_format_msg_json doesn't merge arguments

[phdesign]

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'])

[phdesign]

Adding some more context to this. In the call to format(), the code does call the underlying implementation of record.getMessage

    def format(self, record):
        # Compute attributes handled by parent class.
        super(FluentRecordFormatter, self).format(record)  # <------ record.messge = record.msg % record.args
        # Add ours
        record.hostname = self.hostname

        # Apply format
        data = self._formatter(record)

        self._structuring(data, record)
        return data

which does the merge, however the call to self._structuring() eventually loads the json from the msg property, not the message property which is the merged value. e.g.

json_msg = json.loads(str(msg))

[arcivanov]

Sorry, I've been busy, I'll take a look soon once I get a break.