Reading and writing files (JSON)

Variables vanish when the program ends. To remember expenses between runs, save them to a file. JSON is the standard text format for this, and it's the same format that APIs and LLMs speak.

python

import json

from pathlib import Path

DATA_FILE = Path("expenses.json")

expenses = [{"amount": 250.0, "category": "food", "note": "chai"}]

# write: Python list → JSON text → file

DATA_FILE.write_text(json.dumps(expenses, indent=2, ensure_ascii=False))

# read: file → JSON text → Python list

if DATA_FILE.exists():

    loaded = json.loads(DATA_FILE.read_text())

else:

    loaded = []

json.dumps turns Python into a JSON string, and json.loads turns a JSON string back into Python. (The s stands for string.)
indent=2 makes the file readable to humans.
ensure_ascii=False keeps ₹ and Hindi text readable instead of turning them into ₹.
Path(...).exists() checks for the file so the first run doesn't crash.

This is what expenses.json looks like:

json

[

{

    "amount": 250.0,

    "category": "food",

    "note": "chai"

}

]

JSON's types map directly to Python's: object ↔ dict, array ↔ list, string ↔ str, number ↔ int/float, true/false ↔ True/False, null ↔ None.

The other way to use files is open() with a with block, which closes the file for you automatically:

python

with open("expenses.json", "w", encoding="utf-8") as f:

    json.dump(expenses, f, indent=2)

with open("expenses.json", encoding="utf-8") as f:

    expenses = json.load(f)