code: def get_user(user_or_username):
      
      user = None
      if isinstance(user_or_username, MemberData):
          user = user_or_username
      if isinstance(user_or_username, basestring):
          user = get_member_by_login_name(get_portal(), user_or_username, False)
      return user
docs: Return Plone User
  
      :param user_or_username: Plone user or user id
      :returns: Plone MemberData