Thank’s for helping. Actually; I did’nt even think about creating and using the constraint node, since i was focused on using the openmaya objects.
So i’ll have a look on it and first see how to properly create the node and query the value.
But i’d still like to achieve it throught openmaya if possible 
EDIT : after a few experimentations, you are right, this is definitly related to a world/object coordinate problem, in both situation (openmaya or node).
So I’ll try to play with matrix to correct the problem. I’m not very used to it but OM seems to have great objects and methods to apply and modify thoses.
I think doing it whole in OM would be cleaner.