Python - Dictionaries – Complete Guide for Beginners

Python dictionaries are one of the most important built-in data structures used to store data in key-value pairs.

Unlike lists or sets, dictionaries allow you to store data with a meaningful label (key), making data access fast and organized.

Dictionaries are:

• Ordered (Python 3.7+)
• Mutable
• Indexed by keys
• Do not allow duplicate keys

In this tutorial, you will learn:

• What dictionaries are
• How to create and use them
• Accessing and modifying data
• Adding and removing items
• Looping through dictionaries
• Real-world examples
• Common mistakes and best practices

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What is a Dictionary?

A dictionary stores data in key : value pairs.

Example:

student = {
    "name": "John",
    "age": 20,
    "course": "Python"
}

print(student)

Output:

{'name': 'John', 'age': 20, 'course': 'Python'}

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Creating a Dictionary

Method 1: Using curly braces

car = {
    "brand": "Toyota",
    "model": "Corolla",
    "year": 2024
}

Method 2: Using dict() constructor

car = dict(
    brand="Toyota",
    model="Corolla",
    year=2024
)

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Access Dictionary Items

You can access values using keys.

student = {
    "name": "Alice",
    "age": 22
}

print(student["name"])

Output:

Alice

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Using get() Method (Safe Access)

print(student.get("age"))

If key does not exist:

print(student.get("grade"))

Output:

None

---

Change Dictionary Items

student = {
    "name": "Bob",
    "age": 20
}

student["age"] = 21

print(student)

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Add Dictionary Items

student = {
    "name": "Charlie"
}

student["age"] = 25

print(student)

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Remove Dictionary Items

Using pop()

student.pop("age")

Using del

del student["name"]

Using clear()

student.clear()

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Loop Through a Dictionary

Loop keys

for key in student:
    print(key)

Loop values

for value in student.values():
    print(value)

Loop key-value pairs

for key, value in student.items():
    print(key, value)

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Dictionary Methods

Python provides many useful methods:

Method	Description
keys()	Returns all keys
values()	Returns all values
items()	Returns key-value pairs
get()	Safe access
pop()	Removes item
popitem()	Removes last item
update()	Adds/updates items
clear()	Removes all items
copy()	Copies dictionary

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Using keys(), values(), items()

keys()

print(student.keys())

values()

print(student.values())

items()

print(student.items())

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Nested Dictionaries

Dictionaries can store other dictionaries.

students = {
    "student1": {
        "name": "Alice",
        "age": 20
    },
    "student2": {
        "name": "Bob",
        "age": 22
    }
}

print(students["student1"]["name"])

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Real-World Example: User Profile

user = {
    "username": "john_doe",
    "email": "john@example.com",
    "active": True
}

print(user["email"])

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Real-World Example: Product Inventory

product = {
    "name": "Laptop",
    "price": 1200,
    "stock": 10
}

product["stock"] -= 1

print(product)

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Real-World Example: Student Grades

grades = {
    "Alice": 85,
    "Bob": 90,
    "Charlie": 78
}

print(grades["Bob"])

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Checking if Key Exists

if "name" in student:
    print("Key exists")

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Dictionary Length

print(len(student))

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Copy Dictionary

new_student = student.copy()

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Merge Dictionaries

Using update()

a = {"x": 1, "y": 2}
b = {"z": 3}

a.update(b)

print(a)

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Using | operator (Python 3.9+)

a = {"x": 1}
b = {"y": 2}

result = a | b

print(result)

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Common Mistakes

Mistake 1: Using index

student[0]  # Error

Mistake 2: Duplicate keys

data = {"name": "A", "name": "B"}

Only last value is kept.

Mistake 3: Key error

print(student["grade"])  # may error

Use get() instead.

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Best Practices

• Use meaningful keys
• Use get() for safe access
• Use dictionaries for structured data
• Avoid duplicate keys
• Use items() for looping key-value pairs

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Quick Summary

Task	Method
Create dictionary	{} or dict()
Access value	key or get()
Add item	dict[key] = value
Remove item	pop() / del
Loop dictionary	items()
Copy dictionary	copy()
Merge dictionaries	update() or `	`

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Conclusion

Python dictionaries are one of the most powerful and flexible data structures. They allow you to store data in key-value pairs, making data access fast, readable, and efficient.

By mastering dictionaries, you can build real-world applications like user systems, inventory management, APIs, and data processing tools.

Understanding dictionaries is essential for becoming a strong Python developer.