Implementation:PrefectHQ Prefect Hello World Example

Knowledge Sources	PrefectHQ_Prefect Prefect Docs
Domains	Orchestration, Getting_Started, Examples
Last Updated	2026-02-09 22:00 GMT

Overview

Introductory example demonstrating the most basic Prefect flow using the `@flow` decorator with `log_prints`, parameters, and tags.

Description

The hello_world.py example is the canonical starting point for new Prefect users. It defines a single `hello` function decorated with `@flow(log_prints=True)` that prints a greeting. The example demonstrates calling the flow with default parameters, custom parameters, and iterating over a list of names. It also shows the use of the `tags` context manager for labeling flow runs in the UI. The file includes extensive inline documentation explaining what happens when a function is decorated with `@flow`.

Usage

Use this example as a template when creating your first Prefect flow. It demonstrates the minimum viable Prefect workflow: decorating a Python function with `@flow`, running it, and observing the automatic logging, state tracking, and UI integration.

Code Reference

Source Location

• Repository: PrefectHQ_Prefect
• File: examples/hello_world.py
• Lines: 1-111

Signature

@flow(log_prints=True)
def hello(name: str = "Marvin") -> None:
    """Log a friendly greeting."""
    print(f"Hello, {name}!")

Import

from prefect import flow, tags

I/O Contract

Inputs

Name	Type	Required	Description
name	str	No	Name to greet (default: "Marvin")

Outputs

Name	Type	Description
return	None	No return value; greeting is printed and logged
Flow run	Prefect state	Tracked flow run with logs, state, and metadata

Usage Examples

Basic Hello World

from prefect import flow, tags


@flow(log_prints=True)
def hello(name: str = "Marvin") -> None:
    """Log a friendly greeting."""
    print(f"Hello, {name}!")


if __name__ == "__main__":
    # Run with default parameters
    with tags("test"):
        hello()  # Logs: "Hello, Marvin!"

        # Run with custom parameter
        hello("Marvin")

        # Run for multiple people
        crew = ["Zaphod", "Trillian", "Ford"]
        for name in crew:
            hello(name)

Related Pages

• Environment:PrefectHQ_Prefect_Python_Runtime_Environment