Read and write a JSON file in Python

JSON is a text format for data made of objects, lists, strings, numbers, true/false and null. It maps closely to Python dictionaries and lists, and the json module converts between the two.

The program

import json

profile = {
    "name": "Hedy",
    "age": 35,
    "languages": ["Python", "SQL"],
    "active": True,
    "manager": None,
}

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

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

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

print(loaded)
print("Same data:", loaded == profile)

{
  "name": "Hedy",
  "age": 35,
  "languages": [
    "Python",
    "SQL"
  ],
  "active": true,
  "manager": null
}
{'name': 'Hedy', 'age': 35, 'languages': ['Python', 'SQL'], 'active': True, 'manager': None}
Same data: True

How it works

• json.dump(data, f) writes the data to the file. indent=2 puts each item on its own line, so people can read the file.
• json.load(f) reads the file and gives back Python objects: objects become dictionaries, arrays become lists.
• Look at the file: Python's True and None are written as JSON's true and null. When the file is loaded, they become True and None again.
• dump and load work with files. json.dumps() and json.loads() (with an s, for "string") do the same with strings.

Types that change

Not every Python type has a JSON form. A tuple is saved as a JSON list, so it comes back as a list. Dictionary keys must be strings, so a number key comes back as a string:

import json

data = {"point": (3, 4), 1: "one"}
text = json.dumps(data)
print(text)
print(json.loads(text))

{"point": [3, 4], "1": "one"}
{'point': [3, 4], '1': 'one'}

A set or a datetime cannot be saved directly: json.dumps() raises TypeError. Convert them first, for example with list(my_set) or my_date.isoformat().