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Implementing custom components#

This guide will teach you how to build custom components and integrate them in your dataset.


In the previous tutorial, you learned how to create your first Fondant dataset. While the example demonstrates how to build a workflow from reusable components, this is only the beginning.

Reusable components consume data in a specific format, defined in a data contract. Therefore, it is often necessary to implement custom components to connect the reusable components to your specific data. The easiest way to do this is to implement a Lightweight Component.

In this tutorial, we will guide you through the process of implementing your very own custom component. We will illustrate this by building a transform component that uppercases the alt_text of the image dataset.

If you want to build a complex custom component or share the component within your organization or even the community, take a look at how to build reusable components.

This dataset is an extension of the one introduced in the previous tutorial. Make sure you have completed the tutorial before diving into this one.

In the last tutorial, we implemented this dataset:

from fondant.dataset import Dataset
import pyarrow as pa

dataset = Dataset.create(
        "dataset_name": "fondant-ai/fondant-cc-25m",
        "n_rows_to_load": 100,
      "alt_text": pa.string(),
      "image_url": pa.string(),
      "license_location": pa.string(),
      "license_type": pa.string(),
      "webpage_url": pa.string(),

images = dataset.apply(

english_images = images.apply(
        "language": "en"
        "text": "alt_text"

We want to extend the dataset and apply a simple text transformation to the alt_text. Let's consider that the alt_text is so important that the text has to be transformed into uppercase letters.

Implement your Lightweight component#

Now, it's time to implement the component logic.

We will subclass the PandasTransformComponent offered by Fondant. This is the most basic type of component. The following method should be implemented:

  • transform(): This method receives a chunk of the input data as a Pandas DataFrame. Fondant automatically chunks your data you can process larger-than-memory data, and your component is executed in parallel across the available cores.
"""A component that transform the alt text of the dataframe into uppercase."""
import pandas as pd
from fondant.component import PandasTransformComponent
from fondant.dataset import lightweight_component

class UpperCaseTextComponent(PandasTransformComponent):

    def transform(self, dataframe: pd.DataFrame) -> pd.DataFrame:
        """Transform the alt text into upper case."""
        dataframe["alt_text"] = dataframe["alt_text"].apply(lambda x: x.upper())
        return dataframe


Note that we have used a decorator @lightweight_component. This decorator is necessary to inform Fondant that this class is a lightweight component and can be used as a component in your workflow.

We apply the uppercase transformation to the alt_text column of the dataframe. Afterward, we return the transformed dataframe from the transform method, which Fondant will use to automatically update the index.

The lightweight components provide an easy way to start with your component implementation. However, the lightweight component implementation still allows you to define all advanced component configurations, including installing extra arguments or defining component arguments. These concepts are more advanced and not needed for quick exploration and experiments. You can find more information on these topics in the documentation of the lightweight components.

Using the component#

Now were we have defined our lightweight component we can start using it in our workflow. For instance we can apply this component to the end of the workflow to apply the transformation to the english_images dataset.

uppercase_alt_text = english_images.apply(

Instead of providing the name of the component, as we did with the reusable components, we now provide the component implementation.

Now, you can execute the workflow once more to materialize the dataset and examine the results. In the final output, the alt_text is in uppercase.

Of course, it is debatable whether uppercasing the alt_text is genuinely useful. This is just a constructive and simple example to showcase how to use lightweight components as glue code you might need for the dataset creation, helping you connect reusable components to each other.

Next steps#

We now have a workflow that downloads a dataset from the HuggingFace hub, filters the urls by image type, downloads the images, and filters them by alt text language.

If you want to inspect your final dataset without using the data explorer or use the dataset for further tasks, we recommend to write the final dataset to a destination. We offer write components to perform this task, for instance the write_to_file component, which allows you to export the dataset either to a local file or a remote storage bucket.

uppercase_alt_text.write(ref="write_to_file", arguments={"path": "/data/export"})

You can open the path and use any tools of your choice to inspect the resulting Parquet dataset.