How do you parse and build JSON payloads in Python?

Python's built-in json module converts between JSON strings/files and Python objects. The mapping is: JSON object ↔ Python dict, JSON array ↔ Python list, JSON string ↔ Python str, JSON number ↔ Python int/float, JSON true/false ↔ Python True/False, JSON null ↔ Python None.

import json

# --- Parsing (deserialising) ---

# From a JSON string (e.g., API response body)
json_str = '{"name": "Alice", "age": 30, "tags": ["admin", "user"]}'
data = json.loads(json_str)    # loads = load string
print(data['name'])            # Alice
print(data['tags'][0])         # admin

# From a file
with open('config.json', encoding='utf-8') as f:
    config = json.load(f)      # load (no s) = load file

# --- Building (serialising) ---

payload = {'user_id': 42, 'scores': [95, 87, 100], 'active': True}

# To a string
json_out = json.dumps(payload, indent=2)   # pretty-printed
print(json_out)

# To a file
with open('output.json', 'w', encoding='utf-8') as f:
    json.dump(payload, f, indent=2)

# Handling dates — not natively serialisable
from datetime import date
json.dumps({'date': date.today().isoformat()})  # convert to string first

Common pitfalls: JSON keys are always strings, but Python dicts can have non-string keys — serialising a dict with int keys will convert them to strings, which can break round-trip assumptions. Python's datetime, Decimal, and custom objects are not JSON-serialisable by default — you must either convert them before serialising or provide a custom default= encoder function to json.dumps().

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