Optional value for CLI Options¶

As in Click, providing a value to a CLI option can be made optional, in which case a default value will be used instead.

To make a CLI option's value optional, you can annotate it as a Union of types bool and the parameter type.

Info

You can create a type Union by importing Union from the typing module.

For example Union[bool, str] represents a type that is either a boolean or a string.

You can also use the equivalent notation bool | str

Let's add a CLI option --tone with optional value:

Python 3.8+

import typer
from typing_extensions import Annotated


def main(
    name: str, lastname: str, greeting: Annotated[bool | str, typer.Option()] = "formal"
):
    if not greeting:
        return

    if greeting == "formal":
        print(f"Hello {name} {lastname}")

    elif greeting == "casual":
        print(f"Hi {name} !")

    else:
        raise ValueError(f"Invalid greeting '{greeting}'")


if __name__ == "__main__":
    typer.run(main)

🤓 Other versions and variants

Python 3.8+ - non-Annotated

Tip

Prefer to use the Annotated version if possible.

import typer


def main(name: str, lastname: str, greeting: bool | str = "formal"):
    if not greeting:
        return

    if greeting == "formal":
        print(f"Hello {name} {lastname}")

    elif greeting == "casual":
        print(f"Hi {name} !")

    else:
        raise ValueError(f"Invalid greeting '{greeting}'")


if __name__ == "__main__":
    typer.run(main)

Now, there are three possible configurations:

--greeting is not used, the parameter will receive a value of False.
```
python main.py
```
--greeting is supplied with a value, the parameter will receive the string representation of that value.
```
python main.py --greeting <value>
```
--greeting is used with no value, the parameter will receive the default formal value.
```
python main.py --greeting
```

And test it:

$ python main.py Camila Gutiérrez

// We didn't pass the greeting CLI option, we get no greeting


// Now update it to pass the --greeting CLI option with default value
$ python main.py Camila Gutiérrez --greeting

Hello Camila Gutiérrez

// The above is equivalent to passing the --greeting CLI option with value `formal`
$ python main.py Camila Gutiérrez --greeting formal

Hello Camila Gutiérrez

// But you can select another value
$ python main.py Camila Gutiérrez --greeting casual

Hi Camila !