Lesson

Python JSON Module

Q: What does object_hook in json.loads() do?

object_hook passes every decoded dictionary through a custom function so you can transform keys or turn dicts into typed objects during parsing.

10 min reading

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Python `json` Module: Custom Encoders, Hooks, and Streaming

Beyond basic loads() and dumps(), Python's json module includes hooks for serializing custom objects like datetime or Decimal, formatting compact API payloads, and parsing newline-delimited JSON streams.

Using these options keeps API serializers clean without pulling in heavy external dependencies for simple scripts.

Custom Serialization with `default=`

When json.dumps() encounters an object it does not recognize—such as a datetime or dataclass—it calls your default callback to convert it into a JSON-safe type.

from datetime import date
from decimal import Decimal
import json

def json_fallback(obj):
    if isinstance(obj, date):
        return obj.isoformat()
    if isinstance(obj, Decimal):
        return float(obj)
    raise TypeError(f"Unsupported type: {type(obj).__name__}")

invoice = {"issued": date(2026, 11, 12), "total": Decimal("149.95")}
print(json.dumps(invoice, default=json_fallback, sort_keys=True))

Compact Output and Streaming NDJSON

For network responses, pass separators=(",", ":") to strip whitespace and shrink payload bytes. When processing multi-gigabyte log exports, write one JSON object per line (NDJSON) and iterate line by line instead of loading one giant array.

lines = ['{"id": 1, "ok": true}', '{"id": 2, "ok": false}']
records = [json.loads(line) for line in lines]
print(records[1]["ok"])

Common Mistakes to Avoid

Using float(Decimal(...)) when exact cents matter: Converting money Decimal values to string (str(obj)) preserves exact decimal precision across JSON APIs.
Loading 2GB JSON arrays with json.load(): Full-file loading buffers every object in RAM; use NDJSON lines or streaming parsers like ijson.
Forgetting ensure_ascii=False: By default json.dumps() escapes non-ASCII characters as \uXXXX; set ensure_ascii=False for readable UTF-8 output.