CLI Arguments with Default

We can also use the same typer.Argument() to set a default value.

That way the CLI argument will be optional and also have a default value.

An optional CLI argument with a default

We can also use typer.Argument() to make a CLI argument have a default value other than None:

Python 3.6+Python 3.6+ non-Annotated

import typer
from typing_extensions import Annotated


def main(name: Annotated[str, typer.Argument()] = "Wade Wilson"):
    print(f"Hello {name}")


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

Tip

Prefer to use the Annotated version if possible.

import typer


def main(name: str = typer.Argument("Wade Wilson")):
    print(f"Hello {name}")


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

Tip

Because now the value will be a str passed by the user or the default value of "Wade Wilson" which is also a str, we know the value will never be None, so we don't have to (and shouldn't) use Optional[str].

Have in mind that the Optional[something] tells Python that a value "could be None". But the use of Optional doesn't affect Typer in any way, e.g. it doesn't tell Typer if a value is required or not.

Check it:

// Check the help
$ python main.py --help

// Notice the [default: Wade Wilson] ✨
Usage: main.py [OPTIONS] [NAME]

Arguments:
  [NAME]  [default: Wade Wilson]

Options:
  --help                Show this message and exit.

// With no optional CLI argument
$ python main.py

Hello Wade Wilson

// With one CLI argument
$ python main.py Camila

Hello Camila

Dynamic default value

And we can even make the default value be dynamically generated by passing a function as the default_factory argument:

Python 3.6+Python 3.6+ non-Annotated

import random

import typer
from typing_extensions import Annotated


def get_name():
    return random.choice(["Deadpool", "Rick", "Morty", "Hiro"])


def main(name: Annotated[str, typer.Argument(default_factory=get_name)]):
    print(f"Hello {name}")


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

Tip

Prefer to use the Annotated version if possible.

import random

import typer


def get_name():
    return random.choice(["Deadpool", "Rick", "Morty", "Hiro"])


def main(name: str = typer.Argument(default_factory=get_name)):
    print(f"Hello {name}")


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

In this case, we created the function get_name that will just return a random str each time.

And we pass it as the first function argument to typer.Argument().

Tip

The word "factory" in default_factory is just a fancy way of saying "function that will create the default value".

Check it:

// Check the help
$ python main.py --help

Usage: main.py [OPTIONS] [NAME]

Arguments:
  [NAME]  [default: (dynamic)]

Options:
  --help                Show this message and exit.

// Try it several times, it will use a random default each time
$ python main.py

Hello Deadpool

$ python main.py

Hello Hiro

$ python main.py

Hello Rick

// Now pass a value for the CLI argument
$ python main.py Camila

Hello Camila