Karamba Data manupilation with python component

I made a vierendeel truss and I want to extract the vertical beams (''Posts'') from the list of meshes that can be found in de BeamView output.

In the Element(karamba) component a list that consist out of classes can be found. The list in this component defines which elements are ''Posts''.

In the Python component i made loop that filters out the meshes that are ''Posts''. But this is not working since the list in the Element(karamba) component consist out of ''GrassBeam'' objects and is not a list in a list.

Can someone help me with defining the right code in order to sort out the ''Posts'' Meshes from the list of meshes.

The grasshoppercode can be found in de appendix.