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mathieu.goulet-aubin

July 28, 2026 15:41:43

Is it illegal to create in-memory Usd.Stage or anonymous Sdf.Layer from inside a Python Lop node?
I am looking to know what should be avoided inside a Python Lop node.

Using a snippet similar to this one below from a Python Lop node, everything looks good from inside Houdini.
But once I try to render through Husk, some un-explicable things happen.

from pxr import Sdf, Usd


base_layer = Sdf.Layer.FindOrOpen('file.usd')
top_layer = Sdf.Layer.CreateAnonymous()
#
# Do some edits on the top_layer using the Sdf api.
#

memory_stage = Usd.Stage.CreateInMemory()
root_layer = memory_stage.GetRootLayer()
root_layer.subLayerPaths.append(base_layer.identifier)
root_layer.subLayerPaths.append(top_layer.identifier)
root_layer.defaultPrim = base_layer.defaultPrim


node = hou.pwd()
stage = node.editableStage()
prim = stage.DefinePrim('/my/prim', 'Xform')
prim.GetReferences().AddReference(root_layer.identifier)

Opening the usd file exported by Husk and inspecting the composition of an affected prim in houdini, i see some sublayers identified as "<unknown name>".

Looking forward to get some insights on this.
Thank you in advance!

mathieu.goulet-aubin

July 29, 2026 21:32:41

I was able to find an answer by looking at the hou.LopNode.addHeldLayer() documentation.
The important part is using loputils.createPythonLayer() instead of Sdf.Layer.CreateAnonymous().

From what I can understand it does 2 things:
- It adds some metadata to the layer that helps Houdini know what to do with it on export.
- It prevents the Sdf.Layer to be deleted by USD too early. It does so under the hood by linking it to the hou.LopNode using the hou.LopNode.addHeldLayer() method.

Here is the updated example.
As a bonus of these changes, staying within the Sdf api cooks quicker for my real use case.

import hou
import loputils
from pxr import Sdf


lop_node = hou.pwd()
editable_layer = lop_node.editableLayer()

# Open/Create Layers
base_layer = Sdf.Layer.FindOrOpen('file.usd')
top_layer = loputils.createPythonLayer(lop_node)
#
# Do some edits on the top_layer using the Sdf api.
#

# Merge Layers
root_layer = loputils.createPythonLayer(lop_node)
root_layer.subLayerPaths.append(base_layer.identifier)
root_layer.subLayerPaths.append(top_layer.identifier)
root_layer.defaultPrim = base_layer.defaultPrim

# Define Prim
prim_spec = Sdf.CreatePrimInLayer(editable_layer, '/my/prim')
prim_spec.specifier = Sdf.SpecifierDef
prim_spec.typeName = "Xform"

# Add Reference
reference = Sdf.Reference(root_layer.identifier)
prim_spec.referenceList.appendedItems = [reference]

mathieu.goulet-aubin

July 29, 2026 21:43:24

As a side note, does Usd.Stage.CreateInMemory() has a similar helper function?

Sdf.Layer.CreateAnonymous() -> loputils.createPythonLayer()
Usd.Stage.CreateInMemory() -> ???