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Creating custom components#

Fondant makes it easy to build data preparation pipelines leveraging reusable components. Fondant provides a lot of components out of the box (overview, but you can also define your own custom components.

To make sure components are reusable, they should implement a single logical data processing step (like captioning images or removing Personal Identifiable Information [PII] from text.) If a component grows too large, consider splitting it into multiple separate components each tackling one logical part.

To implement a custom component, a couple of files need to be defined:

Fondant component specification#

Each Fondant component is defined by a specification which describes its interface. This specification is represented by a single fondant_component.yaml file. See the component specification page for info on how to write the specification for your component. script#

The core logic of the component should be implemented in a script in a folder called src. The logic should be implemented as a class, inheriting from one of the base Component classes offered by Fondant. There are three large types of components:

  • LoadComponent: Load data into your pipeline from an external data source
  • TransformComponent: Implement a single transformation step in your pipeline
  • WriteComponent: Write the results of your pipeline to an external data sink

The easiest way to implement a TransformComponent is to subclass the provided PandasTransformComponent. This component streams your data and offers it in memory-sized chunks as pandas dataframes.

import pandas as pd
from fondant.component import PandasTransformComponent

class ExampleComponent(PandasTransformComponent):

    def __init__(self, *, argument1, argument2, **kwargs) -> None:
            argumentX: An argument passed to the component
        # Initialize your component here based on the arguments

    def transform(self, dataframe: pd.DataFrame) -> pd.DataFrame:
        """Implement your custom logic in this single method
            dataframe: A Pandas dataframe containing one partition of your data
            A pandas dataframe containing the transformed data

The __init__ method is called once for each component class with custom arguments defined in the args section of the component specification.) This is a good place to initialize resources and costly initializations such as network connections, models, parsing a config file, etc. By doing so, you can effectively prevent the redundant re-initialization of resources each time the transform method is invoked.

The transform method is called multiple times, each time containing a pandas dataframe with a partition of your data loaded in memory.

The dataframes passed to the transform method contains the data specified in the consumes section of the component specification. If a component defines that it consumes an image field, this data can be accessed using dataframe["image"].

The transform method should return a single dataframe, with the columns complying to the schema defined by the produces section of the component specification.

Note that the script can be split up into several Python scripts in case it would become prohibitively long. See the prompt based LAION retrieval component as an example: the CLIP client itself is defined in a separate script called clip_client, which is then imported in the script.


The Dockerfile defines how to build the component into a Docker image. An example Dockerfile is defined below.

FROM --platform=linux/amd64 python:3.8-slim

# install requirements
COPY requirements.txt ./
RUN pip3 install --no-cache-dir -r requirements.txt

# Set the working directory to the component folder
WORKDIR /component/src

# Copy over src-files and spec of the component
COPY src/ .

ENTRYPOINT ["fondant", "execute", "main"]


A requirements.txt file lists the Python dependencies of the component. Note that any Fondant component will always have Fondant[component] as the minimum requirement. It's important to also pin the version of each dependency to make sure the component remains working as expected. Below is an example of a component that relies on several Python libraries such as Pillow, PyTorch and Transformers.


Refer to this section to find out how to build and publish your components to use them in your own pipelines.