Python

import bcrypt
import getpass
from bcrypt import hashpw, gensalt
from sqlalchemy import *
from sqlalchemy.orm import*
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import exists


from login_table import user, Base


engine = create_engine('sqlite:///sqlalchemy_login.db')
Base.metadata.bind = engine

DBSession = sessionmaker(bind=engine)
session = DBSession()



username_attempt = raw_input('Enter username:')
username_attempt = unicode(username_attempt)
#password_attempt = raw_input('Enter password:')

exist = session.query(exists().where(user.username == username_attempt)).scalar()


while exist == False :
	print 'Incorrect username'
	username_attempt = raw_input('Enter username:')
	username_attempt = unicode(username_attempt)
	exist = session.query(exists().where(user.username == username_attempt)).scalar()