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_format_msg_json doesn't merge arguments

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
52/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Stale
Tech stack
python
Domain
devtools

Research direction

Start in fluent/handler.py at FluentRecordFormatter, especially the json.loads(str(msg)) path, and review tests/test_handler.py. Run the existing handler tests, then cover the reported logger call with arguments and confirm the emitted JSON contains the substituted values.

Written by the indexing model from the issue text.

Description

I'm using the FluentHandler class for Python logging module. I was hoping to pass a json string to the logger and have it interpreted as json, which it does, but the FluentRecordFormatter doesn't merge logged arguments in.

Example

logger = logging.getLogger('fluent.test')
logger.info('{"from": "%(a)s", "to": "%(b)s"}', {"a":"userA","b":"userB"})

Expected result

{"type":"INFO","from": "userA", "to": "userB"}

Actual result

{"type":"INFO","from":"%(a)s","to":"%(b)s"}

I expected the arguments to be merged in based on the behaviour of the default Python formatter which calls record.getMessage which merges in arguments

if self.args:
    msg = msg % self.args

However, the behaviour of the FluentRecordFormatter is to json.loads(str(msg)) the message, and ignore any arguments.

Would it be suitable to merge in the arguments when loading the json? I'd be happy to contribute a PR for it. Here's a test that would cover it.

    def test_json_encoded_message_with_args(self):
        handler = fluent.handler.FluentHandler('app.follow', port=self._port)

        with handler:
            logging.basicConfig(level=logging.INFO)
            log = logging.getLogger('fluent.test')
            handler.setFormatter(fluent.handler.FluentRecordFormatter())
            log.addHandler(handler)

            log.info('{"key": "%(a)s", "param": "value"}', {"a": "hello world!"})

            log.removeHandler(handler)

        data = self.get_data()
        self.assertTrue('key' in data[0][2])
        self.assertEqual('hello world!', data[0][2]['key'])
Dominant language
Python
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