How to Write JSON File in Python: Simple Guide

To write a JSON file in Python, use the json.dump() function from the json module. Open a file in write mode, then pass your Python data and the file object to json.dump() to save the data as JSON.

Syntax

The basic syntax to write JSON data to a file in Python is:

• import json: Imports the JSON module.
• with open('filename.json', 'w') as file:: Opens a file in write mode.
• json.dump(data, file): Writes the Python data to the file in JSON format.

python

import json

data = {}
with open('data.json', 'w') as file:
    json.dump(data, file)

Example

This example shows how to write a Python dictionary to a JSON file named data.json. It demonstrates opening the file, writing the JSON, and closing the file automatically.

python

import json

# Python dictionary to save
person = {
    "name": "Alice",
    "age": 30,
    "city": "New York"
}

# Write JSON file
with open('data.json', 'w') as file:
    json.dump(person, file)

Common Pitfalls

Common mistakes when writing JSON files include:

• Not opening the file in write mode ('w'), which causes errors.
• Trying to write non-serializable Python objects like sets or custom classes without converting them.
• Forgetting to import the json module.

Here is an example of a wrong and right way:

python

# Wrong: opening file in read mode
import json

person = {"name": "Bob"}

try:
    with open('data.json', 'r') as file:
        json.dump(person, file)  # This will raise an error
except Exception as e:
    print(f"Error: {e}")

# Right: open file in write mode
with open('data.json', 'w') as file:
    json.dump(person, file)

Output

Error: not writable

Quick Reference

Tips for writing JSON files in Python:

• Always use with open(filename, 'w') to safely open files.
• Use json.dump() to write Python data to JSON format.
• Ensure data is JSON serializable (e.g., dict, list, str, int).
• Use indent parameter in json.dump() for readable formatting.

python

import json

person = {"name": "Eve", "age": 25}

with open('pretty.json', 'w') as file:
    json.dump(person, file, indent=4)

Key Takeaways

Use the json module's dump() function to write Python data to a JSON file.

Always open the file in write mode ('w') before writing JSON data.

Ensure your Python data is JSON serializable to avoid errors.

Use the with statement to handle files safely and automatically close them.

Add indent parameter in json.dump() for readable JSON formatting.