Make the add-on to be registered in last anywhere

In some cases when Blender starts it is necessary to make the add-on to be registered in last, after all other add-ons have been registered. This may be needed if the add-on conflicts with other add-ons or uses functionality from other add-ons, which should be available already during its registration.

.blend .py files on Patreon

When Blender starts, add-ons are loaded in order, according to the “bpy.context.preferences.addons” list, the order of add-ons in which is determined by they were activated by the user.

We can write the required order of activation of the add-on in its installation instructions or, using the fact that the list of add-ons is already available to us when the add-on is registering, add the functionality we need in the “register” function of the add-on.

Let’s write classes for two simple add-ons: The “First” and the “Second”.

The “First” add-on class:

import bpy

bl_info = {
    'name': 'FIRST',
    'category': 'All',
    'version': (0, 0, 1),
    'blender': (3, 4, 0),
}

class FIRST_OT_main(bpy.types.Operator):
    bl_idname = 'first.main'
    bl_label = 'First main OP'
    bl_options = {"REGISTER"}

    def execute(self, context):
        print('FIRST ADD-ON OPERATOR EXECUTE')
        return {"FINISHED"}

def register():
    print('FIRST add-on loaded')
    bpy.utils.register_class(FIRST_OT_main)

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

if __name__ == '__main__':
    register()

import bpy

bl_info = {

'name': 'FIRST',

'category': 'All',

'version': (0, 0, 1),

'blender': (3, 4, 0),

}

class FIRST_OT_main(bpy.types.Operator):

bl_idname = 'first.main'

bl_label = 'First main OP'

bl_options = {"REGISTER"}

def execute(self, context):

print('FIRST ADD-ON OPERATOR EXECUTE')

return {"FINISHED"}

def register():

print('FIRST add-on loaded')

bpy.utils.register_class(FIRST_OT_main)

def unregister():

bpy.utils.unregister_class(FIRST_OT_main)

if __name__ == '__main__':

The “Second” add-on class:

import bpy

bl_info = {
    'name': 'SECOND',
    'category': 'All',
    'version': (0, 0, 1),
    'blender': (3, 4, 0),
}

class SECOND_OT_main(bpy.types.Operator):
    bl_idname = 'second.main'
    bl_label = 'Second main OP'
    bl_options = {"REGISTER"}

    def execute(self, context):
        print('SECOND ADD-ON OPERATOR EXECUTE')
        return {"FINISHED"}

def register():
    print('SECOND add-on loaded')
    bpy.utils.register_class(SECOND_OT_main)

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

if __name__ == '__main__':
    register()

import bpy

bl_info = {

'name': 'SECOND',

'category': 'All',

'version': (0, 0, 1),

'blender': (3, 4, 0),

}

class SECOND_OT_main(bpy.types.Operator):

bl_idname = 'second.main'

bl_label = 'Second main OP'

bl_options = {"REGISTER"}

def execute(self, context):

print('SECOND ADD-ON OPERATOR EXECUTE')

return {"FINISHED"}

def register():

print('SECOND add-on loaded')

bpy.utils.register_class(SECOND_OT_main)

def unregister():

bpy.utils.unregister_class(SECOND_OT_main)

if __name__ == '__main__':

Pay attention to the call of the “print” command in the “register” function of add-ons. With its help, we will track the actual loading order of our add-ons.

Save the classes in separate .py files and install add-ons from these files to Blender. First, install the “Second” add-on and after it – the “First” add-on. Don’t forget to save the user settings.

Walking through the “bpy.context.preferences.addons” list:

for addon in bpy.context.preferences.addons:
    print(addon)

# ...
# <bpy_struct, Addon("second_addon") at 0x0000014EEE7283A8>
# <bpy_struct, Addon("first_addon") at 0x0000014EEE729128>

for addon in bpy.context.preferences.addons:

print(addon)

# ...

# <bpy_struct, Addon("second_addon") at 0x0000014EEE7283A8>

# <bpy_struct, Addon("first_addon") at 0x0000014EEE729128>

We can see that our add-ons will be loaded in order: the “Second” – the “First”.

We can verify this by running Blender from the system console to see the output from the “register” add-on functions.

Let’s make some changes to the “Second” add-on so that it will always be loaded last.

We can perform manipulations with loading the add-on inside its “register” function.

Let’s add a check to the “register” function code of the “Second” add-on to see if the add-on is in the last place in the “bpy.context.preferences.addons list”.

def register():
    if bpy.context.preferences.addons[-1].module == __name__:
        print('SECOND add-on loaded after all other add-ons')
        bpy.utils.register_class(SECOND_OT_main)

def register():

if bpy.context.preferences.addons[-1].module == __name__:

print('SECOND add-on loaded after all other add-ons')

bpy.utils.register_class(SECOND_OT_main)

If success – everything works as we need, the add-on is loaded in last, and we can register its classes as usual.

