# From argparse

URL: /milo-cli/docs/get-started/migrate-existing-cli/from-argparse/
Section: migrate-existing-cli
Tags: migration, argparse, cli

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argparse is parser-first: you create an ArgumentParser, attach arguments, parse sys.argv, then call your own function. Milo is handler-first: write the function contract once, then let Milo derive argv parsing, MCP schema, and llms.txt. Official reference: argparse documentation. Before import argparse def deploy(environment: str, service: str, version: str) -> dict[str, str]: return {"environment": environment, "service": service, "version": version} parser = argparse.ArgumentParser(prog="deployer") parser.add_argument("environment") parser.add_argument("service") parser.add_argument("--version", default="latest") args = parser.parse_args() print(deploy(args.environment, args.service, args.version)) After from milo import CLI cli = CLI(name="deployer", description="Deploy services") @cli.command("deploy", description="Deploy a service") def deploy(environment: str, service: str, version: str = "latest") -> dict[str, str]: """Deploy a service. Args: environment: Target environment. service: Service name. version: Version or image tag. """ return {"environment": environment, "service": service, "version": version} if __name__ == "__main__": cli.run() Run it: uv run python app.py deploy --environment staging --service api uv run python app.py deploy --environment staging --service api --format json uv run python app.py --llms-txt uv run milo verify app.py Mapping argparse concept Milo equivalent ArgumentParser(prog=..., description=...) CLI(name=..., description=...) add_argument("name") Required typed parameter: name: str add_argument("--name", default="x") Defaulted parameter: name: str = "x" type=int Annotation: count: int choices=[...] Literal[...] or Enum parser.parse_args() cli.run() for processes, cli.invoke([...]) for tests Namespace Function parameters What To Watch Positional arguments become named CLI options by default in Milo command handlers. That makes MCP calls and repair loops clearer. print() output is process output. For agent-facing commands, prefer structured returns and Context output helpers. Parser errors become structured Milo/MCP diagnostics when the command is called through tools/call.

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- Author: lbliii
- Word Count: 249
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