Your first step

With your project set up, you are now ready to begin writing your first ProcessingStep in earnest.

One small step

In this tutorial, you will create a Step that takes an input file, reads it, appends a string, and then outputs to a file. You will need to subclass Step, then implement our step function as a method; file input and output are handled with “dataslots”, indicated with decorators. Adding a version decorator is also strongly recommended.

Note

For all ProcessingSteps, it is mandatory to have parameters and return values with type annotations. Additionally, while versioning is optional, semantic versioning is enforced.

src/small_step.py

from pinexq.procon.dataslots import DataSlot, dataslot, MediaTypes
from pinexq.procon.step import Step, version

class SmallStep(Step):
    @dataslot.input("input_file", media_type=MediaTypes.TEXT)
    @dataslot.returns(media_type=MediaTypes.TEXT)
    @version("0.1.0")
    def append(self, input_file: str, next_line: str) -> str:
        temp_string = input_file + "\n" +  next_line
        return temp_string

# for command line execution
if __name__ == "__main__":
    SmallStep()

A couple notes:

• The block at the bottom allows you to execute this file as a script.
• In order to declare file input and output, you need to use the @dataslot decorators, as shown.
• The mediatypes of these files should always be given; for example, media_type=MediaTypes.TEXT ensures the files are opened in text mode.

Running locally

You can now try your new step. To specify parameters and dataslots through the command line, pass Python-style dictionaries from parameter names to values. Furthermore, because a dataslot can potentially contain more than one file, each dataslot takes a list of files.

For instance, create input.txt in the test directory, containing “Hello World!”. You can then try the following, specifying files for each dataslot input/output and values for each parameter:

foo@bar: ~/my_project$ uv run src/small_step.py --function append --input-dataslots "{'input_file': ['test/input.txt']}" --output-dataslots "{'__returns__': ['test/output.txt']}" --parameters "{'next_line': 'Hello again!'}"
23-01-26 16:00:35 INFO Platform and package information:
OS: Linux-6.6.87.1-microsoft-standard-WSL2-x86_64-with-glibc2.39
Python: CPython 3.14.0
pinexq-procon: 2.3.0
pinexq-client: 1.0.0
None

Note that dataslot.returns is a special output dataslot, with the name __returns__. For full documentation on all options, including short forms, try running uv run src/small_step.py run --help.

As shown above, your function returns None; this is expected, since the return value was directed into a file. You can now check test/output.txt and confirm the contents were appended:

test/output.txt

Hello World!
Hello again!