Python Basics: Reading and Writing JSON Files

JSON is useful for small configuration files and data exchanged between programs. Python’s standard-library json module converts JSON into dictionaries, lists, and other Python values.

These examples use Python 3 on Linux. Run the shell commands in order in the same shell. Start in a fresh temporary directory so the example filenames cannot overwrite your work:

cd "$(mktemp -d)"

Read a JSON file

Create a small practice file, then read it:

printf '{"name": "demo", "retries": 2, "enabled": true}\n' > settings.json

Save as demo_json_1.py:

import json

with open("settings.json", encoding="utf-8") as file:
    settings = json.load(file)

print(settings["name"])
print(settings["retries"] + 1)
print(settings["enabled"])

Run:

python3 demo_json_1.py

Output:

demo
3
True

The JSON object becomes a dictionary. The number stays numeric, and JSON true becomes Python True. The with block closes the file after reading it.

Write an updated copy

This creates updated.json. Mode "w" replaces its contents if it already exists, so use only the practice directory.

Save as demo_json_2.py:

import json

with open("settings.json", encoding="utf-8") as file:
    settings = json.load(file)

settings["retries"] = 3
with open("updated.json", "w", encoding="utf-8") as file:
    json.dump(settings, file)
    file.write("\n")

Run:

python3 demo_json_2.py
cat updated.json

Output:

{"name": "demo", "retries": 3, "enabled": true}

json.dump() writes to a file. json.dumps() instead returns a string; the corresponding reading functions are load() and loads().

Handle invalid input

JSON requires double-quoted keys. This short example catches a syntax error without printing a traceback:

Save as demo_json_3.py:

import json

try:
    json.loads('{name: "demo"}')
except json.JSONDecodeError:
    print("Invalid JSON")

Run:

python3 demo_json_3.py

Output:

Invalid JSON

Parsing successfully does not check your application’s required keys or value types. Validate those before using data from another source, and limit the size of untrusted input.

Reference

Python json documentation

Acknowledgements

This post was created with assistance from AI.