Python JSON

Learn to work with JSON data in simple steps

šŸ”„ What is JSON?

JSON (JavaScript Object Notation) is like a universal language for sharing data between programs. Think of it as a way to organize information that both humans and computers can easily read and understand. 


import json

# A simple JSON example
person_info = '{"name": "Alice", "age": 25, "city": "New York"}'

# Convert JSON to Python dictionary
person = json.loads(person_info)
print(person["name"])  # Prints: Alice
Easy
To Read
Universal
Format
Built-in
Python

What is JSON?

šŸ“„

Text-Based Format

Human-readable data format

Lightweight Readable
šŸŒ

Web APIs

Standard format for REST APIs

HTTP REST
šŸ”„

Data Exchange

Perfect for client-server communication

Serialization Parsing
šŸ

Python Integration

Built-in json module support

Native Simple

Lesson 1: What Does JSON Look Like?

JSON looks very similar to Python dictionaries, but it's stored as text:

JSON vs Python Dictionary 
# Python dictionary (what we know)
student = {
    "name": "Bob",
    "age": 20,
    "subjects": ["Math", "Science"],
    "graduated": False
}

print("Python dictionary:", student)
print("Type:", type(student))

# JSON string (text format)
json_string = '{"name": "Bob", "age": 20, "subjects": ["Math", "Science"], "graduated": false}'

print("\nJSON string:", json_string)
print("Type:", type(json_string))
Key Differences:
  • JSON uses "double quotes" for strings
  • JSON uses true/false (lowercase) for booleans
  • JSON is always text/string format

Lesson 2: Reading JSON Data

Use json.loads() to convert JSON text into Python objects:

Converting JSON to Python 
import json

# JSON string with weather data
weather_json = '''
{
    "city": "London",
    "temperature": 18,
    "condition": "Cloudy",
    "humidity": 65,
    "is_raining": false
}
'''

# Convert JSON to Python dictionary
weather = json.loads(weather_json)

# Now we can use it like a normal dictionary
print(f"City: {weather['city']}")
print(f"Temperature: {weather['temperature']}Ā°C")
print(f"Condition: {weather['condition']}")

if weather['is_raining']:
    print("Don't forget your umbrella!")
else:
    print("No rain expected!")
Working with JSON Lists 
# JSON with a list of students
students_json = '''
[
    {"name": "Alice", "grade": 85},
    {"name": "Bob", "grade": 92},
    {"name": "Charlie", "grade": 78}
]
'''

# Convert to Python list
students = json.loads(students_json)

print("Class Grades:")
for student in students:
    name = student["name"]
    grade = student["grade"]
    print(f"{name}: {grade}%")

Lesson 3: Creating JSON Data

Use json.dumps() to convert Python objects into JSON text:

Converting Python to JSON 
import json

# Create a Python dictionary
my_profile = {
    "username": "coder123",
    "email": "coder@example.com",
    "age": 22,
    "hobbies": ["coding", "gaming", "reading"],
    "is_student": True
}

# Convert to JSON string
profile_json = json.dumps(my_profile)

print("Python dictionary:")
print(my_profile)
print("\nJSON string:")
print(profile_json)
Making JSON Pretty 
# Make JSON easier to read with indentation
book_info = {
    "title": "Python for Beginners",
    "author": "Jane Smith",
    "pages": 250,
    "chapters": ["Introduction", "Variables", "Functions", "Classes"]
}

# Pretty JSON with indentation
pretty_json = json.dumps(book_info, indent=4)

print("Pretty formatted JSON:")
print(pretty_json)

Lesson 4: Saving and Loading JSON Files

You can save JSON data to files and load it back later:

Saving Data to JSON File 
import json

# Create some data to save
my_favorites = {
    "movies": ["The Matrix", "Inception", "Interstellar"],
    "books": ["1984", "To Kill a Mockingbird"],
    "colors": ["blue", "green", "purple"],
    "number": 42
}

# Save to a JSON file
with open("my_favorites.json", "w") as file:
    json.dump(my_favorites, file, indent=4)

print("āœ… Data saved to my_favorites.json!")
Loading Data from JSON File 
import json

# Load data from JSON file
try:
    with open("my_favorites.json", "r") as file:
        loaded_favorites = json.load(file)
    
    print("āœ… Data loaded successfully!")
    print("\nYour favorite movies:")
    for movie in loaded_favorites["movies"]:
        print(f"  šŸŽ¬ {movie}")
    
    print(f"\nYour lucky number: {loaded_favorites['number']}")

except FileNotFoundError:
    print("āŒ File not found. Make sure to save data first!")

