code: def register():
      
      if request.method == "POST":
          username = request.form["username"]
          password = request.form["password"]
          error = None
  
          if not username:
              error = "Username is required."
          elif not password:
              error = "Password is required."
          elif db.session.query(
              User.query.filter_by(username=username).exists()
          ).scalar():
              error = f"User {username} is already registered."
  
          if error is None:
              # the name is available, create the user and go to the login page
              db.session.add(User(username=username, password=password))
              db.session.commit()
              return redirect(url_for("auth.login"))
  
          flash(error)
  
      return render_template("auth/register.html")
docs: Register a new user.
  
      Validates that the username is not already taken. Hashes the
      password for security.