JSON: Persisting Data

Goal: Learn to save and load data in JSON format.

What Is JSON?

JSON (JavaScript Object Notation) is a text format for storing data. It looks like a Python dictionary, is human-readable, and is supported by every programming language.

{
  "name": "Alice",
  "age": 25,
  "balance": 1000.50,
  "transactions": ["deposit", "withdraw"]
}

Python to JSON type mapping

Core functions of the json module

import json
# json.dump()  — save to a file
# json.load()  — load from a file
# json.dumps() — convert to a string
# json.loads() — parse from a string

Saving: json.dump()

import json

user = {
    "name": "Alice",
    "age": 25,
    "balance": 1000
}

with open("user.json", "w", encoding="utf-8") as file:
    json.dump(user, file, indent=4, ensure_ascii=False)

Parameters:
- indent=4 — indentation for readability
- ensure_ascii=False — preserves non-ASCII characters

Result in user.json:

{
    "name": "Alice",
    "age": 25,
    "balance": 1000
}

Loading: json.load()

import json

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

print(user["name"])    # Alice
print(user["balance"]) # 1000

Safe Loading with try/except

The file may not exist or may be corrupted — always wrap your load call:

import json

try:
    with open("user.json", "r", encoding="utf-8") as file:
        user = json.load(file)
    print("Data loaded!")
except FileNotFoundError:
    print("File not found, creating a new profile.")
    user = {"name": "Stranger", "age": 0, "balance": 0}
except json.JSONDecodeError:
    print("File is corrupted, using defaults.")
    user = {"name": "Stranger", "age": 0, "balance": 0}

Practical Example: BankAccount

import json

class BankAccount:
    def __init__(self, name, balance=0):
        self.name = name
        self.balance = balance
        self.transactions = []

    def deposit(self, amount):
        self.balance += amount
        self.transactions.append(f"Deposit: +{amount}")

    def withdraw(self, amount):
        if amount > self.balance:
            print("Insufficient funds!")
            return
        self.balance -= amount
        self.transactions.append(f"Withdrawal: -{amount}")

    def save(self, filename):
        data = {
            "name": self.name,
            "balance": self.balance,
            "transactions": self.transactions
        }
        with open(filename, "w", encoding="utf-8") as file:
            json.dump(data, file, indent=4, ensure_ascii=False)

    @staticmethod
    def load(filename):
        try:
            with open(filename, "r", encoding="utf-8") as file:
                data = json.load(file)
            account = BankAccount(data["name"], data["balance"])
            account.transactions = data["transactions"]
            return account
        except FileNotFoundError:
            print("File not found!")
            return None

# Usage
account = BankAccount("Alice", 1000)
account.deposit(500)
account.withdraw(200)
account.save("alice.json")

loaded = BankAccount.load("alice.json")
print(f"Balance: {loaded.balance}")
print(f"History: {loaded.transactions}")

Contents of alice.json:

{
    "name": "Alice",
    "balance": 1300,
    "transactions": [
        "Deposit: +500",
        "Withdrawal: -200"
    ]
}

json.dumps() and json.loads() — Working with Strings

import json

# dumps() — dict → string (for APIs, networking, logging)
data = {"name": "Alice", "age": 25}
json_string = json.dumps(data, ensure_ascii=False)
print(json_string)        # {"name": "Alice", "age": 25}
print(type(json_string))  # <class 'str'>

# loads() — string → dict (when receiving data from an API)
raw = '{"name": "Bob", "balance": 500}'
parsed = json.loads(raw)
print(parsed["name"])  # Bob
print(type(parsed))    # <class 'dict'>

What Cannot Be Saved to JSON

# Functions → TypeError
data = {"func": print}
json.dump(data, file)  # ❌ TypeError

# Class instances → TypeError
user = User("Alice")
json.dump(user, file)  # ❌ TypeError
# Fix: convert to a dictionary
json.dump({"name": user.name}, file)  # ✅

# Sets → TypeError
data = {"numbers": {1, 2, 3}}
json.dump(data, file)  # ❌ TypeError
# Fix: convert to a list
data = {"numbers": list({1, 2, 3})}  # ✅

Pattern: Config File

import json

def load_config(filename="config.json"):
    try:
        with open(filename, "r", encoding="utf-8") as file:
            return json.load(file)
    except FileNotFoundError:
        return {
            "app_name": "MyApp",
            "version": "1.0",
            "debug": False,
            "max_accounts": 10
        }

config = load_config()
print(f"App: {config['app_name']} v{config['version']}")

Common Mistakes

1. Forgot ensure_ascii=False:

# ❌ Non-ASCII chars become \uXXXX escape sequences
json.dump(data, file)
# ✅ Correct
json.dump(data, file, ensure_ascii=False)

2. Tried to serialize a class instance directly:

# ❌ TypeError
json.dump(my_object, file)
# ✅ Convert to a dict first
json.dump(my_object.__dict__, file)

Summary

• json.dump(data, file, indent=4, ensure_ascii=False) — save to a file
• json.load(file) — load from a file
• json.dumps(data) — convert to a string
• json.loads(string) — parse from a string
• Always use try/except when loading
• Serialize class instances as dictionaries

Related Topics

• Try/Except: Error Handling
• with: Working with Files
• OOP: Basics