ROS 2 python type support and rclpy interaction

ros_python_typesupport.md

Doing import std_msgs.msg.String or from std_msgs.msg import String imports a generated Python class.

Type support

The typesupport for the String type exists on a metaclass: String.__class__._TYPE_SUPPORT. This value starts as None. It later becomes a pycapsule (with no name) when someone calls String.__class__.__import_type_support__(). This is done in rclpy by check_for_type_support(msg_type). It's called by the Node in create_...(). Afterwards inside C code the function rclpy_common_get_type_support(pymsg_type). blindly assumes _TYPE_SUPPORT is a pycapsule. The pointer from that capsule is passed to rcl_thing_init().

Deserializing

Rclpy can deserialize a ROS message to a Python instance using rclpy_deserialize(). It does this in these steps:

1. Get the type support for the message type being deserialized
2. Create some kind of instance of the message type.
   • It's a void * returned by a function stored in a pycapsule String.__class__._CREATE_ROS_MESSAGE.
   • It's the C type support for the message. The python type support converts to/from the C type support
3. Call rmw_deserialize() to populate that message.
4. Convert the C message type to a python type
   • It calls a function in a pycapsule String.__class__._CONVERT_FROM_PY to convert from void * to PyObject *.

Serializing

rclpy can serialize a Python message to bytes using rclpy_serialize(). It does so using these steps:

1. Get the type support of the message type being serialized.
2. Converts the Python message to a C message.
3. Creates a serialized message object to hold the serialized data.
4. Calls rmw_serialzie() to serialize the C message.