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Core Module

Logging in general refers to the practise of recording events activities over time. Having proper logging in your applications can be extremely beneficial for a few reasons:

  • Debugging becomes easier because we in a more structure way can output information about the state of our program, variables, values etc. to help identify and fix bugs or unexpected behavior.

  • When we move into a more production environment, proper logging is essential for monitoring the health and performance of our application.

  • It can help in auditing as logging info about specific activities etc. can help keeping a record of who did what and when.

  • Having proper logging means that info is saved for later, that can be analysed to gain insight into the behavior of our application, such as trends.

We are in this course going to divide the kind of logging we can do into categories: application logging and experiment logging. In general application logging is important regardless of the kind of application you are developing, whereas experiment logging is important machine learning based projects where we are doing experiments.

Application logging

The most basic form of logging in Python applications is the good old print statement:

for batch_idx, batch in enumerate(dataloader):
    print(f"Processing batch {batch_idx} out of {len(dataloader)}")

This will keep a "record" of the events happening in our script, in this case how far we have progressed. We could even change the print to include something like batch.shape to also have information about the current data being processed.

Using print statements is fine for small applications, but to have proper logging we need a bit more functionality than what print can offer. Python actually comes with a great logging module, that defines functions for flexible logging. It is exactly this we are going to look at in this module.

The four main components to the Python logging module are:

  1. Logger: The main entry point for using the logging system. You create instances of the Logger class to emit log messages.

  2. Handler: Defines where the log messages go. Handlers send the log messages to specific destinations, such as the console or a file.

  3. Formatter: Specifies the layout of the log messages. Formatters determine the structure of the log records, including details like timestamps and log message content.

  4. Level: Specifies the severity of a log message.

Especially, the last point is important to understand. Levels essentially allows of to get rid of statements like this:

if debug:

where the logging is conditional on the variable debug which we can set a runtime. Thus, it is something we can disable for users of our application (debug=False) but have enabled when we develop the application (debug=True). And it makes sense that not all things logged, should be available to all stakeholders of a codebase. We as developers probably always wants the highest level of logging, whereas users of the our code need less info and we may want to differentiate this based on users.


It is also important to understand the different between logging and error handling. Error handling Python is done using raise statements and try/catch like:

def f(x: int):
    if not isinstance(x, int):
        raise ValueError("Expected an integer")
    return 2 * x

except ValueError:
    print("I failed to do a thing, but continuing.")

Why would we evere need log warning, error, critical levels of information, if we are just going to handle it? The reason is that raising exceptions are meant to change the program flow at runtime e.g. things we do not want the user to do, but we can deal with in some way. Logging is always for after a program have run, to inspect what went wrong. Sometimes you need one, sometimes the other, sometimes both.

❔ Exercises

Exercises are inspired by this made with ml module on the same topic. If you need help for the exercises you can find a simple solution script here.

  1. As logging is a built-in module in Python, nothing needs to be installed. Instead start a new file called and start out with the following code:

    import logging
    import sys
    # Create super basic logger
    logging.basicConfig(stream=sys.stdout, level=logging.DEBUG)
    logger = logging.getLogger(__name__) # (1)
    # Logging levels (from lowest to highest priority)
    logger.debug("Used for debugging your code.")"Informative messages from your code.")
    logger.warning("Everything works but there is something to be aware of.")
    logger.error("There's been a mistake with the process.")
    logger.critical("There is something terribly wrong and process may terminate.")
    1. 🙋‍♂️ The built-in variable __name__ always contains the record of the script or module that is currently being run. Therefore if we initialize our logger base using this variable, it will always be unique to our application and not conflict with logger setup by any third-party package.

    Try running the code. Than try changing the argument level when creating the logger. What happens when you do that?

  2. Instead of sending logs to the terminal, we may also want to send them to a file. This can be beneficial, such that only warning level logs and higher are available to the user, but debug and info is still saved when the application is running.

    1. Try adding the following dict to your file:

      logging_config = {
          "version": 1,
          "formatters": { # (1)
              "minimal": {"format": "%(message)s"},
              "detailed": {
                  "format": "%(levelname)s %(asctime)s [%(name)s:%(filename)s:%(funcName)s:%(lineno)d]\n%(message)s\n"
          "handlers": { # (2)
              "console": {
                  "class": "logging.StreamHandler",
                  "stream": sys.stdout,
                  "formatter": "minimal",
                  "level": logging.DEBUG,
              "info": {
                  "class": "logging.handlers.RotatingFileHandler",
                  "filename": Path(LOGS_DIR, "info.log"),
                  "maxBytes": 10485760,  # 1 MB
                  "backupCount": 10,
                  "formatter": "detailed",
                  "level": logging.INFO,
              "error": {
                  "class": "logging.handlers.RotatingFileHandler",
                  "filename": Path(LOGS_DIR, "error.log"),
                  "maxBytes": 10485760,  # 1 MB
                  "backupCount": 10,
                  "formatter": "detailed",
                  "level": logging.ERROR,
          "root": {
              "handlers": ["console", "info", "error"],
              "level": logging.INFO,
              "propagate": True,
      1. 🙋‍♂️ The formatter section determines how logs should be formatted. Here we define two separate formatters, called minimal and detailed which we can use in the next part of the code.

      2. 🙋‍♂️ The handlers is in charge of what should happen to different level of logging. console uses the minimal format we defined and sens logs to the stdout stream for messages of level DEBUG and higher. The info handler uses the detailed format and sends messages of level INFO and higher to a separate info.log file. The error handler does the same for messages of level ERROR and higher to a file called error.log.

      you will need to set the LOGS_DIR variable and also figure out how to add this logging_config using the logging config submodule to your logger.

    2. When the code successfully runs, check the LOGS_DIR folder and make sure that a info.log and error.log file was created with the appropriate content.

