wtform_fields.py

from flask_wtf import FlaskForm
from wtforms import StringField,PasswordField,SubmitField
from wtforms.validators import InputRequired, Length, EqualTo,ValidationError
from models import User


def invalid_credentials(form, field):
    """ Username and password checker """

    password_entered = field.data
    username_entered = form.username.data

    # Check username is invalid
    user_object = User.query.filter_by(username=username_entered).first()
    if user_object is None:
        raise ValidationError("Username or password is incorrect")

    # Check password in invalid
    elif password_entered != user_object.password:
        raise ValidationError("Username or password is incorrect")

class RegistrationForm(FlaskForm):
    """ Registration form"""

    username = StringField('username', validators=[InputRequired(message="Username required"), Length(min=4, max=25, message="Username must be between 4 and 25 characters")])
    password = PasswordField('password', validators=[InputRequired(message="Password required"), Length(min=4, max=25, message="Password must be between 4 and 25 characters")])
    confirm_pswd = PasswordField('confirm_pswd', validators=[InputRequired(message="Password required"), EqualTo('password', message="Passwords must match")])

    submit_button = SubmitField('Create')

    def validate_username(self,username):
        user_object = User.query.filter_by(username=username.data).first()
        if user_object:
            raise ValidationError("Username already taken")

class LoginForm(FlaskForm):
    """ Login form """

    username = StringField('username', validators=[InputRequired(message="Username required")])
    password = PasswordField('password', validators=[InputRequired(message="Password required"), invalid_credentials])

    submit_button = SubmitField('Login')