How to read and write JSON files in Python

Python's built-in json module handles all JSON serialization and deserialization. No third-party packages are needed for standard JSON work.

Reading JSON from a string

import json

raw = '{"name": "Ada", "age": 30, "active": true}'
data = json.loads(raw)   # loads = load string

print(data["name"])   # Ada
print(data["age"])    # 30
print(type(data))     # <class 'dict'>

Reading a JSON file

import json

with open("data.json", encoding="utf-8") as f:
    data = json.load(f)   # load (no 's') reads from a file object

print(data)              # Python dict or list

Always specify encoding='utf-8' to avoid platform-dependent encoding issues, especially on Windows.

Writing JSON to a string

import json

data = {"name": "Ada", "scores": [98, 87, 95], "active": True}

compact = json.dumps(data)
# '{"name": "Ada", "scores": [98, 87, 95], "active": true}'

pretty = json.dumps(data, indent=2)
# {
#   "name": "Ada",
#   "scores": [98, 87, 95],
#   "active": true
# }

Writing a JSON file

import json

data = {"name": "Ada", "age": 30}

with open("output.json", "w", encoding="utf-8") as f:
    json.dump(data, f, indent=2)   # dump (no 's') writes to a file object

Handling JSONDecodeError

import json

def safe_parse(text: str):
    try:
        return json.loads(text)
    except json.JSONDecodeError as e:
        print(f"Invalid JSON at line {e.lineno}, col {e.colno}: {e.msg}")
        return None

result = safe_parse('{"broken": }')
# Invalid JSON at line 1, col 12: Expecting value

Type mapping: JSON to Python

# JSON        → Python
# object       → dict
# array        → list
# string       → str
# number (int) → int
# number (float)→ float
# true         → True
# false        → False
# null         → None

Sorting keys and controlling output

json.dumps(data, sort_keys=True)           # alphabetical key order
json.dumps(data, indent=2, sort_keys=True) # pretty + sorted
json.dumps(data, ensure_ascii=False)       # allow non-ASCII chars (e.g. emoji, CJK)
json.dumps(data, separators=(',', ':'))    # compact, no spaces

Custom serialization with default

import json
from datetime import datetime, date

def json_default(obj):
    if isinstance(obj, (datetime, date)):
        return obj.isoformat()
    raise TypeError(f"Object of type {type(obj)} is not JSON serializable")

data = {"name": "Ada", "joined": datetime(2024, 1, 15)}
json.dumps(data, default=json_default)
# '{"name": "Ada", "joined": "2024-01-15T00:00:00"}'