Document use of extra with json logging

[kislyuk]

logger.info(..., extra={...})

[alexminza]

@kislyuk extra parameters currently are not sent to CloudWatch Logs:

import logging
import watchtower

logging.basicConfig(level="DEBUG")
logger = logging.getLogger()
logger.setLevel("DEBUG")

cloudwatch_handler = watchtower.CloudWatchLogHandler(log_group_name="log_group_name")
cloudwatch_handler.formatter.add_log_record_attrs=["name", "levelname", "filename", "lineno", "funcName", "message"]
logger.addHandler(cloudwatch_handler)

test_extra = {"test": 'test}
logging.info("TEST JSON LOGGING", extra={"test_extra": test_extra})

[alexminza]

Here's the solution implemented in Powertools for AWS Lambda

https://github.com/aws-powertools/powertools-lambda-python/blob/develop/aws_lambda_powertools/logging/formatter.py#L380

[alexminza]

Here's a basic monkey patch to get extra parameters logged:

class CloudWatchLogMonkeyPatchedFormatter(watchtower.CloudWatchLogFormatter):
    #https://github.com/aws-powertools/powertools-lambda-python/blob/develop/aws_lambda_powertools/logging/formatter.py
    RESERVED_LOG_ATTRS = (
        "name",
        "msg",
        "message",
        "args",
        "level",
        "levelname",
        "levelno",
        "pathname",
        "filename",
        "module",
        "exc_info",
        "exc_text",
        "stack_info",
        "lineno",
        "funcName",
        "created",
        "msecs",
        "relativeCreated",
        "thread",
        "threadName",
        "processName",
        "process",
        "asctime",
        "location",
        "timestamp",
    )

    def format(self, message):
        extras = {k: v for k, v in message.__dict__.items() if k not in self.RESERVED_LOG_ATTRS}
        extras['msg'] = message.msg
        message.msg = extras
        format_message = super().format(message)
        return format_message

watchtower_handler = watchtower.CloudWatchLogHandler(log_group_name='log_group_name')
watchtower_formatter = CloudWatchLogMonkeyPatchedFormatter()
watchtower_formatter.add_log_record_attrs=["name", "levelname", "filename", "lineno", "funcName", "message"]
watchtower_handler.setFormatter(watchtower_formatter)
logger.addHandler(watchtower_handler)

[alexminza]

Or even simpler - log whole record:

class CloudWatchLogJSONFormatter(watchtower.CloudWatchLogFormatter):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def format(self, message):
        formatted_message = json.dumps(message.__dict__, default=self.json_serialize_default)
        return formatted_message