Use Console Stream with MCAP Logging

In this tutorial, we will look into how we can combine different log handlers to have console output and MCAP log at the same time.

Setup our tutorial project

In this tutorial we will use uv.

Let's create a new uv project and add the MCAP Logger package as dependency.

uv init mcap_logger_tutorial
cd mcap_logger_tutorial
uv add mcap-logger

After this we should have the following elements in the project's folder:

.
├── .python-version
├── README.md
├── hello.py
└── pyproject.toml

We will change the hello.py as shown below.

import logging
from pathlib import Path

from mcap_logger.mcap_handler import McapHandler


def main():  # noqa: ANN201
    logger = logging.getLogger("mcap_logger")
    log_file = Path("hello.mcap")
    mcap_handler = McapHandler(log_file)
    mcap_handler.setLevel("DEBUG")

    logger.addHandler(mcap_handler)
    logger.setLevel("DEBUG")

    logger.info("Hello from mcap-logger-tutorial!")


if __name__ == "__main__":
    main()

Configure console logging

We can see that in our code, we create a logger instance, and we add our McapHandler to it as a handler. A Python logger can have multiple handlers. It can be beneficial for an application to log all messages of all severities to a file while simultaneously logging warning or above to the console. To set this up, we have to add a StreamHandler to our logger instance as follows.

stream_handler = logging.StreamHandler()
stream_handler.setLevel("WARNING")

logger.addHandler(stream_handler)

Note, that we can set the logging level separately from other handlers and from the logger. In this case, becasue the level of the logger is DEBUG, it will forward all log messages to the handlers. The mcap_handler has also DEBUG level, so it will log every messages to the log file. The stream_handler has WARNING log level, which means that it will only print messages that are WARNING or prior to WARNING.

Logging Flow

Here you can check the flowdiagram of the logging.

We can add a warning message to our hello.py script to see the behaviour described above. After the modifications our hello.py script should look like this.

import logging
from pathlib import Path

from mcap_logger.mcap_handler import McapHandler


def main():  # noqa: ANN201
    logger = logging.getLogger("mcap_logger")
    log_file = Path("hello.mcap")
    mcap_handler = McapHandler(log_file)
    mcap_handler.setLevel("DEBUG")

    stream_handler = logging.StreamHandler()
    stream_handler.setLevel("WARNING")

    logger.addHandler(mcap_handler)
    logger.addHandler(stream_handler)
    logger.setLevel("DEBUG")

    logger.info("Hello from mcap-logger-tutorial!")
    logger.warning("This is a warning")


if __name__ == "__main__":
    main()

Running the script

uv run hello.py

When we run the command above, we should see that the warning message is printed on our console (and the info message is not), and also that we have a hello.mcap file in our folder that contains both the warning message and the info message.

Code structuring

Our code is functional, however it is really verbose and suppress the essential parts of the main function (the two log messages 😄). We can move the technical details of the logger to its own function, so that all the logger configuration can live in one place. We should also move the logger variable out to the global scope so that any function in the script can access it.

import logging
from pathlib import Path

from mcap_logger.mcap_handler import McapHandler


def get_logger(name: str, file: Path) -> logging.Logger:
    logger = logging.getLogger(name)
    mcap_handler = McapHandler(file)
    mcap_handler.setLevel("DEBUG")

    stream_handler = logging.StreamHandler()
    stream_handler.setLevel("WARNING")

    logger.addHandler(mcap_handler)
    logger.addHandler(stream_handler)
    logger.setLevel("DEBUG")
    return logger


log_file = Path("hello.mcap")
log = get_logger("mcap_logger", log_file)


def main():  # noqa: ANN201
    log.info("Hello from mcap-logger-tutorial!")
    log.warning("This is a warning")


if __name__ == "__main__":
    main()

With this, we have a simple example that how we can configure a logger with both console and MCAP logging.

Configuring Logging

To learn about more advanced logging configuration, please check out the Logging HOWTO - Configuring Logging section