Validation

[darrenburns]

Here's an example which covers lots of the new additions (video and code below):

• The validation framework
• Using multiple validators with a widget
• Writing a custom validator (Palindrome)
• Handling validation successes and failures and updating the UI correspondingly.
• Customising error messages ("That's not a palindrome :/")
• The -valid and -invalid styles applied to Input. Note that only -invalid has styling by default. -valid has no default styling - but we attach it anyway so that users have the flexibility to change styling when validation explicitly passes. In the example below, when validation passes, the border of the Input goes green.

validation-23may23.mov

from textual import on
from textual.app import App, ComposeResult
from textual.validation import Function, Number, ValidationResult, Validator
from textual.widgets import Input, Label, Pretty


class InputApp(App):
    CSS = """
    Input.-valid {
        border: tall $success 60%;
    }
    Input.-valid:focus {
        border: tall $success;
    }
    Input {
        margin: 1 1;
    }
    Label {
        margin: 1 2;
    }
    Pretty {
        margin: 1 2;
    }
    """

    def compose(self) -> ComposeResult:
        yield Label("Enter an even number between 1 and 100 that is also a palindrome.")
        yield Input(
            placeholder="Enter a number...",
            validators=[
                Number(minimum=1, maximum=100),
                Function(is_even, "Value is not even."),
                Palindrome(),
            ],
        )
        yield Pretty([])

    @on(Input.Changed)
    def show_invalid_reasons(self, event: Input.Changed) -> None:
        # Updating the UI to show the reasons why validation failed
        if not event.validation_result:
            self.query_one(Pretty).update(event.validation_result.failure_descriptions)
        else:
            self.query_one(Pretty).update([])


def is_even(value: str) -> bool:
    try:
        return int(value) % 2 == 0
    except ValueError:
        return False


# A custom validator
class Palindrome(Validator):
    def validate(self, value: str) -> ValidationResult:
        """Check a string is equal to its reverse."""
        if self.is_palindrome(value):
            return self.success()
        else:
            return self.failure("That's not a palindrome :/")

    @staticmethod
    def is_palindrome(value: str) -> bool:
        return value == value[::-1]


app = InputApp()

if __name__ == "__main__":
    app.run()

[willmcgugan]

Just some comments. Will do a review tomorrow.

[willmcgugan]

Great work. Some comments, and maybe-issues.

[willmcgugan]

self.validator can never be None according the the type.

I gather this sets description if its None. But that means that description is still type as being | None, which it never will be from the POV of the Textual developer.

Can this logic be moved to where a Failure is constructed? Thus removing the need to type description as potentially being None ?

[darrenburns]

I'm not sure this is possible without having to duplicate this logic across all validators (meaning users writing custom validators would also have to duplicate this logic). I think things will become much messier and the trade-off isn't worth it unless you can suggest another approach.

Is the description being None bad? Maybe as a developer who wants a custom Validator, I just have no need to describe my validation failure because I won't be using the description anyway.

[willmcgugan]

Some requests re error messages

[willmcgugan]

LGTM