[Bf-python] Addon's can no longer access context on activation.

Thu Dec 20 15:31:15 CET 2012

On Thu, Dec 20, 2012 at 2:13 PM, dimitris chloupis <thekilon at yahoo.co.uk> wrote:
>
> I have to agree with you. Thats not necessary because loading those presets
> back is not
> automatic, the users has to press the restore_history button as is shown in
> the second link.
>
> At first the load was automatic , but blender was very buggy with this
> approach and so
> I made it manual. Also this way I gave more power to the user to decide what
> is loaded
> and what is not. But I would love to add the automatic restore back if that
> was possible.
>
> The question is do I access on load on the first place. As you can see my
> access does not
> happen in the register function but on my class definition. Actually nothing
> exiting
> happens in my registers and unregisters anyway. Because I find this approach
> very
> "unpythonic" to do stuff in the register functions and instead I use my own
> classes.
> So this is why I am not sure if
> the issue you refering to is relevant to Gyes or not. Or if Gyes is affected
> at all.
>
> If it is, I could try to move everything in the button function but I am not
> sure if that would be entirely possible.
> And will most certainly make the code uglier and more difficult to
> understand.
> ________________________________
> From: Campbell Barton <ideasman42 at gmail.com>
> To: dimitris chloupis <thekilon at yahoo.co.uk>; Blender Foundation Python list
> <bf-python at blender.org>
> Sent: Thursday, 20 December 2012, 16:04
>
> Subject: Re: [Bf-python] Addon's can no longer access context on activation.
>
> From what I can tell you dont even need to access the scene on load.
>
> On Thu, Dec 20, 2012 at 12:55 PM, dimitris chloupis
> <thekilon at yahoo.co.uk> wrote:
>> No idea what you talking about . No idea how to access bpy.context.scene
>> while the addon loads.
>>
>> my access to bpy.context.scene happens here ->
>>
>> https://github.com/kilon/Gyes/blob/master/gyes/random_material_generator.py#L67
> Quite sure you can replace:
> if hasattr(bpy.context.scene , "historybak")==False:
> with...
> if hasattr(bpy.types.Scene, "historybak")==False:
>
>> and here ->
>>
>> https://github.com/kilon/Gyes/blob/master/gyes/random_material_generator.py#L622
> This will continue to work without problems.
>
>>
>> in first case I check whether there is a historybak prop in scene , so
>> there
>> is no assumptions here
>> and blender has no reason to crash. Because I check if it exists in the
>> first place.
>> If the prop does not exists it gets created in the process.
>>
>> in second case the history is restored when the user clicks the button.
>>
>> In any cases there is no chance that the addon would access non existant
>> properties
>> because I like to leave nothing to chance ;)
>>
>> unless you try to tell me something else here that I dont understand
>> or I missed a possible scenario . I am all ears :)
>>
>> ________________________________
>> From: Campbell Barton <ideasman42 at gmail.com>
>> To: dimitris chloupis <thekilon at yahoo.co.uk>; Blender Foundation Python
>> list
>> <bf-python at blender.org>
>> Sent: Thursday, 20 December 2012, 14:33
>> Subject: Re: [Bf-python] Addon's can no longer access context on
>> activation.
>>
>> On Thu, Dec 20, 2012 at 12:09 PM, dimitris chloupis
>> <thekilon at yahoo.co.uk> wrote:
>>> I am the developer of Gyes ( random material and texture generator). The
>>> reason why Gyes
>>> use bpy.context.scene is to store a property that store presets of
>>> randomised materials / textures.
>>> I wanted the data to be stored/saved with the blend file and be
>>> associated
>>> with the scene curently loaded
>>> as global scene presets and not specific presets to objects or any other
>>> 3d
>>> asset.
>>>
>>> I am not sure if removing bpy.context.scene is a good idea in the first
>>> place, unless there is another way to save
>>> , store scene presets together with the blend file. If so , then gladly I
>>> will update my addon. But if not , it basically
>>> cripples / makes useless my history tool ->
>>>
>>>
>>> http://wiki.blender.org/index.php/Extensions:2.6/Py/Scripts/System/Gyes/History-Tool
>>>
>>>
>>> ________________________________
>>> From: Campbell Barton <ideasman42 at gmail.com>
>>> To: Blender Foundation Python list <bf-python at blender.org>; bf-blender
>>> developers <bf-committers at blender.org>
>>> Sent: Thursday, 20 December 2012, 4:38
>>> Subject: [Bf-python] Addon's can no longer access context on activation.
>>>
>>> Recently I've noticed addons using bpy.context.scene when being
>>> imported or in the register() function.
>>>
>>> This is really bad practice and will cause inconsistency if the addon
>>> is enabled and a different blend file is loaded.
>>> In the worst case accessing a removed datablock can crash which is
>>> likely if a datablock is stored as a global variable on activation -
>>> object_laplace_lightning.py does this.
>>>
>>> I've changed addon loading so that they only get access to a
>>> restricted context which only exposes 'window_manager' - needed for
>>> adding keymaps.
>>>
>>>
>>> Accessing other attributes now errors out with...
>>>
>>>  AttributeError: '_RestrictedContext' object has no attribute 'scene'
>>>
>>> The addons distributed with blender don't do this but some contrib
>>> addons do, so they will need updating
>>> (release/scripts/addons_contrib).
>>>
>>> ----
>>>    add_mesh_clusters
>>>    gyes
>>>    io_atomblend_utilities
>>>    io_export_marmalade
>>>    io_import_LRO_Lola_MGS_Mola_img
>>>    io_import_lipSync_Importer
>>>    mesh_edgetools
>>>    mesh_fiber
>>>    mesh_show_vgroup_weights
>>>    node_categories
>>>    object_laplace_lightning
>>>    online_mat_lib
>>>    sequencer_extra_actions
>>>    space_view3d_quickPrefs
>>>
>>> If any devs want some help updating their addons you can ask on
>>> bf-python mailing list, though in all cases I've seen its quite
>>> straightforward.
>>>
>>> --
>>> - Campbell
>>
>> Access has only been removed on loading the addon.
>>
>> The solution is just not to access presets when the addon loads and
>> register()'s,
>> during execution you still have regular access to the context
>> (operators, panels etc).
>>
>> Since the scene at load time may very well not be the same scene when
>> the script executes (if the user loads a new file), whatever changes
>> made at load time are likely lost.
>>
>> --
>> - Campbell
>>
>>
>>
>> _______________________________________________
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>> Bf-python at blender.org
>> http://lists.blender.org/mailman/listinfo/bf-python
>>
>
>
>
> --
> - Campbell

An instance of random_material_class is created on load here, this is
causing the problem, so you can choose to create it at runtime, or to
have it not touch context.scene.

https://github.com/kilon/Gyes/blob/master/gyes/random_material_generator.py#L441

Both register and creating an instance of random_material_class()
happen - both have the same limitation.

The purpose of register() is so you can unregister(), without it there
would be no good way to disable/enable addons.

I don't understand your second last comment, '''The question is do I
access on load on the first place.''' - can you give an example?

Suggest you get the script running without errors first - think its
only a few lines to change, then check on ways to improve.

-- 
- Campbell