Working with JSON files in Python - Mini Project: Build & Apply

Working with JSON files

📖 Scenario: You are managing a small library's book records. The library stores book information in JSON files. You want to read, update, and save this data using Python.

🎯 Goal: Learn how to load JSON data from a file, update it, and save it back to a JSON file using Python.

📋 What You'll Learn

Create a JSON data structure representing books

Load JSON data from a file

Update the JSON data by adding a new book

Save the updated JSON data back to a file

Print the updated JSON data

💡 Why This Matters

🌍 Real World

Many applications store data in JSON files because JSON is easy to read and write. Managing JSON files is common in web apps, configuration files, and data exchange.

💼 Career

Knowing how to work with JSON files is important for software developers, data analysts, and anyone working with APIs or configuration management.

1

Create initial JSON data

Create a variable called books that holds a list of dictionaries. Each dictionary should represent a book with these exact keys and values: {"title": "The Hobbit", "author": "J.R.R. Tolkien", "year": 1937} and {"title": "1984", "author": "George Orwell", "year": 1949}.

Python

# Create the list of books with two dictionaries
# Your code here

Hint

Use a list with two dictionaries inside. Each dictionary has keys: title, author, and year.

2

Save JSON data to a file

Import the json module. Then open a file called library.json in write mode and save the books list to this file using json.dump().

Python

books = [
    {"title": "The Hobbit", "author": "J.R.R. Tolkien", "year": 1937},
    {"title": "1984", "author": "George Orwell", "year": 1949}
]

# Import json and save books to library.json
# Your code here

Hint

Use import json. Then open the file with open("library.json", "w") and use json.dump(books, file) to save.

3

Load JSON data from the file and update it

Open the file library.json in read mode and load the JSON data into a variable called loaded_books using json.load(). Then add a new book dictionary {"title": "To Kill a Mockingbird", "author": "Harper Lee", "year": 1960} to loaded_books.

Python

import json

books = [
    {"title": "The Hobbit", "author": "J.R.R. Tolkien", "year": 1937},
    {"title": "1984", "author": "George Orwell", "year": 1949}
]

with open("library.json", "w") as file:
    json.dump(books, file)

# Load JSON data from library.json and add a new book
# Your code here

Hint

Use with open("library.json", "r") as file and loaded_books = json.load(file). Then use loaded_books.append(...) to add the new book.

4

Save updated data and print it

Open library.json in write mode again and save the updated loaded_books list using json.dump(). Then print loaded_books to show the updated list.

Python

import json

books = [
    {"title": "The Hobbit", "author": "J.R.R. Tolkien", "year": 1937},
    {"title": "1984", "author": "George Orwell", "year": 1949}
]

with open("library.json", "w") as file:
    json.dump(books, file)

with open("library.json", "r") as file:
    loaded_books = json.load(file)

loaded_books.append({"title": "To Kill a Mockingbird", "author": "Harper Lee", "year": 1960})

# Save updated books and print loaded_books
# Your code here

Hint

Open the file with open("library.json", "w") and save loaded_books with json.dump(). Then print loaded_books.

Practice

1.

What is the main purpose of using JSON files in Python programs?

easy

A. To create graphical user interfaces

B. To execute Python code faster

C. To save and load data in a simple text format

D. To compile Python programs into executables

Solution

1. Step 1: Understand JSON file usage

   JSON files store data in a text format that is easy to read and share.

2. Step 2: Identify the correct purpose

   Saving and loading data simply matches the main use of JSON files in Python.

3. Final Answer:

   To save and load data in a simple text format -> Option C

4. Quick Check:

   JSON files = data storage [OK]

Hint: JSON files store data simply as text [OK]

Common Mistakes:

• Thinking JSON speeds up code execution
• Confusing JSON with GUI creation
• Assuming JSON compiles code

2.

Which of the following is the correct way to open a JSON file named data.json for reading in Python?

easy

A. open('data.json', 'r')

B. open('data.json', 'w')

C. open('data.json', 'a')

D. open('data.json', 'x')

Solution

1. Step 1: Understand file modes

   'r' mode opens a file for reading, 'w' for writing, 'a' for appending, 'x' for creating new file.

