Piou

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

Quick Example

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()

Installation

pip install piou

Or with uv:

uv add piou

Or with conda:

conda install piou -c conda-forge

Raw Formatter

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

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

Documentation

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

Features

• 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
• Interactive TUI mode with command suggestions and history

Why Piou?

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.

Async Commands

Commands can be async functions — piou detects coroutines and runs them automatically, no manual asyncio.run() needed:

from piou import Cli, Option
cli = Cli(description='Async example')
@cli.command(cmd='fetch', help='Fetch data')
async def fetch(url: str = Option(help='URL to fetch')):
import niquests
async with niquests.AsyncSession() as client:
r = await client.get(url)
print(r.status_code)
if name == 'main':
cli.run()

This works the same way for commands inside command groups.

Interactive TUI Mode

Piou includes an optional interactive TUI (Text User Interface) mode powered by Textual. This provides a rich terminal experience with command suggestions, history, and inline completions.

Installation

pip install piou[tui]
With auto-reload support for development
pip install piou[tui-reload]

Usage

Enable TUI mode by setting tui=True when creating your CLI:

from piou import Cli, Option
cli = Cli(description='My Interactive CLI', tui=True)
@cli.command(cmd='hello', help='Say hello')
def hello(name: str = Option(..., help='Name to greet')):
print(f'Hello, {name}!')
if name == 'main':
cli.run()

Or via the --tui flag:

python my_cli.py --tui

Or via the PIOU_TUI=1 environment variable:

PIOU_TUI=1 python my_cli.py

TUI Features

• Command suggestions: Type / to see available commands with descriptions
• Subcommand navigation: Use : to navigate subcommands (e.g., /stats:uploads)
• Inline completions: See argument placeholders as you type
• Command history: Navigate previous commands with up/down arrows (persisted across sessions)
• Rich output: ANSI colors and formatting preserved in output
• Keyboard shortcuts:
  • Tab - Confirm selected suggestion
  • Up/Down - Navigate suggestions or history
  • Ctrl+C - Clear input (press twice to exit)
  • Escape - Quit
• Dev mode: Auto-reload commands when source files change (see below)

Dev Mode with Auto-Reload

For faster development iteration, enable dev mode to automatically reload your commands when source files change:

pip install piou[tui-reload]

Then use the --tui-reload flag:

python my_cli.py --tui-reload

Or via environment variable:

PIOU_TUI_DEV=1 python my_cli.py --tui

When enabled, Piou watches your command source files and hot-reloads them on save. You can also toggle reload mode at runtime with the /tui-reload command.

To run custom code after each reload (e.g., refresh cached data), use the @cli.tui_on_reload decorator:

@cli.tui_on_reload
def on_reload():
    print('Code reloaded!')

Advanced Example: HTTP Benchmark

The TUI mode supports mounting custom Textual widgets for rich interactive displays. This example benchmarks HTTP libraries with live progress grids:

See examples/http_bench_tui.py for the full implementation using TuiContext and custom widgets.

Development

Running Tests

uv run pytest

Generating Documentation

# Build docs
uv run --group docs mkdocs build
Serve locally
uv run --group docs mkdocs serve

Generating Screenshots and GIFs

Terminal recordings are created with VHS. Install it first:

# Ubuntu/Debian
sudo apt install vhs ttyd
macOS
brew install vhs
Or via Go
go install github.com/charmbracelet/vhs@latest

Then generate recordings from tape files:

vhs docs/static/tui-demo.tape

Tape files are located in docs/static/ and define scripted terminal sessions that produce GIFs.