A simple program that shows usage of the jsonlogger package.

JsonFormatterTest.py

import logging
import pythonjsonlogger.jsonlogger
import datetime


def testJsonFormatter():
    supported_keys = [
        'asctime',
        'created',
        'filename',
        'funcName',
        'levelname',
        'levelno',
        'lineno',
        'module',
        'msecs',
        'message',
        'name',
        'pathname',
        'process',
        'processName',
        'relativeCreated',
        'thread',
        'threadName'
    ]

    log_format = lambda x: ['%({0:s})'.format(i) for i in x]
    custom_format = ' '.join(log_format(supported_keys))

    fr = jsonlogger.JsonFormatter(custom_format)
    logHandler.setFormatter(fr)

    # msg = "testing logging format" # This also works. 
    # In the following message, default json encoder will be used for encoding datetime objects
    msg = { "message": "You've got message",
            "text": "testing logging", "num": 1, 5: "9",
            "adate": datetime.datetime(1999, 12, 31, 23, 59),
            "otherdate": datetime.date(1789, 7, 14),
            "otherdatetime": datetime.datetime(1789, 7, 14, 23, 59),
            "otherdatetimeagain": datetime.datetime(1900, 1, 1),
            "nested": {"more": "data"}
        }
    extra = {"text": "testing logging with an extra", "num": 1, 5: "five", "nested": {"some more": "data"}}
    logger.info(msg, extra = extra)

    # Testing with unknown format key: 
    fr = jsonlogger.JsonFormatter('%(unknown_key)s %(message)s')
    logHandler.setFormatter(fr)
    msg = "testing unknown logging format"
    logger.info(msg)
    # The log output will be: {"unknown_key": null, "message": "testing unknown logging format"}

    try:
        raise Exception('test')
    except Exception:
        logger.exception("hello")

    # Using a custom encoder for encoding non-standard objects into json
    def custom(o):
        return "very custom"
    fr = jsonlogger.JsonFormatter(json_default=custom)
    logHandler.setFormatter(fr)

    # In the following message, the 'custom' function above will be called for the 'adate' field. 
    msg = {"message": "The message",
            "adate": datetime.datetime(1999, 12, 31, 23, 59),
            "normal": "value", 
            "int":5,
            "list": [1, 2]}
    logger.info(msg)

    # Custom logic: Adding fields to the output message. 
    class CustomJsonFormatter(jsonlogger.JsonFormatter):
        def process_log_record(self, log_record):
            log_record["custom"] = "a custom value"
            # Old Style "super" since Python 2.6's logging.Formatter is old
            # style
            return jsonlogger.JsonFormatter.process_log_record(self, log_record)

    logHandler.setFormatter(CustomJsonFormatter())
    logger.info("A message with a custom json formatter")

    logHandler.setFormatter(jsonlogger.JsonFormatter(prefix='{'))
    logger.info("A prefixed message")


if __name__ == '__main__':
    logger = logging.getLogger('logging-test')
    logger.setLevel(logging.DEBUG)
    logHandler = logging.FileHandler("log.json")
    logger.addHandler(logHandler)

    testFormatKeys()

jsonlogger allows standard python logging mechanism to output log data as json objects. With JSON logs will be more readable by machines.
The package is hosted on GitHub: https://github.com/madzak/python-json-logger/ .