# vtk-9.2.5 python wrapper vtkProp3DCollection.GetNextProp3D() return null
**URL:** https://discourse.vtk.org/t/vtk-9-2-5-python-wrapper-vtkprop3dcollection-getnextprop3d-return-null/10504
**Category:** Support
**Tags:** training, python, code
**Created:** [January 19, 2023, 2:37pm UTC](https://discourse.vtk.org/t/vtk-9-2-5-python-wrapper-vtkprop3dcollection-getnextprop3d-return-null/10504 "2023-01-19T14:37:07Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@TartaPise](https://discourse.vtk.org/u/TartaPise)
#### Post date: [January 19, 2023, 2:37pm UTC](https://discourse.vtk.org/t/vtk-9-2-5-python-wrapper-vtkprop3dcollection-getnextprop3d-return-null/10504/1 "2023-01-19T14:37:07Z")
Hello everyone, I’m new to the Forum, please treat me well and let me know if I do something wrong.
I’m having issues writing a simple python function to save the centers of the actors in a vtkassembly.
Here is the function:
def get\_spheres\_centers(assembly):
print(assembly)
collection=vtk.vtkProp3DCollection()
assembly.GetActors(collection)
print(collection)
```
for i in range(collection.GetNumberOfItems()):
actor=collection.GetNextProp3D()
print(actor)
centers[i-1,:]=actor.GetCenter()
return centers
```
GetNextProp3D() always returns NULL, i’ve tried using
actor=collection.GetLastProp3D()
and this work but of course always return the last actor of the collection. Any idea why?
---
### Author:  [@TartaPise](https://discourse.vtk.org/u/TartaPise)
#### Post date: [January 19, 2023, 2:42pm UTC](https://discourse.vtk.org/t/vtk-9-2-5-python-wrapper-vtkprop3dcollection-getnextprop3d-return-null/10504/2 "2023-01-19T14:42:01Z")
I’m so dumb sorry. I just needed to initialize the position inside the collection. I’ll leave here the solution just in case anyone else look for this:
def get\_spheres\_centers(assembly):
collection=vtk.vtkProp3DCollection()
assembly.GetActors(collection)
collection.InitTraversal()
for i in range(collection.GetNumberOfItems()):
actor=collection.GetNextProp3D()
centers[i-1,:]=actor.GetCenter()
return centers
---
### Author:  [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [January 19, 2023, 2:51pm UTC](https://discourse.vtk.org/t/vtk-9-2-5-python-wrapper-vtkprop3dcollection-getnextprop3d-return-null/10504/3 "2023-01-19T14:51:21Z")
Even the VTK developers themselves have trouble with the “Collection” classes.
From Python, the easiest way to use them is like this:
```
for i in range(collection.GetNumberOfItems()):
actor = collection.GetItemAsObject(i)
```
---
### Author:  [@TartaPise](https://discourse.vtk.org/u/TartaPise)
#### Post date: [January 19, 2023, 2:57pm UTC](https://discourse.vtk.org/t/vtk-9-2-5-python-wrapper-vtkprop3dcollection-getnextprop3d-return-null/10504/4 "2023-01-19T14:57:40Z")
Thank you mate. My next issue is that changing the pose of the vtkassembly using RotateX() ecc does not change the center of each single actor, any idea why?
edit: I just found another post with the same issue with no answer here [https://vtk.org/pipermail/vtkusers/2018-March/101327.html](https://vtk.org/pipermail/vtkusers/2018-March/101327.html)
---
### Author:  [@dgobbi](https://discourse.vtk.org/u/dgobbi)
#### Post date: [January 19, 2023, 3:02pm UTC](https://discourse.vtk.org/t/vtk-9-2-5-python-wrapper-vtkprop3dcollection-getnextprop3d-return-null/10504/5 "2023-01-19T15:02:13Z")
> My next issue is that changing the pose of the vtkassembly using RotateX() ecc does not change the center of each single actor
@will.schroeder might know the answer to that one.
---
### Author:  [@ben.boeckel](https://discourse.vtk.org/u/ben.boeckel)
#### Post date: [January 19, 2023, 4:52pm UTC](https://discourse.vtk.org/t/vtk-9-2-5-python-wrapper-vtkprop3dcollection-getnextprop3d-return-null/10504/6 "2023-01-19T16:52:57Z")
> [@dgobbi](#):
>
> Even the VTK developers themselves have trouble with the “Collection” classes.
Yes, the stateful iteration is annoying (basically, a call to `InitTraversal()` is required before `GetNext*` is “useful”). Also note that any collection can contain any object because of the `vtkCollection::AddItem(vtkObject*)` method.