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What are the new class naming conventions in Blender's 2.80 Python API for add-ons?

It says here:

This constraint applies to the bl_idname of each class (or the class name which uses it if no bl_idname is defined in the class).

These are: UPPER_CASE_{SEPARATOR}_mixed_case, in the case of a menu the regular expression is:

[A-Z][A-Z0-9_]*_MT_[A-Za-z0-9_]+

The separator for each class is listed below:

  • Header -> _HT_
  • Menu -> _MT_
  • Operator -> _OT_
  • Panel -> _PT_
  • UIList -> _UL_

Valid Examples:

  • OBJECT_OT_fancy_tool
  • SOME_HEADER_HT_my_header
  • PANEL123_PT_myPanel (lower case is preferred but mixed case is supported).

At the time of writing this, names that don't conform to this convention will produce a warning on startup. Eventually we will make this into an error, eg:

Warning: 'Oscurart Files Tools' doesn't contain '_PT_' with prefix & suffix
Warning: 'Oscurart Overrides' doesn't contain '_PT_' with prefix & suffix
Warning: 'Oscurart Animation Tools' doesn't contain '_PT_' with prefix & suffix

So this might mean that the bl_idname should actually be something like:

"object.MY_ADDON_OT_some_object_operator"

It still requires a '.' to be there so object.OBJECT_OT_something_something does not seem to make much sense to me. Does this actually apply to bl_idname? Could someone explain in more detail the idea behind this and how these conventions should be followed in practice?

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  • $\begingroup$ Good question. I thought you only add the "." when accessing the name, not when assigning. $\endgroup$ – ACopeLan Dec 11 '18 at 7:23
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It seems bl_idname of operators is an exception to this new convention

Looking through Python scripts that come with Blender 2.80 I can see that the convention is followed in bl_idname and class name as described with:

  • headers( _HT_ )
  • menus( _MT_ )
  • panels ( _PT_ )
  • UI lists ( _UL_ )

but not with operators since that is not possible. Operators sometimes follow this convention in their class names, but never in bl_idname, where they follow the usual categorization with the dot, like bl_idname = "object.randomize_transform".

That was the confusing part to me since OBJECT_OT_fancy_tool was given as an example without clarification and it would definitely not work as a bl_idname for an operator.

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    $\begingroup$ I think what they refer to with bl_idname is the class name. If you go to the Python console in Blender, and run bpy.ops.object.vertex_group_add.idname(), the result will be OBJECT_OT_vertex_group_add. It's definitely something that needs work in the documentation. But for any operator, the convention is to use the structure from the bl_idname in the class name. Like MESH_OT_myop for something that has mesh.myop as bl_idname, OBJECT_OT_myop for object.myop bl_idname and so on... $\endgroup$ – aliasguru Dec 11 '18 at 8:25
  • $\begingroup$ The bl_idname from an operator can by the way be retrieved using bpy.ops.object.vertex_group_add.idname_py() $\endgroup$ – aliasguru Dec 11 '18 at 8:26
  • $\begingroup$ Also worth mentioning that the class name convention has been there since 2.61 at least. I remember Sebastian Koenig mentioning it in a tutorial series of his. The change with 2.80 now is that you are forced to follow it. $\endgroup$ – aliasguru Dec 11 '18 at 8:30
  • $\begingroup$ They seem to refer to bl_idname with bl_idname. bl_idnames are following the convention as well as class names for headers, menus, panels and UI lists. It is also more serious with 2.80 since not following that will produce a warning now and it is going to become an error in the future. That's why I am so interested in this. I would not care otherwise. $\endgroup$ – Martynas Žiemys Dec 11 '18 at 8:47
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    $\begingroup$ It is still not very well defined and confusing since it is not even consistent in the templates that come with Blender in the Text Editor's Templates menu. And it is not consistent in the Python code that comes with Blender. Browsing the web it seems people are not sure. Nobody answered my question as well... Somewhat clear definition of it seems to be new. $\endgroup$ – Martynas Žiemys Dec 12 '18 at 6:21

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