Optional CLI Arguments

We said before that by default:

• CLI options are optional
• CLI arguments are required

Again, that's how they work by default, and that's the convention in many CLI programs and systems.

But you can change that.

In fact, it's very common to have optional CLI arguments, it's way more common than having required CLI options.

As an example of how it could be useful, let's see how the ls CLI program works.

// If you just type
$ ls

// ls will "list" the files and directories in the current directory
typer  tests  README.md  LICENSE

// But it also receives an optional CLI argument
$ ls ./tests/

// And then ls will list the files and directories inside of that directory from the CLI argument
__init__.py  test_tutorial

An alternative CLI argument declaration

In the First Steps you saw how to add a CLI argument:

import typer


def main(name: str):
    print(f"Hello {name}")


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

Now let's see an alternative way to create the same CLI argument:

import typer
from typing_extensions import Annotated


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


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

Info

Typer added support for Annotated (and started recommending it) in version 0.9.0.

If you have an older version, you would get errors when trying to use Annotated.

Make sure you Upgrade the Typer version to at least 0.9.0 before using Annotated.

Before, you had this function parameter:

name: str

And now we wrap it with Annotated:

name: Annotated[str]

Both of these versions mean the same thing, Annotated is part of standard Python and is there for this.

But the second version using Annotated allows us to pass additional metadata that can be used by Typer:

name: Annotated[str, typer.Argument()]

Now we are being explicit that name is a CLI argument. It's still a str and it's still required (it doesn't have a default value).

All we did there achieves the same thing as before, a required CLI argument:

$ python main.py

Usage: main.py [OPTIONS] NAME
Try "main.py --help" for help.

Error: Missing argument 'NAME'.

It's still not very useful, but it works correctly.

And being able to declare a required CLI argument using

name: Annoated[str, typer.Argument()]

...that works exactly the same as

name: str

...will come handy later.

Make an optional CLI argument

Now, finally what we came for, an optional CLI argument.

To make a CLI argument optional, use typer.Argument() and pass a different "default" as the first parameter to typer.Argument(), for example None:

from typing import Optional

import typer
from typing_extensions import Annotated


def main(name: Annotated[Optional[str], typer.Argument()] = None):
    if name is None:
        print("Hello World!")
    else:
        print(f"Hello {name}")


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

Now we have:

name: Annotated[Optional[str], typer.Argument()] = None

Because we are using typer.Argument() Typer will know that this is a CLI argument (no matter if required or optional).

Tip

By using Optional your editor will be able to know that the value could be None, and will be able to warn you if you do something assuming it is a str that would break if it was None.

Check the help:

// First check the help
$ python main.py --help

Usage: main.py [OPTIONS] [NAME]

Arguments:
  [NAME]

Options:
  --help                Show this message and exit.

Tip

Notice that NAME is still a CLI argument, it's shown up there in the "Usage: main.py ...".

Also notice that now [NAME] has brackets ("[" and "]") around (before it was just NAME) to denote that it's optional, not required.

Now run it and test it:

// With no CLI argument
$ python main.py

Hello World!

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

Hello Camila

Tip

Notice that "Camila" here is an optional CLI argument, not a CLI option, because we didn't use something like "--name Camila", we just passed "Camila" directly to the program.

Alternative (old) typer.Argument() as the default value

Typer also supports another older alternative syntax for declaring CLI arguments with additional metadata.

Instead of using Annotated, you can use typer.Argument() as the default value:

import typer


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


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

Tip

Prefer to use the Annotated version if possible.

Before, because name didn't have any default value it would be a required parameter for the Python function, in Python terms.

When using typer.Argument() as the default value Typer does the same and makes it a required CLI argument.

We changed it to:

name: str = typer.Argument()

But now as typer.Argument() is the "default value" of the function's parameter, it would mean that "it is no longer required" (in Python terms).

As we no longer have the Python function default value (or its absence) to tell if something is required or not and what is the default value, typer.Argument() receives a first parameter default that serves the same purpose of defining that default value, or making it required.

Not passing any value to the default argument is the same as marking it as required. But you can also explicitly mark it as required by passing ... as the default argument, passed to typer.Argument(default=...).

name: str = typer.Argument(default=...)

Info

If you hadn't seen that ... before: it is a special single value, it is part of Python and is called "Ellipsis".

import typer


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


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

And the same way, you can make it optional by passing a different default value, for example None:

from typing import Optional

import typer


def main(name: Optional[str] = typer.Argument(default=None)):
    if name is None:
        print("Hello World!")
    else:
        print(f"Hello {name}")


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

Because the first parameter passed to typer.Argument(default=None) (the new "default" value) is None, Typer knows that this is an optional CLI argument, if no value is provided when calling it in the command line, it will have that default value of None.

The default argument is the first one, so it's possible that you see code that passes the value without explicitly using default=, like:

name: str = typer.Argument(...)

...or like:

name: str = typer.Argument(None)

...but again, try to use Annotated if possible, that way your code in terms of Python will mean the same thing as with Typer and you won't have to remember any of these details.