How to Write JSON Data to a File in Python

To write JSON data to a file in Python, open the file in write mode and call json.dump(data, f). Add indent=2 for readable output, and open the file with encoding="utf-8" so names with accents survive:

with open("orders.json", "w", encoding="utf-8") as f:
    json.dump(orders, f, indent=2, ensure_ascii=False)

These snippets ran on Python 3.12.5 in a Windows Command Prompt, where encoding mistakes show up fast.

You’ll see json.dump with a list of records, the ensure_ascii setting, the datetime TypeError and its fix, appending to a JSON file, and writing without corrupting the file if something fails.

Write JSON to a file with json.dump()

json.dump() writes straight to a file object; json.dumps() returns a string instead. Here a list of order dictionaries goes to disk and is read back:

import json
from pathlib import Path

orders = [
    {"id": 1001, "customer": "Emily Carter", "city": "Austin", "total": 129.99, "paid": True},
    {"id": 1002, "customer": "José Ramírez", "city": "San Antonio", "total": 54.5, "paid": False},
]

with open("orders.json", "w", encoding="utf-8") as f:
    json.dump(orders, f, indent=2, ensure_ascii=False)

print(Path("orders.json").read_text(encoding="utf-8"))
print(json.load(open("orders.json", encoding="utf-8"))[1]["customer"])

Output:

[
  {
    "id": 1001,
    "customer": "Emily Carter",
    "city": "Austin",
    "total": 129.99,
    "paid": true
  },
  {
    "id": 1002,
    "customer": "José Ramírez",
    "city": "San Antonio",
    "total": 54.5,
    "paid": false
  }
]
José Ramírez

Python’s True becomes true and None becomes null. The mapping goes the other way too, as shown in converting a Python dict to JSON.

Save a Python dictionary as a JSON file and load it back

A single dict works the same way. The surprise comes when you read it back: JSON object keys are always strings, so integer keys don’t round-trip:

import json
from pathlib import Path

settings = {"store": "Austin #12", "tax_rate": 0.0825, "open_days": ["Mon", "Tue", "Wed"],
            1001: "Emily Carter", 1002: "James Wilson"}          # int keys

Path("settings.json").write_text(json.dumps(settings, indent=2, sort_keys=False), encoding="utf-8")
loaded = json.loads(Path("settings.json").read_text(encoding="utf-8"))

print(loaded == settings, list(loaded)[-2:])        # int keys came back as strings
fixed = {int(k) if k.isdigit() else k: v for k, v in loaded.items()}
print(fixed == settings)

try:
    json.dumps({("TX", "Austin"): 12})
except TypeError as err:
    print("TypeError:", err)

Output:

False ['1001', '1002']
True
TypeError: keys must be str, int, float, bool or None, not tuple

Tuple keys can’t be saved at all. Convert them to strings first, or store a list of records instead. Reading JSON text into a dict is covered in converting a JSON string to a dictionary.

Keep accented characters readable with ensure_ascii

By default, json escapes every non-ASCII character. That’s valid JSON, but hard to read:

import json

row = {"customer": "José Ramírez", "note": "Café ☕"}
print(json.dumps(row))                       # default: ensure_ascii=True
print(json.dumps(row, ensure_ascii=False))   # readable, needs a UTF-8 file
print(json.dumps(row, separators=(",", ":"), ensure_ascii=False))   # compact

Output:

{"customer": "Jos\u00e9 Ram\u00edrez", "note": "Caf\u00e9 \u2615"}
{"customer": "José Ramírez", "note": "Café ☕"}
{"customer":"José Ramírez","note":"Café ☕"}

With ensure_ascii=False you must write the file as UTF-8, or Windows may fall back to another code page. For spacing and sorting options, see pretty printing JSON in Python.

Why do I get “Object of type datetime is not JSON serializable”?

