Dictionaries

What are dictionaries?

Dictionaries store data in key-value pairs. Think of them like a real dictionary where you look up a word (key) to find its definition (value). Real-world examples:

Phone book: name > phone number
Menu: dish > price
User profile: username > user info

Creating Dictionaries

Python provides several ways to create dictionaries depending on your data source and style:

1. Using Literal Curly Braces `{}`

The most common way to create a dictionary is using curly braces with key-value pairs separated by colons:

# Empty dictionary
my_dict = {}

# With initial data
person = {
    "name": "Aarav",
    "age": 30,
    "city": "Mumbai"
}

2. Using the `dict()` Constructor

You can construct dictionaries using the built-in dict() constructor in two ways:

Keyword arguments: Keys are passed as keyword arguments (keys are automatically converted to strings):
scores = dict(math=95, english=87, science=92)
List of key-value tuples: Useful when converting zipped or tabular sequences into a dictionary:
pairs = [("math", 95), ("english", 87), ("science", 92)] scores = dict(pairs)

3. Using `dict.fromkeys()`

Useful when you want to initialize a dictionary with a set of keys mapping to the same default value:

keys = ["math", "english", "science"]
scores = dict.fromkeys(keys, 0)  # {'math': 0, 'english': 0, 'science': 0}

4. Dictionary Comprehension

A concise way to construct dictionaries dynamically using loops:

# Create a dictionary of squares: {1: 1, 2: 4, 3: 9}
squares = {x: x**2 for x in range(1, 4)}

Keys must be unique and of an immutable type (like strings, numbers, or tuples). Values can be of any data type and can repeat.

Accessing values

person = {"name": "Alice", "age": 30, "city": "New York"}

# Get values by key
print(person["name"])       # "Alice"
print(person["age"])        # 30

# Safer with get()
print(person.get("job"))    # None (no error)
print(person.get("job", "Unknown"))  # "Unknown" (default)

Changing dictionaries

person = {"name": "Alice", "age": 30}

# Add or update
person["email"] = "[email protected]"  # Add new
person["age"] = 31                   # Update existing

# Remove items
del person["email"]              # Remove by key
age = person.pop("age")          # Remove and return
person.clear()                   # Remove all items

Dictionary methods

person = {"name": "Alice", "age": 30, "city": "New York"}

# Get all keys, values, or items
print(person.keys())    # dict_keys(['name', 'age', 'city'])
print(person.values())  # dict_values(['Alice', 30, 'New York'])
print(person.items())   # dict_items([('name', 'Alice'), ...])

# Check if key exists
if "name" in person:
    print("Name found!")

# Update multiple values
person.update({"age": 31, "job": "Engineer"})

Nested dictionaries

# Dictionary of dictionaries
students = {
    "alice": {"age": 20, "grade": "A"},
    "bob": {"age": 21, "grade": "B"},
    "charlie": {"age": 19, "grade": "A"}
}

# Access nested data
print(students["alice"]["grade"])  # "A"

Common mistakes

KeyError when key doesn't exist

# Wrong
person = {"name": "Alice"}
print(person["age"])  # KeyError!

# Right - use get()
print(person.get("age", 0))  # Returns 0 if missing

Using mutable keys

# Wrong - lists can't be keys
bad_dict = {[1, 2]: "value"}  # TypeError!

# Right - use immutable types
good_dict = {(1, 2): "value"}  # Tuple is OK
good_dict = {"1,2": "value"}   # String is OK

Practice & Exercises

To reinforce what you’ve learned in this section (Dictionaries, creation patterns, key accesses, and updates), practice with these interactive notebooks:

Follow-Along Practice

Practice initializing dictionaries in various ways, looking up values, and editing key-value pairs.💻 VS Code | 🚀 Colab | 📥 Download

Practice Exercises

Test your knowledge with hands-on exercises on school grades databases and safe lookups.💻 VS Code | 🚀 Colab | 📥 Download

What’s next?

Finally, let’s explore Sets - collections of unique items!

Sets

Learn about unique values

Getting started

Data types

Working with data

Control flow

Data structures

Packing & Unpacking

What are dictionaries?

Creating Dictionaries

1. Using Literal Curly Braces `{}`

2. Using the `dict()` Constructor

3. Using `dict.fromkeys()`

4. Dictionary Comprehension

Accessing values

Changing dictionaries

Dictionary methods

Nested dictionaries

Common mistakes

Practice & Exercises

Follow-Along Practice

Practice Exercises

What’s next?

Sets

​What are dictionaries?

​Creating Dictionaries

​1. Using Literal Curly Braces {}

​2. Using the dict() Constructor

​3. Using dict.fromkeys()

​4. Dictionary Comprehension

​Accessing values

​Changing dictionaries

​Dictionary methods

​Nested dictionaries

​Common mistakes

​Practice & Exercises

Follow-Along Practice

Practice Exercises

​What’s next?

Sets

What are dictionaries?

Creating Dictionaries

1. Using Literal Curly Braces `{}`

2. Using the `dict()` Constructor

3. Using `dict.fromkeys()`

4. Dictionary Comprehension

Accessing values

Changing dictionaries

Dictionary methods

Nested dictionaries

Common mistakes

Practice & Exercises

What’s next?