Can't create classes inheriting from node without explicitly importing rclpy.node.Node

[mikaelarguedas]

To settle on before tagging the release.

Since #147, all classes inheriting from rclpy.Node need to explicitly import rclpy.node.Node (see ros2/demos#196).

It would be great to allow users to use that class without having to explicitly import it as it was the case before.

@dirk-thomas could you provide more information about the need of delaying importing the singleton and the possible alternatives to restore previous usability?

[dirk-thomas]

could you provide more information about the need of delaying importing the singleton and the possible alternatives to restore previous usability?

The import of the singleton needs to be delayed in order to allow to override the logic where it should be loaded from.

The Python module rclpy.node would need to be importable without importing rclpy.impl.implementation_singleton. Then it could be imported in the rclpy/__init__.py to make it accessible as before.

[dhood]

As a note: rclpy.logging is not being imported automatically either (it could be imported manually but it also imports a C extension and FWIU that's to be avoided): ros2/demos#210