object oriented – Python Back-end registration validation

I am working on a personal project (Flask CRUD app) and I am currently building the user service. I am trying to use as less as libraries as possible (that’s why I do not use WTF-forms for example, just for learning purposes). I am really not sure about a couple of things.

I am aware that form validation should be handled in both front and back-end (or at the very least on the back-end). In order to validate a new user, I am (currently) checking 3 things:

1) email does not already exist on the db (can only be verified on the back-end)

2) password is strong enough (can be done in both front and back end)

3) password and password_confirm matches (can be done in both front and back end)

My goal is to display a small error message on the registration page if either at least one those errors appears. Currently, I am checking option 1, 2 and 3 and the back-end but only return True if the registration is valid otherwise, False.

Since case 1 and 2 can directly be handle also on the front-end, can I display the error message using JavaScript and not from the back-end? I am still confused on what would be the best return for validate_registration (currently a Boolean).

Also, I am not quite sure my code is over-engineered, I made a RegistrationForm class just to validate a registration form. I did that because I am able to pick validations methods from my validators.py which may be used in other Classes as well.

So this is what I did so for for the registration form:

user/blueprints/routes.py

@user.route('/new-user',methods = ('POST'))
def register_user():
    form_email = request.form.get('email')
    form_password = request.form.get('psw')
    form_password_repeat = request.form.get('psw-repeat')
    registration_form = RegistrationForm(form_email, form_password, form_password_repeat).validate_registration()
    
    if registration_form:
        new_user = UserService().register_user(form_email, form_password)
        user_repository = UserRepository(conn, 'users')
        user_repository.add_user(new_user)
        user_repository.save()
        return "ok" #will probably change return statements later on
    return "not ok" #will probably change return statements later on

user/blueprints/forms.py

from prepsmarter.blueprints.user.validators import email_already_in_use, password_matching

class RegistrationForm():
    def __init__(self, email, pwd_1, pwd_2):
        self.email = email
        self.pwd_1 = pwd_1
        self.pwd_2 = pwd_2

    def validate_registration(self):
        is_valid = email_already_in_use(self.email) and is_strong_password(self.pwd_1) and password_matching(self.pwd_1, self.pwd_2)
        return is_valid

class LoginForm():
 # to do

user/blueprints/validators.py

import re 
from email_validator import validate_email, EmailNotValidError
from prepsmarter.extensions import conn

def is_strong_password(password):
    length_error = len(password) < 8
    digit_error = re.search(r"d", password) is None
    uppercase_error = re.search(r"(A-Z)", password) is None
    return length_error and digit_error and uppercase_error
    
def is_email_formated_correctly(email):
    is_correct = True
    try:
        validate_email(email)
    except EmailNotValidError:
        is_correct = False
    return is_correct

def password_matching(pwd_1, pwd_2):
    return pwd_1 == pwd_2

    
def email_already_in_use(email):
    sql = "SELECT CASE WHEN EXISTS ( SELECT * FROM users WHERE email = (%s)) THEN 1 ELSE 0 END;"
    cursor = conn.cursor()
    cursor.execute(sql, (email))
    res = cursor.fetchall()
    return res == 1