What are the best practices for WTForms validation?

What are the best practices for WTForms validation?

When using WTForms, it's important to ensure that your validation logic is clear and maintainable. Here are some best practices:

1. Use built-in validators whenever possible.
2. Create custom validators for complex rules.
3. Keep validation logic separate from business logic.
4. Provide user-friendly error messages.
5. Validate data on both client and server sides.

Can you give an example of a custom validator?

Sure! Here's a simple example of a custom validator that checks if a field contains only alphabetic characters:

from wtforms import ValidationError

class Alphabetic(object):
    def __call__(self, form, field):
        if not field.data.isalpha():
            raise ValidationError('Field must contain only alphabetic characters.')

How do I implement this custom validator in a form?

You can implement the custom validator in your form like this:

from wtforms import Form, StringField

class MyForm(Form):
    name = StringField('Name', [Alphabetic()])

What should I do if a validation fails?

If validation fails, you should display the error messages to the user. WTForms handles this automatically if you render the form in your template. You can access the errors using form.name.errors for the 'name' field.