# Flask-Security - How to extend a form and its validator

I am currently building a Python application, which let me use:

• Flask - a simple and very well documented web Framework
• Flask-Security - an extension dedicated to user management.
While it is a really great extension, there is some documentation missing or quite hard to find on search engines or StackOverflow.

### How to add fields to a Flask’s security form, with a custom validator.

First, we need to declare a new class, which extends RegisterForm, then add the new fields we want on the form. If we want a custom validator for that field, we need to define it, but, to avoid losing Flask-Security validator, just call it with Form.validate(self) The last thing needed is to tell Flask-Security to use our form when instantiating it.

Instantiation

security = Security(app, user_datastore, register_form=ExtendedRegisterForm)


Form.py

from flask.ext.wtf import Form
from wtforms import StringField, BooleanField
from wtforms.validators import DataRequired
from .models import User

class ExtendedRegisterForm(RegisterForm):
""" Add nickname field to the register's class
"""
nickname = StringField('Nickname', [DataRequired()])

def validate(self):

:return: True is the form is valid
"""
# Use standard validator
validation = Form.validate(self)
if not validation:
return False

# Check if nickname already exists
user = User.query.filter_by(
nickname=self.nickname.data).first()
if user is not None:
# Text displayed to the user