A CLI tool to build beautiful command-line interfaces with type validation.

from piou import Cli, Option

cli = Cli(description='A CLI tool')


@cli.command(cmd='foo', help='Run foo command')
def foo_main(
        bar: int = Option(..., help='Bar positional argument (required)'),
        baz: str = Option(..., '-b', '--baz', help='Baz keyword argument (required)'),
        foo: str | None = Option(None, '--foo', help='Foo keyword argument'),
):
    """
    A longer description on what the function is doing.
    """
    pass


if __name__ == '__main__':
    cli.run()

pip install piou

Or with uv:

uv add piou

Or with conda:

conda install piou -c conda-forge

By default, Piou uses Rich for beautiful terminal output. If you prefer plain text output or want to minimize dependencies, you can use the raw formatter:

# Install without Rich formatting (Rich is still installed but not used)
pip install piou[raw]

# Or force raw output via environment variable
PIOU_FORMATTER=raw python your_cli.py --help

Full documentation is available at andarius.github.io/piou.

• FastAPI-like developer experience with type hints
• Custom formatters (Rich-based by default)
• Nested command groups / sub-commands
• Derived options for reusable argument patterns
• Async command support
• Type validation and casting

I could not find a library that provided:

• The same developer experience as FastAPI
• Customization of the interface (to build a CLI similar to Poetry)
• Type validation / casting

Typer is the closest alternative but lacks the possibility to format the output in a custom way using external libraries (like Rich).

Piou provides all these possibilities and lets you define your own Formatter.