【Python】JSONで保存できない値をどう扱う？日時・数値・日本語の受け渡し

from datetime import datetime, timezone
import json
import math
import numpy as np

def encode_extra(value):
    if isinstance(value, datetime):
        if value.tzinfo is None or value.utcoffset() is None:
            raise ValueError("timezone required")
        return value.isoformat()
    if isinstance(value, np.integer):
        return int(value)
    if isinstance(value, np.floating):
        return float(value)
    if isinstance(value, np.ndarray):
        return value.tolist()
    raise TypeError(f"unsupported type: {type(value).__name__}")

def reject_constant(value):
    raise ValueError("nonstandard JSON constant: " + value)

def validate(data):
    keys = {"schema_version", "measured_at", "count", "values", "note", "missing"}
    if not isinstance(data, dict) or set(data) != keys:
        raise ValueError("unexpected keys")
    if type(data["schema_version"]) is not int or data["schema_version"] != 1:
        raise ValueError("unknown schema version")
    if type(data["count"]) is not int or data["count"] < 0:
        raise ValueError("count must be a nonnegative integer")
    if not isinstance(data["values"], list) or not all(type(x) in (int, float) and math.isfinite(x) for x in data["values"]):
        raise ValueError("values must be finite numbers")
    if data["count"] != len(data["values"]):
        raise ValueError("count does not match values")
    if not isinstance(data["measured_at"], str):
        raise ValueError("timestamp must be text")
    stamp = datetime.fromisoformat(data["measured_at"])
    if stamp.tzinfo is None or stamp.utcoffset() is None:
        raise ValueError("timezone required")
    if not isinstance(data["note"], str) or data["missing"] is not None:
        raise ValueError("invalid note or missing marker")
    return stamp

payload = {"schema_version": 1,
           "measured_at": datetime(2026, 10, 2, 12, 0, tzinfo=timezone.utc),
           "count": np.int64(2), "values": np.array([1.5, 2.5]),
           "note": "測定A", "missing": None}
text = json.dumps(payload, ensure_ascii=False, allow_nan=False,
                  default=encode_extra, separators=(",", ":"))
loaded = json.loads(text, parse_constant=reject_constant)
assert validate(loaded) == payload["measured_at"]
assert loaded["count"] == 2 and loaded["values"] == [1.5, 2.5]
print(text)
for label, value, error in [("NaN", float("nan"), ValueError),
                             ("set", {1, 2}, TypeError),
                             ("naive datetime", datetime(2026, 10, 2), ValueError)]:
    try:
        json.dumps(value, allow_nan=False, default=encode_extra)
    except error:
        print("rejected:", label)
    else:
        raise AssertionError(label)
try:
    json.loads('{"value":NaN}', parse_constant=reject_constant)
except ValueError:
    print("rejected: nonstandard NaN on input")
else:
    raise AssertionError("NaN accepted")
try:
    validate({**loaded, "count": True})
except ValueError:
    print("rejected: boolean count")
else:
    raise AssertionError("invalid count accepted")