How do I write JSON data to a file?

How to Write JSON Data to a File in Python

Learn the essential steps to serialize Python dictionaries and lists into JSON format and save them to a file, ensuring data persistence and interoperability.

JSON (JavaScript Object Notation) is a lightweight data-interchange format that is easy for humans to read and write and easy for machines to parse and generate. It's widely used for web services, APIs, and configuration files due to its simplicity and language-agnostic nature. This article will guide you through the process of writing JSON data to a file using Python, covering basic serialization and formatting options.

Understanding Python's json Module

Python's standard library includes a built-in json module that provides methods for working with JSON data. The two primary functions for writing JSON are json.dump() and json.dumps().

• json.dump(): This function directly writes a Python object (like a dictionary or list) to a file-like object (e.g., a file opened in write mode).
• json.dumps(): This function serializes a Python object into a JSON formatted string. You would then write this string to a file manually.

Flowchart of JSON serialization to file

Basic JSON Serialization to a File

The most straightforward way to write JSON data to a file is by using the json.dump() function. This function takes two main arguments: the Python object you want to serialize and the file object where you want to write the data. Remember to open the file in write mode ('w') or append mode ('a').

import json

data = {
    "name": "Alice",
    "age": 30,
    "isStudent": False,
    "courses": ["Math", "Science", "History"]
}

file_path = "output.json"

with open(file_path, 'w') as json_file:
    json.dump(data, json_file)

print(f"Data successfully written to {file_path}")

Basic example of writing a Python dictionary to a JSON file.

Formatting JSON Output for Readability

By default, json.dump() writes JSON in a compact format, which is efficient for machines but hard for humans to read. You can make the output more readable by using the indent parameter. The indent parameter specifies the number of spaces to use for indentation.

import json

data = {
    "product_id": "P12345",
    "name": "Laptop Pro",
    "price": 1200.00,
    "features": [
        "16GB RAM",
        "512GB SSD",
        "14-inch Display"
    ],
    "available": True
}

file_path = "formatted_output.json"

with open(file_path, 'w') as json_file:
    json.dump(data, json_file, indent=4)

print(f"Formatted data successfully written to {file_path}")

Writing JSON data with 4-space indentation for improved readability.

Handling Non-ASCII Characters and Sorting Keys

When dealing with international characters or wanting a consistent order for your JSON keys, json.dump() offers additional parameters:

• ensure_ascii=False: Allows non-ASCII characters to be written directly to the file, rather than being escaped (e.g., \u00e9).
• sort_keys=True: Sorts the keys in dictionaries alphabetically before writing them. This ensures a consistent output order, which can be useful for version control or comparisons.

import json

data = {
    "city": "München",
    "country": "Germany",
    "population": 1500000,
    "details": {
        "language": "German",
        "currency": "Euro"
    }
}

file_path = "advanced_output.json"

with open(file_path, 'w', encoding='utf-8') as json_file:
    json.dump(data, json_file, indent=4, ensure_ascii=False, sort_keys=True)

print(f"Advanced data successfully written to {file_path}")

Writing JSON with non-ASCII characters and sorted keys.

Practical Steps to Write JSON

Here's a summary of the steps involved in writing JSON data to a file in Python:

1. Step 1

Prepare your Python data: Create a dictionary or list that you want to convert to JSON.

2. Step 2

Import the json module: Add import json at the beginning of your script.

3. Step 3

Open a file for writing: Use with open('filename.json', 'w', encoding='utf-8') as file: to safely open a file.

4. Step 4

Use json.dump(): Call json.dump(your_data, file, indent=4, ensure_ascii=False) to write the data, applying desired formatting and character handling.

5. Step 5

Verify the output: Open the generated .json file in a text editor to confirm the data and formatting are correct.