How to Effectively Manage Multiple Outputs from a `with_outputs` in a PTransform

What will you learn?

In this tutorial, you will master the art of handling multiple outputs generated by a PTransform using the powerful with_outputs method in Python. This skill is essential for efficiently managing and processing distinct output collections within your data pipelines.

Introduction to the Problem and Solution

When working with data processing frameworks like Apache Beam, it’s common to encounter scenarios where a single transformation yields multiple output collections. The with_outputs method empowers us to explicitly define these outputs during transformation definition. By leveraging this method, we can effectively manage and process these diverse output collections separately within our pipeline.

To adeptly handle multiple outputs from a with_outputs, it’s crucial to grasp how to define and access these outputs within our pipeline. This involves accurately specifying output tags’ names when creating custom transform classes and subsequently extracting these tagged outputs for further downstream processing in the pipeline.


import apache_beam as beam

class MyCustomTransform(beam.PTransform):
    def expand(self, pcoll):
        main_output = pcoll | 'Some Main Processing' >> beam.Map(lambda x: some_main_processing(x))
        additional_output = pcoll | 'Some Additional Processing' >> beam.Map(lambda x: some_additional_processing(x))

        return (main_output, additional_output)

# Using with_outputs to manage multiple outputs
output_results = input_data | 'Apply Custom Transform' >> MyCustomTransform().with_outputs('additional'='additional_output')

# Accessing individual output collections
main_collection = output_results['']  # accessing main collection (default)
additional_collection = output_results['additional']  # accessing additional collection

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  • The code snippet showcases defining a custom transform class MyCustomTransform that extends beam.PTransform.
  • Within this class, two distinct processing steps are applied to the input data: one for main processing and another for additional processing.
  • By using the .with_outputs() method after applying our custom transform on input data (input_data), we can specify named tags for each output collection generated by our transformation.
  • Tagging outputs during transformation application enables accessing separate output streams by referencing corresponding tags within the returned dictionary-like object (output_results).
  • The example demonstrates accessing both main and additional output collections using their respective tags (”, ‘additional’) as keys in the dictionary returned by .with_outputs().
    How many named outputs can be specified using with_outputs?

    You can specify any number of named outputs while applying .with_outputs(). Each name corresponds to a distinct collection produced by the transformation.

    Can I have unnamed default outputs along with named ones using with_outputs?

    Yes, you can. Using an empty string (”) as a name in .with_outputs() represents an unnamed/default output stream from your transformation.

    Is it necessary for all PTransforms to have only one default output?

    No, PTransforms are not limited to producing only one primary result. With methods like .with_outputs(), you can create PTransforms that generate multiple distinct results based on your requirements.

    How do I handle errors specific to each named output in Apache Beam?

    Error handling specific to individual named outputs can be implemented through appropriate exception handling logic within transformations or subsequent DoFns responsible for processing those streams.

    Can I dynamically route elements into different branches based on their content in Apache Beam?

    Yes, conditional branching based on element content is achievable within Apache Beam pipelines. Elements can be dynamically routed into various branches depending on their attributes or values during processing stages like ParDo operations.


    Effectively managing multiple outputs from a PTransform via .width_ouputs() provides enhanced flexibility and control over distinct result streams within Apache Beam pipelines. Embracing best practices such as clear naming conventions and structured error handling tailored towards individual branches streamlines development workflows while ensuring scalability and maintainability across complex data processing tasks.

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