Typer — Build Python CLIs from Type Hints

What it is

Typer is a Python library that turns regular Python functions into command-line interface commands using only type hints. Built on top of Click, Typer removes the boilerplate of argument parsing, help text generation, and shell completion setup.

Typer is created by the same developer behind FastAPI. It follows the same philosophy: use standard Python type annotations to define the interface, and let the framework handle the rest.

How it saves time or tokens

Traditional CLI libraries like argparse require verbose argument definitions separate from the function logic. Typer eliminates this duplication. The function signature is the CLI definition. A single function with type-annotated parameters becomes a complete CLI command with help text, validation, and tab completion.

For AI agents generating CLI tools, Typer is the shortest path from a Python function to a working command. The type-hint-driven approach means LLMs can produce correct CLI code with minimal context.

How to use

1. Install Typer:

pip install typer

1. Write a function with type hints:

import typer

def greet(name: str, count: int = 1, formal: bool = False):
    greeting = 'Good day' if formal else 'Hello'
    for _ in range(count):
        print(f'{greeting}, {name}')

if __name__ == '__main__':
    typer.run(greet)

1. Run it from the terminal:

python greet.py --name World --count 3 --formal

Typer auto-generates --help, validates types, and provides shell completion.

Example

import typer

app = typer.Typer()

@app.command()
def deploy(env: str = typer.Argument(..., help='Target environment'),
           dry_run: bool = typer.Option(False, help='Preview without applying')):
    if dry_run:
        print(f'Would deploy to {env}')
    else:
        print(f'Deploying to {env}...')

@app.command()
def rollback(env: str, steps: int = 1):
    print(f'Rolling back {steps} step(s) in {env}')

if __name__ == '__main__':
    app()

This creates a CLI with deploy and rollback subcommands, each with typed arguments and auto-generated help.

Related on TokRepo

• AI Tools for Coding — AI tools that generate CLI applications
• AI Tools for Automation — Automation tools that pair with CLI workflows

Common pitfalls

• Forgetting that Typer requires Python 3.7+ for proper type hint support. Older Python versions will not work correctly with the annotation-driven approach.
• Not installing the shellingham and click-completion extras for shell completion. Run pip install 'typer[all]' to get the full feature set.
• Using mutable default arguments (like lists or dicts) as parameter defaults. Use typer.Option(default_factory=list) instead to avoid shared state bugs.
• Failing to review community discussions and changelogs before upgrading. Breaking changes in major versions can disrupt existing workflows. Pin versions in production and test upgrades in staging first.