How to Use Python Dictionaries (dict)

Creating a dictionary

A dictionary (dict) is a type that stores values as key-value pairs.

Where a list is a box you pull values from by position (0, 1, 2…), a dict is a box you pull values from by name (key) — that's the easiest mental model.

# Declare a dictionary
car = {
    "brand": "Toyota",
    "model": "Prius",
    "year": 2015,
}

print(car)            # {'brand': 'Toyota', 'model': 'Prius', 'year': 2015}
print(type(car))      # <class 'dict'>

# Keys don't have to be strings (numbers work too)
score_by_id = {1: 80, 2: 95, 3: 72}
print(score_by_id[2]) # 95

When to use list vs dict

If order or position matters, use list. If you want to look up by name, use dict.

For example, 'a ranking of 3 scores' is a list; 'scores per user' is a dict.

Even for the same data, how you want to retrieve it decides which one to pick.

Let's manage a user profile with a dictionary.

① Create a user dict with these 3 key-value pairs:

- "name": "Alice"

- "age": 28

- "email": "alice@example.com"

② Use user["name"] to print the name and user["age"] to print the age with print().

Retrieving values — [] vs .get()

There are two ways to pull a value out of a dict, and the key difference is how they behave when the key doesn't exist.

Try pulling a non-existent key out of the car dict with [] to see what happens.

① Print car["brand"] with print().

② Print car["color"] with print() and confirm that KeyError is raised.

.get() lets you pass a default value as the second argument — that's the handy part.

You can say 'return this if the key isn't there' in a single line.

car = {"brand": "Toyota", "model": "Prius", "year": 2015}

# ① Access with []
print(car["brand"])        # Toyota
# print(car["color"])      # KeyError: 'color'

# ② Access with .get()
print(car.get("brand"))              # Toyota
print(car.get("color"))              # None
print(car.get("color", "Unknown"))  # Unknown (the second arg is returned)

.get() to keep running, [] to stay strict

If a missing key means something is broken, [] is safer — it surfaces the problem early as an error.

If it's fine either way (e.g., optional settings), passing a default to .get() reads better and cuts down on exception handling (covered later).

A user settings dict config = {"theme": "dark", "lang": "en"} is ready.

① Pull the current theme with config["theme"] and print it.

② Use .get() to fetch "font_size", falling back to 16 if it's not set.

Add, update, delete

Dicts, like lists, can be modified after they're created.

The simplest way to add, update, and delete is direct assignment.

car = {"brand": "Toyota", "model": "Prius", "year": 2015}

# Add (new key)
car["color"] = "white"
print(car)
# {'brand': 'Toyota', 'model': 'Prius', 'year': 2015, 'color': 'white'}

# Update (existing key)
car["year"] = 2024
print(car["year"])   # 2024

# Update multiple at once
car.update({"country": "JP", "model": "Camry"})
print(car["model"])    # Camry (overwritten)
print(car["country"])  # JP (newly added)

# Pop and delete
old_year = car.pop("year")
print(old_year)        # 2024
# print(car["year"])   # KeyError (gone now)

# Empty it out
car.clear()
print(car)             # {}

Edit a user info dict.

user = {"name": "Alice", "age": 28} is ready.

① Update "age" to 29.

② Add "email": "alice@example.com" as a new entry.

③ Use update() to apply "country": "JP" and "age": 30 at once, then print the final user.

Listing keys/values & checking existence

Last up: the tools for inspecting a dict.

There are 4 main ones — keys() / values() / items() / the in operator.

• keys() — returns the list of keys (dict_keys(['brand', 'model', ...]))
• values() — returns the list of values
• items() — returns the list of key-value pairs (usually paired with a for loop)
• "key" in dict — returns True / False depending on whether the key exists

car = {"brand": "Toyota", "model": "Prius", "year": 2015}

print(car.keys())     # dict_keys(['brand', 'model', 'year'])
print(car.values())   # dict_values(['Toyota', 'Prius', 2015])
print(car.items())    # dict_items([('brand', 'Toyota'), ...])

# Check for key existence (typically combined with if)
print("brand" in car)  # True
print("color" in car)  # False

if "brand" in car:
    print("Maker: " + car["brand"])
else:
    print("No maker info")

Combining for with items() (reference)

To iterate over a dict one item at a time, the standard form is for key, value in car.items():.

We'll cover for in a later chapter, so we won't dive deep here — but remember just this: items() lets you pull the key and value out together.

From the stock list stock = {"apple": 5, "banana": 0, "grape": 12}, write code that checks whether a given fruit is stocked and lists the keys.

① Use in to check if "banana" is a key in stock and print the result.

② For target = "lemon", print "{fruit} in stock: {count}" if it exists, or "Not carried" otherwise (the if template is in the comments).

③ Use stock.keys() to grab just the keys, wrap it with list(), and print it (the list of fruit names).

In this article you learned how to declare a dict, how [] and .get() differ when retrieving, how to add, update, and delete entries, and how to check key existence and list contents.

Dicts show up everywhere in real work — user info, settings, stock, summary results. Once you internalize the image of a box you look up by key, you'll feel right at home with the data processing that comes next.