TerryHancock, 9 months ago

@CWernerArt
I'm not sure and I'm not testing this right now, but IIRC, bpy collection data types can behave a little oddly for Python programmer expectations.

You might want to check if those data types actually return "false" when they are empty.

If not, you might need to use len() to see if they are 0-length, instead.

CWernerArt, 9 months ago

@TerryHancock You mean like this?

if len(reference_mesh.edges) == 0 and len(reference_mesh.polygons) == 0:

This is also not working. I tried that already.

CWernerArt, 9 months ago

@TerryHancock Maybe it helps if I add: when using empty plane objects for example, then all works fine.

But if you use an empty cube (without faces just wireframe), then it doesn't work.

TerryHancock, 9 months ago

@CWernerArt
Yeah, I just tried deleting the faces from the default cube, and it still reports 6 polygons in the mesh.

I'm not sure why that is.

Maybe "face" and "polygon" are not actually the same thing?

I see that the mesh type doesn't have a "faces" element. But "polygons" doesn't seem to respect whether there are actually faces defined or not.

CWernerArt, 9 months ago

@TerryHancock Yeah. Actually I also thought we can check for polygons by using the .polygons suffix.

CWernerArt, 9 months ago

@TerryHancock Maybe it's a bug?

TerryHancock, 9 months ago

@CWernerArt

Okay, I just looked this up, and there is a weird thing: if you are in Edit Mode, the mesh may not be up to date!

Changing to Object Mode, and THEN inspecting the list(my_mesh.polygons) reports [] if the faces have been deleted.

https://docs.blender.org/api/current/info_gotcha.html#info-gotcha-mesh-faces

TerryHancock, 9 months ago

@CWernerArt
I mean, I just checked this with a couple of cube meshes.

The link suggests a couple of different ways to deal with this. You may be able to force an update even while in edit mode.

CWernerArt, 9 months ago

@TerryHancock Yeah. But I'm always not in Edit mode. I just check for objects that already are wireframe and some not. Or do I misunderstand something? I'm still new to blender python...

TerryHancock, 9 months ago

@CWernerArt

I don't know, but when I did tests above, I had left Blender in edit mode, so I was getting stale results from querying the cube objects.

When I exited to object mode and then ran the tests, I got the expected behavior that the mesh with faces deleted returned an empty list from list(cubemesh.polygons), while the one with faces reported 6 polygons.

Tested in Blender 2.79 and 3.30 just now.

CWernerArt, 9 months ago

@TerryHancock I'm in 3.6 and will check my routine in 3.3.

CWernerArt, 9 months ago

@TerryHancock Same problem in 3.3.

Two cubes. If I delete just the faces of one cube and exit the edit mode, both cubes are recognized as containing polygons and selected...

TerryHancock, 9 months ago

@CWernerArt
Demo of the test I did in Blender 3.3.

Created two cubes. Deleted the faces on one. Examined it with Python Console, and I find the data is stale.

But if I then switch to Object Mode, the data is updated, and the cube without faces returns an empty list of polygons, while the other shows 6, as expected.

I hope this helps you figure out what's going on in your case.

Demoing a "gotcha" I found mentioned in the Blender documentation. In Blender 3.3, copy the default cube. Delete the faces on the original ('Cube'), while leaving the copy ('Cube.001') alone. Locate the objects in Python Console, and label their meshes 'cube1mesh' and 'cube2mesh' respectively. 'cube1mesh.polygons' still shows 6 polys, as does 'cube2mesh.polygons'. Check 3D view -- I'm still in Edit Mode. Switch to Object Mode. Check again, and now 'cube1mesh.polygons' is empty. (I cast these to list() to make sure there wasn't anything funny going on with Blender datatypes. I don't know if that was necessary).

CWernerArt, 9 months ago

@TerryHancock Thank you a lot for your efforts. I guess I've found a solution for my situation, in the meantime. Now all works as expected. 🙂