How to Fix a Python JSONDecodeError in a Config File

What the error looks like

An app that loads config.json at startup and fails usually logs something like this:

json.decoder.JSONDecodeError: Expecting property name enclosed in double quotes: line 9 column 3 (char 160)

The message has two useful parts: what the parser expected, and where it stopped (line, column, and character offset). The real mistake is usually on that line or at the end of the line just before it.

Newer Python versions word some of these messages differently (for example, they may say "Illegal trailing comma"), but the line and column work the same.

Step 1: validate the file yourself

You do not need to start the app to see the error. Python ships a JSON validator:

python3 -m json.tool config.json

Valid JSON prints back formatted. Invalid JSON prints the same error with the same position. If jq is installed, jq . config.json does the same job.

Step 2: find the mistake

JSON is much stricter than JavaScript or Python. These are the common mistakes in hand-edited config files:

{
  "features": {
    "logging": true,
    "cache": true,
  },
  "allowed_origins": [
    "https://app.example.com",
  ]
}

Both , characters before } and ] are trailing commas, and JSON does not allow them. The corrected file:

{
  "features": {
    "logging": true,
    "cache": true
  },
  "allowed_origins": [
    "https://app.example.com"
  ]
}

Other rules that break configs:

• Strings and keys must use double quotes. 'debug': false is invalid.
• Keys must be quoted. {debug: false} is invalid.
• No comments. // and /* */ are not part of JSON.
• Booleans and null are lowercase: true, false, null (not True or None).

Correct only the syntax. Keep the same keys and values, so the app behaves the same as before the edit.

Step 3: re-validate and reload

python3 -m json.tool config.json > /dev/null && echo valid

If the app reads the config only once at startup, it keeps the broken state in memory until it restarts. Restart it (or let a file-watching reloader do it), then check the endpoint:

curl -i http://localhost:8080/api/status    # expect HTTP 200

Make the app report the problem clearly

A startup loader that catches the error and continues with config = None produces a 500 on every request and hides the real cause. Fail loudly instead:

import json, sys

try:
    with open('config.json') as f:
        config = json.load(f)
except json.JSONDecodeError as e:
    sys.exit(f'FATAL: config.json is not valid JSON: {e}')

Now the process exits at deploy time with the exact line and column in the log.

Stop bad configs before they reach production

• Run python3 -m json.tool or jq on every JSON file in CI.
• Validate structure, not only syntax, with a JSON Schema check.
• Add a pre-commit hook such as check-json so the mistake never leaves the developer's machine.

A one-character config change can take a service down. Treat config changes with the same review and checks as code changes.