JSONDecodeError — find the first invalid token, not the last symptom

Meaning

json.loads expects one valid JSON value. A trailing comma, single-quoted key, HTML error page or empty response violates that grammar. The decoder reports line, column and absolute character offset.

import json
from json import JSONDecodeError

bad = '{"enabled": true,}'
try:
    json.loads(bad)
except JSONDecodeError as exc:
    assert exc.lineno == 1
    print("invalid position:", exc.pos, "near:", repr(bad[max(0, exc.pos-8):exc.pos+8]))

Repair the producer and protect the consumer

Check HTTP status and Content-Type before parsing a remote response; an HTTP 500 with an HTML body should not be treated as JSON. Correct the producer to emit double-quoted strings without trailing commas. If a file can be empty, define what empty means before introducing a default object.

valid = json.loads('{"enabled": true}')
assert valid["enabled"] is True

Lab

Test empty text, 'null', an array instead of an object, and a valid object missing the 'enabled' key. Notice that valid JSON can still violate application schema. Validate the decoded type and required fields after decoding. Never patch broken JSON by blindly deleting punctuation from arbitrary input. Reference: https://docs.python.org/3/library/json.html

Separate decoding from validation

import json
def read_config(raw):
    obj = json.loads(raw)
    if not isinstance(obj, dict) or not isinstance(obj.get("enabled"), bool):
        raise ValueError("config.enabled must be boolean")
    return obj

The input "null" is valid JSON but has the wrong shape, while "{invalid}" must fail during decoding. For an HTTP response, verify status and media type before assuming the body is JSON; an HTML error page produces a very different root cause from a missing config key. Avoid echoing untrusted response bodies into error logs with credentials.