And we will get into the “else” condition if Blender is already loading the add-on, but there are more add-ons in the “bpy.context.preferences.addons list” after it.

def register():
    if bpy.context.preferences.addons[-1].module == __name__:
        print('SECOND add-on loaded after all other add-ons ->> OK')
        bpy.utils.register_class(SECOND_OT_main)
    else:
        print('SECOND add-on loaded from the exist place in the bpy.context.preferences.addons list ->> SKIPPING')
        # remove from existed place
        addon_prefs_ptp = bpy.context.preferences.addons.get(__name__)
        addon_prefs_ptp_module = addon_prefs_ptp.module
        bpy.context.preferences.addons.remove(addon_prefs_ptp)
        # and add to the last place
        addon_prefs_new_ptp = bpy.context.preferences.addons.new()
        addon_prefs_new_ptp.module = addon_prefs_ptp_module

def register():

if bpy.context.preferences.addons[-1].module == __name__:

print('SECOND add-on loaded after all other add-ons ->> OK')

bpy.utils.register_class(SECOND_OT_main)

else:

print('SECOND add-on loaded from the exist place in the bpy.context.preferences.addons list ->> SKIPPING')

# remove from existed place

addon_prefs_ptp = bpy.context.preferences.addons.get(__name__)

addon_prefs_ptp_module = addon_prefs_ptp.module

bpy.context.preferences.addons.remove(addon_prefs_ptp)

# and add to the last place

addon_prefs_new_ptp = bpy.context.preferences.addons.new()

addon_prefs_new_ptp.module = addon_prefs_ptp_module

In this case, we get a pointer to the add-on in the “bpy.context.preferences.addons” list and use it to remove the add-on from its current position in the list.

Then we create a pointer to the new add-on in the list, it will be added to the end, and we assign it the name of our add-on.

When Blender reaches the end of the list, and the last element of the list is again the pointer to our “Second” add-on, it will try to register our add-on again, and since this time the add-on is registered from the last place in the list, its registration will succeed.

In order for the add-on not to throw an error when calling the “unregister” function, we add the additional “second_registered” variable, in which we will mark whether the add-on is really registered or not.

second_registered = False

def register():
    if bpy.context.preferences.addons[-1].module == __name__:
        print('SECOND add-on loaded after all other add-ons ->> OK')
        bpy.utils.register_class(SECOND_OT_main)
        # mark as registered
        global second_registered
        second_registered = True
    else:
        print('SECOND add-on loaded from the exist place in the bpy.context.preferences.addons list ->> SKIPPING')
        # remove from existed place
        addon_prefs_ptp = bpy.context.preferences.addons.get(__name__)
        addon_prefs_ptp_module = addon_prefs_ptp.module
        bpy.context.preferences.addons.remove(addon_prefs_ptp)
        # and add to the last place
        addon_prefs_new_ptp = bpy.context.preferences.addons.new()
        addon_prefs_new_ptp.module = addon_prefs_ptp_module

def unregister():
    global second_registered
    if second_registered:
        bpy.utils.unregister_class(SECOND_OT_main)
        second_registered = False

second_registered = False

def register():

if bpy.context.preferences.addons[-1].module == __name__:

print('SECOND add-on loaded after all other add-ons ->> OK')

bpy.utils.register_class(SECOND_OT_main)

# mark as registered

global second_registered

second_registered = True

else:

print('SECOND add-on loaded from the exist place in the bpy.context.preferences.addons list ->> SKIPPING')

# remove from existed place

addon_prefs_ptp = bpy.context.preferences.addons.get(__name__)

addon_prefs_ptp_module = addon_prefs_ptp.module

bpy.context.preferences.addons.remove(addon_prefs_ptp)

# and add to the last place

addon_prefs_new_ptp = bpy.context.preferences.addons.new()

addon_prefs_new_ptp.module = addon_prefs_ptp_module

def unregister():

global second_registered

if second_registered:

bpy.utils.unregister_class(SECOND_OT_main)

second_registered = False

Let’s reinstall our add-ons, again in order, first “Second”, then – “First”.

Now if we launch Blender from the console, we will see the following output:

Blender will first try to register the “Second” add-on from its current location, registration is skipped, and the add-on is moved to the last place in the list. Then the “First” add-on is registered. After that, the “Second” add-on is really registered, the last one in the order.