Why can I not access the custom EnumProperty which I added to my Panel class?

Question

I'm working on a Panel where a user shall be able to choose between different options using a dropdown box. What I'd like to do for convenience reasons is to define an EnumProperty directly in the Panel class. Here is an example, I modified the UI Panel Simple template that ships with Blender slightly and added Addon definition:

import bpy

bl_info = {
    "name": "Hello World Panel with Enum Prop",
    "author": "Rainer Trummer",
    "version": (0, 1, 0),
    "blender": (2, 76, 0),
    "description": "Enum Property is registered, but cannot be accessed",
    "category": "Interface"
}

class HelloWorldPanel(bpy.types.Panel):
    """Creates a Panel in the Object properties window"""
    bl_label = "Hello World Panel"
    bl_idname = "VIEW3D_PT_hello"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_context = "object"

    # EnumProperty definition
    test = bpy.props.EnumProperty(items =
        [
            ('ONE', 'one', 'first one'),
            ('TWO', 'two', 'second one')
        ]
    )

    def draw(self, context):
        layout = self.layout
        row = layout.row()
        row.label(text = "Hello world!", icon = 'WORLD_DATA')

        # the following line fails when hovering over the panel
        # with the mouse cursor
        row.prop(self, 'test')


def register():
    bpy.utils.register_class(HelloWorldPanel)


def unregister():
    bpy.utils.unregister_class(HelloWorldPanel)


if __name__ == "__main__":
    register()

You can run this code from the text editor. There are no errors thrown at first, but the Property never appears in the UI. When you hover over the Panel in the 3D View, the console prints the following error:

However, I can happily access the property from the console:

but only if the panel code is within an addon. Running JUST the panel code without the bl_info block does not register the property.

I know I can put the Enum somewhere else, like into bpy.types.scene, but I don't understand why this approach here would not work. It DOES work with operators, doesn't it? Why not with Panels?

Answer 1

Finally I found the hint in the Blender Reference manual on this page: https://www.blender.org/api/blender_python_api_2_77_3/bpy.types.bpy_struct.html#bpy.types.bpy_struct :

Note Only bpy.types.ID, bpy.types.Bone and bpy.types.PoseBone classes support custom properties

and on top of the https://www.blender.org/api/blender_python_api_2_77_3/bpy.types.ID.html#bpy.types.ID page, the different ID classes are mentioned:

subclasses — Action, Armature, Brush, CacheFile, Camera, Curve, FreestyleLineStyle, GreasePencil, Group, Image, Key, Lamp, Lattice, Library, Mask, Material, Mesh, MetaBall, MovieClip, NodeTree, Object, PaintCurve, Palette, ParticleSettings, Scene, Screen, Sound, Speaker, Text, Texture, VectorFont, WindowManager, World

So, since the Panel class is not an ID class, but another derivate of bpy_struct, it cannot support custom properties by definition.

Answer 2

items = (('OANS', 'oans', 'description', 0))

This is a tuple of four. (x) just evaluates to x. If you want a tuple that contains x, you have to write (x, ). Or just use a list. Either of those two should work:

items = (('OANS', 'oans', 'description', 0), )

items = [('OANS', 'oans', 'description', 0)]