Create Our First Log (new version)

In this tutorial, we will log a message into a hello.mcap file.

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 can run our "Hello World" program with the following command:

uv run hello.py

This should produce the following output on your console:

Hello from mcap-logger-tutorial!

We will use this "Hello World" program as a base for our tutorial.

Create our first log

First, let's import the logging package of Python and the McapHandler class into the hello.py script. After we did the import, we will create our Logger entity with the getLogger function. We will call the logger mcap_logger.

hello.py
import logging
from mcap_logger.mcap_handler import McapHandler


def main():
    logger = logging.getLogger("mcap_logger")
    print("Hello from mcap-logger-tutorial!")

Then we will define the log file we want to create. To do that, we need to import Path and specify the log file's name and path. In this case the log file will be called hello.mcap and we will place it in the project's directory.

After this, we can create our mcap log handler by initializing it with the path of the log file.

Info

If the parent directory of the defined log doesn't exist, the it will be created.

hello.py
import logging
from pathlib import Path

from mcap_logger.mcap_handler import McapHandler


def main():
    logger = logging.getLogger("mcap_logger")
    log_file = Path("hello.mcap")
    mcap_handler = McapHandler(log_file)
    print("Hello from mcap-logger-tutorial!")

When we created our handler then we have to add it to the logger. This means that when the logger logs a message then it will hand it to our MCAP handler, which will put this into our log file.

We will set the level of logging to be DEBUG level both for the logger and for the handler. This basically means that all log messages will be logged.

We will remove the original print statement and replace it with an info level log call.

hello.py
import logging
from pathlib import Path

from mcap_logger.mcap_handler import McapHandler


def main():
    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()

Create the log file

Now that we made modification to the hello.py, we can run our example again and see the result.

uv run hello.py

Note

There will be no console logs, because we only added and McapHandler to the logger.

Notice that we have a new file in our project directory called hello.mcap. This file is serialized with Protobuf, so we can not open it with a text editor and look at the content of it. To do that we will use Foxglove Studio.

Open our log file

Open Foxglove Studio and use the Open local file... command to open our hello.mcap log file. Our log messages is using Foxglove's Log schema, and we can visualise it with a Log panel.

We can add a panel following this guide, and after that we need to configure it to use the /log topic.

After this we should see our info message on the log panel.

And Voilà! We made our first log with mcap-logger.

Filtering logs

Notice that in Foxglove Studio, you can filter the logs by their level in the log panel.