2. Step 2: Choose mode for reading JSON

   To read JSON data, the file must be opened in 'r' mode.

3. Final Answer:

   open('data.json', 'r') -> Option A

4. Quick Check:

   Read mode = 'r' [OK]

Hint: Use 'r' mode to read files [OK]

Common Mistakes:

• Using 'w' which overwrites the file
• Using 'a' which appends data
• Using 'x' which fails if file exists

3.

What will be the output of the following Python code?

import json

with open('data.json', 'w') as f:
    json.dump({'name': 'Alice', 'age': 30}, f)

with open('data.json', 'r') as f:
    data = json.load(f)
print(data['age'])

medium

A. Alice

B. 30

C. {'name': 'Alice', 'age': 30}

D. Error: file not found

Solution

1. Step 1: Write dictionary to JSON file

   The code writes {'name': 'Alice', 'age': 30} to 'data.json' using json.dump.

2. Step 2: Read JSON file and access 'age'

   json.load reads the file back as a dictionary, then data['age'] accesses the value 30.

3. Final Answer:

   30 -> Option B

4. Quick Check:

   data['age'] = 30 [OK]

Hint: json.load returns Python dict from JSON file [OK]

Common Mistakes:

• Expecting string output instead of integer
• Confusing json.dump and json.load usage
• Assuming file read fails without writing first

4.

Identify the error in this code snippet that tries to read JSON data from a file:

import json

f = open('data.json', 'r')
data = json.load(f)
f.close()
print(data)

medium

A. Missing import statement for json

B. File not opened in write mode

C. File not closed properly

D. No error, code works correctly

Solution

1. Step 1: Check import and file open

   The code imports json and opens the file in 'r' mode correctly.

2. Step 2: Verify json.load and file close

   json.load reads JSON data properly, and file is closed with f.close().

3. Final Answer:

   No error, code works correctly -> Option D

4. Quick Check:

   Code reads JSON file correctly [OK]

Hint: json.load needs file opened in 'r' mode [OK]

Common Mistakes:

• Forgetting to import json
• Opening file in wrong mode
• Not closing the file after reading

5.

You want to save a Python dictionary {'a': 1, 'b': 2, 'c': 3} to a JSON file and then read it back. Which code snippet correctly does this and prints the value of key 'b'?

hard

A. import json
with open('file.json', 'w') as f:
json.dump({'a': 1, 'b': 2, 'c': 3}, f)
with open('file.json', 'r') as f:
data = json.load(f)
print(data['b'])

B. import json
with open('file.json', 'r') as f:
json.dump({'a': 1, 'b': 2, 'c': 3}, f)
with open('file.json', 'w') as f:
data = json.load(f)
print(data['b'])

C. import json
f = open('file.json', 'w')
json.load({'a': 1, 'b': 2, 'c': 3}, f)
f.close()
f = open('file.json', 'r')
data = json.dump(f)
f.close()
print(data['b'])

D. import json
with open('file.json', 'w') as f:
json.load({'a': 1, 'b': 2, 'c': 3}, f)
with open('file.json', 'r') as f:
data = json.dump(f)
print(data['b'])

Solution

1. Step 1: Write dictionary to JSON file correctly

   json.dump writes Python dict to file opened in 'w' mode.

   import json
   with open('file.json', 'w') as f:
       json.dump({'a': 1, 'b': 2, 'c': 3}, f)
   with open('file.json', 'r') as f:
       data = json.load(f)
   print(data['b'])

   does this correctly.

2. Step 2: Read JSON file and access key 'b'

   json.load reads JSON back from file opened in 'r' mode, then data['b'] prints 2.

3. Final Answer:

   Code in Option A correctly saves and reads JSON, printing 2 -> Option A

4. Quick Check:

   json.dump to write, json.load to read [OK]

Hint: Use json.dump to write, json.load to read JSON files [OK]

Common Mistakes:

• Using json.load to write data
• Using json.dump to read data
• Opening files in wrong modes