Arguments¶

Validation with annotations¶

Argvard can use function annotations to validate and convert arguments:

from argvard import Argvard, annotations

application = Argvard()

@application.main('number')
def main(number:float):
    assert isinstance(number, float)
    print('Hooray!')

Since Python 2 doesn’t have function annotations, you can explicitly use the argvard.annotations() function decorator:

from argvard import Argvard, annotations

application = Argvard()

@application.main('number')
@annotations(number=float)
def main(number):
    ...

Any callable which returns the new value or raises ValueError on invalid input can be used instead of float.

Most builtin types should just work, but some of them are special-cased to be more liberal in input, and to provide nicer error messages:

bool: Accepts {"y", "yes", "true"} for True and {"n", "no", "false"} for False. Case-insensitive.
float, int: Same accepted values as the builtins, but nicer error messages.

If you want number to be an optional argument, you would have to write it like this:

@application.main('number')
@annotations()
def main(number:float = 1.0):
    ...

The argvard.annotations() decorator also guesses the type of variables by their default value. In the above example you wouldn’t have to specify number to be a float:

@application.main('number')
@annotations()
def main(number=1.0):
    ...

Simple validation¶

If for some reason you don’t want to or can’t use annotations, you can still do what the decorator does under the hood and raise argvard.UsageError in your functions to show a error message and exit:

from argvard import Argvard, UsageError

application = Argvard()

@application.main('number')
def main(number):
    try:
        number = float(number)
    except ValueError:
        raise UsageError('This is not a number.')