How can I implement advanced validation in Flask-WTF forms?

How can I implement advanced validation in Flask-WTF forms?

You can implement advanced validation in Flask-WTF by creating custom validators. Would you like an example?

Yes, please provide an example of a custom validator.

Sure! You can define a custom validator like this:

from wtforms import ValidationError

def validate_username(form, field):
    if not field.data.isalnum():
        raise ValidationError('Username must be alphanumeric.')

Then, you can use it in your form field.

How do I attach this validator to a form field?

You can attach it by using the validators parameter in your form field definition:

username = StringField('Username', validators=[DataRequired(), validate_username])