JSON only knows strings, numbers, booleans, null, arrays and objects. A datetime, Decimal or set stops json.dump():

import json
from datetime import datetime

order = {"id": 1003, "placed": datetime(2026, 10, 6, 9, 30), "total": 89.0}

try:
    with open("order.json", "w", encoding="utf-8") as f:
        json.dump(order, f)
except TypeError as err:
    print("TypeError:", err)

print("left behind:", repr(open("order.json", encoding="utf-8").read()))

Output:

TypeError: Object of type datetime is not JSON serializable
left behind: '{"id": 1003, "placed": '

Pass a default function. json calls it for anything it can’t handle, and you return something it can:

import json
from datetime import date, datetime
from decimal import Decimal
from pathlib import Path

def to_json(value):
    if isinstance(value, (datetime, date)):
        return value.isoformat()
    if isinstance(value, Decimal):
        return str(value)            # keep every cent exactly
    if isinstance(value, set):
        return sorted(value)
    raise TypeError(f"Can't serialize {type(value).__name__}")

order = {"id": 1003, "placed": datetime(2026, 10, 6, 9, 30), "ship_by": date(2026, 10, 9),
         "total": Decimal("89.10"), "tags": {"gift", "express"}}

with open("order.json", "w", encoding="utf-8") as f:
    json.dump(order, f, indent=2, default=to_json)

print(Path("order.json").read_text(encoding="utf-8"))

Output:

{
  "id": 1003,
  "placed": "2026-10-06T09:30:00",
  "ship_by": "2026-10-09",
  "total": "89.10",
  "tags": [
    "express",
    "gift"
  ]
}

The failed run above left a partial order.json behind, since the file was opened before the error. The safe-write pattern below avoids that. Before storing timestamps, read naive vs aware datetime in Python so the ISO strings carry a time zone when they need one.

Append data to a JSON file

A JSON file holds one value, so you can’t just append to it. Either load it, change it and rewrite it, or switch to JSON Lines: one JSON object per line, which works with append mode:

import json
from pathlib import Path

Path("events.jsonl").unlink(missing_ok=True)

def log_event(event):
    with open("events.jsonl", "a", encoding="utf-8") as f:      # append one line per record
        f.write(json.dumps(event) + "\n")

log_event({"user": "olivia", "action": "login", "at": "2026-10-06T08:01:00"})
log_event({"user": "james", "action": "checkout", "amount": 42.75})
log_event({"user": "olivia", "action": "logout"})

print(Path("events.jsonl").read_text(encoding="utf-8"))
events = [json.loads(line) for line in open("events.jsonl", encoding="utf-8")]
print(len(events), "events; checkouts:", sum(e.get("amount", 0) for e in events))

Output:

{"user": "olivia", "action": "login", "at": "2026-10-06T08:01:00"}
{"user": "james", "action": "checkout", "amount": 42.75}
{"user": "olivia", "action": "logout"}

3 events; checkouts: 42.75

I use JSON Lines for logs and exports that grow over time. If you’re writing other text formats, see writing a list to a file in Python.

Write a JSON file safely with os.replace()

If dump() fails halfway, the old file is already truncated. Write to a temporary file in the same folder, then swap it in with os.replace():

import json
import os
import tempfile
from pathlib import Path

def write_json_atomic(path, data):
    path = Path(path)
    fd, tmp = tempfile.mkstemp(dir=path.parent, suffix=".tmp")
    try:
        with os.fdopen(fd, "w", encoding="utf-8") as f:
            json.dump(data, f, indent=2)
            f.flush()
            os.fsync(f.fileno())
        os.replace(tmp, path)          # swap in one step
    except BaseException:
        os.remove(tmp)
        raise

write_json_atomic("settings.json", {"theme": "dark", "currency": "USD"})
try:
    write_json_atomic("settings.json", {"theme": "light", "bad": {1, 2}})
except TypeError as err:
    print("write failed:", err)
print("file still holds:", Path("settings.json").read_text(encoding="utf-8").replace("\n", " "))

Output:

write failed: Object of type set is not JSON serializable
file still holds: {   "theme": "dark",   "currency": "USD" }

The second write fails on the set, and the original settings survive untouched. To replace an existing file on purpose, see overwriting a file in Python.

JSON writing options compared

Need	Code
Write to a file	json.dump(data, f)
Get a string	json.dumps(data)
Readable file	indent=2, ensure_ascii=False + encoding="utf-8"
datetime / Decimal / set	default=to_json
Add records over time	JSON Lines with mode "a"
Never leave a broken file	Temp file + os.replace()

Working with JSON regularly? These guides are next:

• Write a dictionary to a text file in Python
• Work with JSON data in Python
• Convert a dictionary to JSON in Python
• Convert a pandas DataFrame to JSON

Frequently asked questions