How to retrieve session data with Flask?

I have flask+wtforms application. I can see in login() user object stored as

  if user:
   if user.verify_password(form.password.data):
    flash('You have been logged in')
    user.logins += 1

    db.session.add(History(user.uid))
    db.session.commit()

    session['user'] = user

Now I wanted to retrieve the user

if 'user' in session:
       User=session.get('user')
       print User.nickname ###<< how to retrieve specific object member?

It fails with message like :

Instance <User at 0x8e5a64c> is not bound to a Session; attribute refresh operation cannot proceed