Lesson 5: Building a Simple Contact Book

Let's create a contact book that saves data in JSON format:

Simple Contact Book 
import json
import os

class SimpleContactBook:
    def __init__(self):
        self.filename = "contacts.json"
        self.contacts = self.load_contacts()
    
    def load_contacts(self):
        """Load contacts from JSON file"""
        if os.path.exists(self.filename):
            try:
                with open(self.filename, "r") as file:
                    return json.load(file)
            except:
                print("āš ļø Error loading contacts. Starting fresh.")
                return []
        return []
    
    def save_contacts(self):
        """Save contacts to JSON file"""
        try:
            with open(self.filename, "w") as file:
                json.dump(self.contacts, file, indent=4)
            print("āœ… Contacts saved!")
            return True
        except:
            print("āŒ Error saving contacts!")
            return False
    
    def add_contact(self, name, phone, email=""):
        """Add a new contact"""
        contact = {
            "name": name,
            "phone": phone,
            "email": email
        }
        self.contacts.append(contact)
        self.save_contacts()
        print(f"āœ… Added {name} to contacts!")
    
    def show_all_contacts(self):
        """Display all contacts"""
        if not self.contacts:
            print("šŸ“­ No contacts found!")
            return
        
        print(f"\nšŸ“ž Your Contacts ({len(self.contacts)} total):")
        print("-" * 40)
        
        for i, contact in enumerate(self.contacts, 1):
            print(f"{i}. {contact['name']}")
            print(f"   šŸ“± {contact['phone']}")
            if contact['email']:
                print(f"   šŸ“§ {contact['email']}")
            print()
    
    def find_contact(self, name):
        """Find a contact by name"""
        for contact in self.contacts:
            if name.lower() in contact['name'].lower():
                return contact
        return None

# Demo the contact book
def demo_contact_book():
    book = SimpleContactBook()
    
    print("šŸ“ž Simple Contact Book Demo")
    print("=" * 30)
    
    # Add some sample contacts
    book.add_contact("Alice Johnson", "555-0123", "alice@email.com")
    book.add_contact("Bob Smith", "555-0456")
    book.add_contact("Charlie Brown", "555-0789", "charlie@email.com")
    
    # Show all contacts
    book.show_all_contacts()
    
    # Find a specific contact
    found = book.find_contact("Alice")
    if found:
        print(f"šŸ” Found: {found['name']} - {found['phone']}")

# Run the demo
demo_contact_book()

Quick Reference: JSON Methods

json.loads()

Convert JSON string to Python object 

# JSON string ā†’ Python
data = json.loads('{"name": "Alice"}')
print(data["name"])  # Alice

json.dumps()

Convert Python object to JSON string 

# Python ā†’ JSON string
data = {"name": "Alice"}
json_str = json.dumps(data)
print(json_str)  # {"name": "Alice"}

json.load()

Read JSON from a file 

# Read from file
with open("data.json", "r") as f:
    data = json.load(f)

json.dump()

Write JSON to a file 

# Write to file
with open("data.json", "w") as f:
    json.dump(data, f, indent=4)

Common JSON Errors and Solutions

šŸš« JSONDecodeError

Problem: Invalid JSON format 

# Wrong: Single quotes
bad_json = "{'name': 'Alice'}"

# Right: Double quotes
good_json = '{"name": "Alice"}'

# Always use try-except for safety
try:
    data = json.loads(json_string)
except json.JSONDecodeError:
    print("Invalid JSON format!")

šŸš« File Not Found

Problem: JSON file doesn't exist 

# Check if file exists first
import os

if os.path.exists("data.json"):
    with open("data.json", "r") as f:
        data = json.load(f)
else:
    print("File not found!")

šŸš« KeyError

Problem: Trying to access a key that doesn't exist 

# Use .get() method for safety
data = {"name": "Alice"}

# Wrong: Might cause KeyError
# age = data["age"]

# Right: Safe with default value
age = data.get("age", "Unknown")
print(f"Age: {age}")

šŸ§  Test Your Knowledge

Which method converts JSON string to Python object?

What makes JSON output more readable?

Which method saves data to a JSON file?