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:
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:
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