prefect.projects.steps.core

Core primitives for running Prefect project steps.

Project steps are YAML representations of Python functions along with their inputs.

Whenever a step is run, the following actions are taken:

• The step's inputs and block / variable references are resolved (see the projects concepts documentation for more details)
• The step's function is imported; if it cannot be found, the requires keyword is used to install the necessary packages
• The step's function is called with the resolved inputs
• The step's output is returned and used to resolve inputs for subsequent steps

run_step async

Runs a step, returns the step's output.

Steps are assumed to be in the format {"importable.func.name": {"kwarg1": "value1", ...}}.

The 'requires' keyword is reserved for specific purposes and will be removed from the inputs before passing to the step function:

This keyword is used to specify packages that should be installed before running the step.

Source code in src/prefect/projects/steps/core.py

async def run_step(step: dict) -> dict:
    """
    Runs a step, returns the step's output.

    Steps are assumed to be in the format `{"importable.func.name": {"kwarg1": "value1", ...}}`.

    The 'requires' keyword is reserved for specific purposes and will be removed from the
    inputs before passing to the step function:

    This keyword is used to specify packages that should be installed before running the step.
    """
    fqn, inputs = step.popitem()

    if step:
        raise ValueError(
            f"Step has unexpected additional keys: {', '.join(step.keys())}"
        )

    keywords = {
        keyword: inputs.pop(keyword)
        for keyword in RESERVED_KEYWORDS
        if keyword in inputs
    }

    inputs = await resolve_block_document_references(inputs)
    inputs = await resolve_variables(inputs)

    step_func = _get_function_for_step(fqn, requires=keywords.get("requires"))
    return step_func(**inputs)