Layered architecture.py

#pseudo code
#option 1

class User:
  screen_name = ''
  email = ''

class UserManager:
  def validate(user):
    if user.screen_name == '':
      raise CustomException(100,'Screen name can not be empty')

#in the GUI
user(screen_name='',email='')
try:
  user_manager.insert(user)
except CustomException,e:
  display_error_message(e.code)

#option 2
class User:
  screen_name = StringProperty(required=True,allow_empty=False)
  email = EmailProperty()

#in GUI
try:
 user(screen_name='',email='you@example')
except TypeException,e:
  display_error_message(e.message) #string can not be empty