code: def get_device_id(handler_input):
      # type: (HandlerInput) -> Optional[AnyStr]
      
      device = handler_input.request_envelope.context.system.device
      if device:
          return device.device_id
      else:
          return None
docs: Return the device id from the input request.
  
      The method retrieves the `deviceId` property from the input request.
      This value uniquely identifies the device and is generally used as
      input for some Alexa-specific API calls. More information about this
      can be found here :
      https://developer.amazon.com/docs/custom-skills/request-and-response-json-reference.html#system-object
  
      If there is no device information in the input request, then a ``None``
      is returned.
  
      :param handler_input: The handler input instance that is generally
          passed in the sdk's request and exception components
      :type handler_input: ask_sdk_core.handler_input.HandlerInput
      :return: Unique device id of the device used to send the alexa request
          or `None` if device information is not present
      :rtype: Optional[str]