code: def get_app_hostname():
    
    if not is_running_on_app_engine() or is_running_on_localhost():
      return None
  
    app_id = app_identity.get_application_id()
  
    prefix = get_hostname_prefix()
    suffix = 'appspot.com'
  
    if ':' in app_id:
      tokens = app_id.split(':')
      api_name = tokens[1]
      if tokens[0] == 'google.com':
        suffix = 'googleplex.com'
    else:
      api_name = app_id
  
    return '{0}{1}.{2}'.format(prefix, api_name, suffix)
docs: Return hostname of a running Endpoints service.
  
    Returns hostname of an running Endpoints API. It can be 1) "localhost:PORT"
    if running on development server, or 2) "app_id.appspot.com" if running on
    external app engine prod, or "app_id.googleplex.com" if running as Google
    first-party Endpoints API, or 4) None if not running on App Engine
    (e.g. Tornado Endpoints API).
  
    Returns:
      A string representing the hostname of the service.