Parsing JSON in Python

Quick answer: Python's built-in json module converts between JSON text and Python objects. Use json.loads() to parse a JSON string into a dict/list, and json.dumps() to serialize a Python object back to a JSON string. The load/dump variants (no "s") work on files. To check that a string is valid before parsing, run it through the JSON Validator.

The "s" means "string"; without it, the function works on a file object:

import json

# string → Python object
data = json.loads('{"name": "Ada", "age": 36}')   # {'name': 'Ada', 'age': 36}

# Python object → string
text = json.dumps(data)                            # '{"name": "Ada", "age": 36}'

# file → Python object
with open("data.json") as f:
    data = json.load(f)

# Python object → file
with open("out.json", "w") as f:
    json.dump(data, f)

JSON and Python types line up closely, with a few names changed:

Two things to watch: JSON object keys are always strings, so json.loads('{"1": "a"}') gives {"1": "a"}, not {1: "a"}. And json.dumps turns dict keys into strings, so an int key round-trips as a string.

json.dumps(data, indent=2)            # pretty-print
json.dumps(data, sort_keys=True)      # deterministic key order
json.dumps(data, ensure_ascii=False)  # keep é, 你好, emoji as-is (not \uXXXX)
json.dumps(data, separators=(",", ":"))  # minify — no spaces

ensure_ascii=True (the default) escapes non-ASCII characters as \uXXXX. Setting it False writes real UTF-8, which is usually what you want for readable output — see Escaping Special Characters.

• Printing a dict isn't JSON. print(d) and str(d) use single quotes and True/None — invalid JSON. Always use json.dumps(). See Single Quotes in JSON.
• Non-serializable types raise TypeError. datetime, set, Decimal, and bytes aren't JSON types. Convert first (e.g. dt.isoformat() — see Dates in JSON) or pass a default= function to dumps.
• Big integers are fine in Python (arbitrary precision) but may lose precision once a JavaScript client parses them — see Big Numbers in JSON.
• json.loads raises json.JSONDecodeError with a line, column, and position on invalid input — catch it and inspect with the JSON Debugger.

What's the difference between loads and load in Python? loads parses a JSON string; load reads from a file object. The same pattern applies to dumps (to string) and dump (to file).

How do I convert a Python dict to JSON? Use json.dumps(my_dict) for a string, or json.dump(my_dict, file) to write to a file. Never use str(dict) — it produces invalid JSON with single quotes.

Why do my JSON number keys become strings? JSON object keys must be strings, so json.dumps converts dict keys to strings and json.loads always returns string keys. Convert them back to int in your code if needed.

How do I serialize a datetime to JSON in Python? datetime isn't JSON-serializable. Convert it to an ISO 8601 string with .isoformat() first, or pass a default= function to json.dumps that handles it.