  3. Finally, lets try to add a little bit of style and color to our logging. For this we can use the great package rich which is a great package for rich text and beautiful formatting in terminals. Install rich and add the following line to your script:

    logger.root.handlers[0] = RichHandler(markup=True)  # set rich handler

    and try re-running the script. Hopefully you should see something beautiful in your terminal like this:


  4. (Optional) We already briefly touched on logging during the module on config files using hydra. If you want to configure hydra to use custom logging scheme as the one we setup in the last two exercises, you can take a look at this page. In hydra you will need to provide the configuration of the logger as config file. You can find examples of such config file here.

Experiment logging

When most people think machine learning, we think about the training phase. Being able to track and log experiments is an important part of understanding what is going on with your model while you are training. It can help you debug your model and help tweak your models to perfection. Without proper logging of experiments, it can be really hard to iterate on the model because you do not know what changes lead to increase or decrease in performance.

The most basic logging we can do when running experiments is writing the metrics that our model is producing e.g. the loss or the accuracy to the terminal or a file for later inspection. We can then also use tools such as matplotlib for plotting the progression of our metrics over time. This kind of workflow may be enough when doing smaller experiments or working alone on a project, but there is no way around using a proper experiment tracker and visualizer when doing large scale experiments in collaboration with others. It especially becomes important when you want to compare performance between different runs.

There exist many tools for logging your experiments, with some of them being:

All of the frameworks offers many of the same functionalities, you can see a (bias) review here. We are going to use Weights and Bias (wandb), as it support everything we need in this course. Additionally, it is an excellent tool for collaboration and sharing of results.


Using the Weights and Bias (wandb) dashboard we can quickly get an overview and compare many runs over different metrics. This allows for better iteration of models and training procedure.

❔ Exercises

  1. Start by creating an account at wandb. I recommend using your github account but feel free to choose what you want. When you are logged in you should get an API key of length 40. Copy this for later use (HINT: if you forgot to copy the API key, you can find it under settings).

  2. Next install wandb on your laptop

    pip install wandb
  3. Now connect to your wandb account

    wandb login

    you will be asked to provide the 40 length API key. The connection should be remain open to the wandb server even when you close the terminal, such that you do not have to login each time. If using wandb in a notebook you need to manually close the connection using wandb.finish().

  4. With it all setup we are now ready to incorporate wandb into our code. The interface is fairly simple, and this guide should give enough hints to get you through the exercise. (HINT: the two methods you need to call are wandb.init and wandb.log). To start with, logging the training loss of your model will be enough.

  5. After running your model, checkout the webpage. Hopefully you should be able to see at least one run with something logged.

  6. Now log something else than scalar values. This could be a image, a histogram or a matplotlib figure. In all cases the logging is still going to use wandb.log but you need extra calls to wandb.Image etc. depending on what you choose to log.

  7. Finally, lets create a report that you can share. Click the Create report button and include some of the graphs/plots/images that you have generated in the report.

  8. To make sure that you have completed todays exercises, make the report shareable by clicking the Share button and create view-only-link. Send the link to my email, so I can checkout your awesome work 😃

  9. When calling wandb.init you have two arguments called project and entity. Make sure that you understand these and try them out. It will come in handy for your group work as they essentially allows multiple users to upload their own runs to the same project in wandb.

  10. Wandb also comes with build in feature for doing hyperparameter sweeping which can be beneficial to get a better working model. Look through the documentation on how to do a hyperparameter sweep in Wandb. You at least need to create a new file called sweep.yaml and make sure that you call wandb.log in your code on an appropriate value. Note: if you want hydra and wandb to work together you will need to change the command config in your sweep.yaml file, see this page.

  11. In the future it will be important for us to be able to run Wandb inside a docker container (together with whatever training or inference we specify). The problem here is that we cannot authenticate Wandb in the same way as the previous exercise, it needs to happen automatically. Lets therefore look into how we can do that.

    1. First we need to generate an authentication key, or more precise an API key. This is in general the way any service (like a docker container) can authenticate. Start by going, click your profile icon in the upper right corner and then go to settings. Scroll down to the danger zone and generate a new API key and finally copy it.

    2. Next create a new docker file called wandb.docker and add the following code

      FROM python:3.9
      RUN apt update && \
          apt install --no-install-recommends -y build-essential gcc && \
          apt clean && rm -rf /var/lib/apt/lists/*
      RUN pip install wandb
      COPY s4_debugging_and_logging/exercise_files/
      ENTRYPOINT ["python", "-u", ""]

      please take a look at the script being copied into the image and afterwards build the docker image.

    3. When we want to run the image, what we need to do is including a environment variables that contains the API key we generated. This will then authenticate the docker container with the wandb server:

      docker run -e WANDB_API_KEY=<your-api-key> wandb:latest

      Try running it an confirm that the results are uploaded to the wandb server.

  12. Feel free to experiment more with wandb as it is a great tool for logging, organizing and sharing experiments.

That is the module on logging. Please note that at this point in the course you will begin to see some overlap between the different frameworks. While we mainly used hydra for configuring our python scripts it can also be used to save metrics and hyperparameters similar to how wandb can. Similar arguments holds for dvc which can also be used to log metrics. In our opinion wandb just offers a better experience when interacting with the results after logging. We want to stress that the combination of tools presented in this course may not be the best for all your future projects, and we recommend finding a setup that fits you. That said, each framework provide specific features that the others does not.

Finally, we want to note that we during the course really try to showcase a lot of open source frameworks, Wandb is not one. It is free to use for personal usage (with a few restrictions) but for enterprise it does require a license. If you are eager to only work with open-source tools we highly recommend trying out MLFlow which offers the same overall functionalities as Wandb.