps a key (roll number) straight to a value (marks), with no searching through positions.

🙋 Questions learners ask

List or dictionary: how do I choose?

If you access items by position or just go through them in order, use a list. If you look items up by a name or ID, use a dictionary.

What is JSON and how does it relate?

JSON is the text format APIs use. It maps directly to Python dictionaries and lists: json.loads(text) gives you dicts and lists, json.dumps(data) turns them back into text.

📌 Remember

List = ordered line of items. Dictionary = look up by name (key).

Learn this as part of a full course

Track your progress, take the quick checks and follow the lessons in the right order. This lesson is part of:

AI Foundations Path · AI Product Engineer · LLM / Agent Engineer · AI Forward Deployed Engineer

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