How can I create custom validators in Flask?

How can I create custom validators in Flask?

You can create custom validators in Flask by defining a function that checks the input and raising a validation error if needed.

Can you show me an example of a custom validator?

Sure! Here's a simple example:

from flask_wtf import FlaskForm
from wtforms import StringField, ValidationError

class MyForm(FlaskForm):
    name = StringField('Name')

    def validate_name(form, field):
        if len(field.data) < 3:
            raise ValidationError('Name must be at least 3 characters long.')

What if I want to validate an email format?

You can use regex for that! Here's an example:

import re

def validate_email(form, field):
    if not re.match(r'^[\w\.-]+@[\w\.-]+\.\w+$', field.data):
        raise ValidationError('Invalid email format.')

Can I use multiple validators on a single field?

Yes, you can chain multiple validators by adding them to the field definition. For example:

email = StringField('Email', validators=[DataRequired(), validate_email])