Dictionaries: Key-Value Mapping

15 min · Beginner

A dictionary (dict) is an unordered-by-nature (insertion-ordered in Python 3.7+) collection of key-value pairs. Dictionaries provide average O(1) constant-time lookups.

Creating dictionaries

Dictionary is a collection that stores data as key-value pairs. You look up a value by its key, not by a position number, and the lookup stays fast even for a very large dictionary.

Enclose key-value pairs separated by colons : inside curly braces {}.

Python

1student = {2    "id": 101,3    "name": "Kabir",4    "gpa": 3.95}6print(student["name"])

Accessing student["name"] retrieves 'Kabir' instantly in O(1) time.

Safe lookups with get()

KeyError is the error Python raises when you ask a dictionary for a key that is not in it. The method get() avoids it by returning a fallback value instead.

Accessing a missing key with square brackets raises a KeyError. The .get() method returns None or a custom fallback instead.

Python

config = {"port": 8000, "debug": True}print(config.get("host", "127.0.0.1"))print(config.get("timeout"))

Example 1 · Selling one unit

Python

product = {"sku": "SKU-99", "price": 450, "stock": 12}# Check inventoryif product.get("stock", 0) > 0:    product["stock"] -= 1    print(f"Purchased 1 unit. Remaining: {product['stock']}")

Output

Purchased 1 unit. Remaining: 11

What this code does

Uses .get('stock', 0) to inspect inventory safely.
Deducts 1 unit from stock, printing remaining count 11.

Because 'host' was not found, .get() smoothly returns the default '127.0.0.1'.

Adding and updating keys

Assigning to a key adds it if it does not exist, or updates its value if it does.

Python

user = {"name": "Tara"}user["role"] = "Developer"user["role"] = "Lead"print(user)

Example 2 · Updating an order record

Python

order = {"id": 5012, "status": "packed"}order["status"] = "shipped"order["courier"] = "Express"print(order)print(len(order))

Output

{'id': 5012, 'status': 'shipped', 'courier': 'Express'}
3

What this code does

order["status"] already exists, so the assignment replaces "packed" with "shipped".
order["courier"] is a new key, so the assignment adds it at the end.
len(order) counts the keys: three.

user['role'] = 'Developer' inserts the new key, and assigning 'Lead' updates it in place.

Checking keys and removing entries

Hashable is a value whose hash code never changes while it lives, so Python can use it as a dictionary key. Strings, numbers and tuples of them are hashable. Lists and dictionaries are not.

The in operator checks whether a key exists. The method pop() removes a key and gives back its value, and del removes a key without returning it.

Python

stock = {"pen": 40, "book": 12, "bag": 7}print("pen" in stock)removed = stock.pop("book")del stock["bag"]print(removed, stock, len(stock))

Example 3 · Cancelling a cart line

Python

1cart = {"laptop": 1, "mouse": 2, "cable": 3}2removed = cart.pop("cable")3print(f"Removed {removed} cables")4print("cable" in cart)5print(cart.pop("hub", 0))6print(cart)

Output

Removed 3 cables
False
0
{'laptop': 1, 'mouse': 2}

What this code does

pop("cable") deletes the key and returns its value, 3.
After that "cable" in cart is False.
pop("hub", 0) gives the fallback 0 because hub is not there. Without a fallback it would raise a KeyError.

in looks only at keys. pop returned 12 and del returned nothing, so stock is left with the one key pen.

Next step

Check your understanding

9 quick questions

Checking keys and removing entries

Hashable is a value whose hash code never changes while it lives, so Python can use it as a dictionary key. Strings, numbers and tuples of them are hashable. Lists and dictionaries are not.

The in operator checks whether a key exists. The method pop() removes a key and gives back its value, and del removes a key without returning it.