python-services

Python Services & CLI

Modern Tooling

• uv init --package myproject for distributable packages, uv init for apps
• uv add <pkg>, uv add --group dev <pkg>, never edit pyproject.toml deps manually
• uv run <cmd> instead of activating venvs
• uv.lock goes in version control
• Use [dependency-groups] (PEP 735) for dev/test/docs, not [project.optional-dependencies]
• PEP 723 inline metadata for standalone scripts with deps
• ruff check --fix . && ruff format . for lint+format in one pass

CLI Tools

Entry points in pyproject.toml:

[project.scripts]
my-tool = "my_package.cli:main"

Click (recommended for complex CLIs):

import click

@click.group()
@click.version_option()
def cli(): ...

@cli.command()
@click.argument("name")
@click.option("--count", default=1, type=int)
def greet(name: str, count: int):
    for _ in range(count):
        click.echo(f"Hello, {name}!")

def main():
    cli()

argparse for simple CLIs — subparsers for subcommands, parser.add_argument("--output", "-o").

Use src/ layout. Include py.typed for type hints. importlib.resources.files() for package data access.

Parallelism

Key rule: Stay fully sync or fully async within a call path.

asyncio patterns:

• asyncio.gather(*tasks) for concurrent I/O — use return_exceptions=True for partial failure tolerance
• asyncio.Semaphore(n) to limit concurrency (rate limiting external APIs)
• asyncio.wait_for(coro, timeout=N) for timeouts
• asyncio.Queue for producer-consumer
• asyncio.Lock when coroutines share mutable state
• Never block the event loop: asyncio.to_thread(sync_fn) for sync libs, aiohttp/httpx.AsyncClient for HTTP
• Handle CancelledError — always re-raise after cleanup
• Async generators (async for) for streaming/pagination