Why is globalString not being updated by localUpdate in this add-on?

Question

From the print statements in localUpdate it appears that changing the value of localString in the Redo panel updates globalString but globalString is not actually updated. What am I missing? (This doesn't seem to be a Blender bug, but something I'm not understanding about StringProperty or properties in general.)

NOTE: This is a a question about why the update function doesn't work, not about how to update the global without using the update function. I've simplified a fairly complex setup that has mixins that modify the local to get to this example, so while updating the global in execute would work in the example, it won't work in the final system.

To see the problem, invoke the operator by using the button in the tla Panel. When the Last Operator Adjustment (redo) panel shows up, change the value of localString in the panel. Messages will appear on the system console showing that localUpdate has run and modified globalString but if you check bpy.context.scene.globalString in the Python console after making the change, it will still have its original value of "global". Further, setting its value in the Python console causes it to change in the panel.

Here is the add-on. Copy it into the text editor and hit run.

import bpy
from bpy.types import Operator, Panel
from bpy.props import StringProperty

bl_info = {
    "name" : "Op Test",
    "description" : "a test",
    "author" : "Marty",
    "version" : (0, 0, 1),
    "blender" : (2, 80, 0),
    "location" : "View3D",
    "support" : "COMMUNITY",
    "category" : "3D View"
}

class TLA_OT_message(Operator):
    bl_idname = "tla.message"
    bl_label = "An Op"
    bl_options = {"REGISTER", "UNDO"}
    
    def localUpdate(self, context):
        print("localUpdate called")
        print(self.localString)
        print(f'before {bpy.context.scene.globalString}')
        context.scene.globalString = self.localString
        print(f' after {bpy.context.scene.globalString}')

    localString : bpy.props.StringProperty(
        name="localString",
        default="local",
        update=localUpdate)
    
    def execute(self, context):
        self.report({"INFO"}, f"local string is {self.localString}")
        return {"FINISHED"}
        
class TL_PT_panel(Panel):
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"
    bl_category = "tla"
    bl_label = "Panel"
    bl_options = set()
    
    def draw(self, context):
        col = self.layout.column()
        col.operator(TLA_OT_message.bl_idname, text = "push me")
        col.prop(context.scene, "globalString", text="Global")

def anUpdater(self, context):
    print("an Updater called")

def register():
    bpy.utils.register_class(TLA_OT_message)
    bpy.utils.register_class(TL_PT_panel)
    bpy.types.Scene.globalString = bpy.props.StringProperty(
        name="globalString",
        default = "global",
        update=anUpdater,
    )
    
def unregister():
    del bpy.types.Scene.globalString
    bpy.utils.unregister_class(TL_PT_panel)
    bpy.utils.unregister_class(TLA_OT_message)
    
if __name__ == "__main__":
    register()

Here is what the 3D Viewport looks like after installing the addon, opening the side panel, selecting the 'tla' tab, and pushing the button:

Change "local" to "other" and the info message in the info window changes:

bpy.ops.text.run_script()
bpy.ops.tla.message()
local string is local
bpy.data.window_managers["WinMan"].(null) = "other"
bpy.ops.tla.message(localString="other")
local string is other

(Icons don't copy so here's a screenshot)

and the content of the system console at this point:

Read prefs: C:\Users\stupi\AppData\Roaming\Blender Foundation\Blender\2.93\config\userpref.blend
Read blend: D:\stupi\blender\blends\stack exchange answers\updater test.blend
Info: local string is local

localUpdate called
other
before global
an Updater called
 after other
Info: local string is other

Finally, checking the value of bpy.context.scene.globalString after all of that:

PYTHON INTERACTIVE CONSOLE 3.9.2 (default, Mar  1 2021, 08:18:55) [MSC v.1916 64 bit (AMD64)]

Builtin Modules:       bpy, bpy.data, bpy.ops, bpy.props, bpy.types, bpy.context, bpy.utils, bgl, blf, mathutils
Convenience Imports:   from mathutils import *; from math import *
Convenience Variables: C = bpy.context, D = bpy.data

>>> bpy.context.scene.globalString
'global'

>>> 

and for completeness: my test file:

Answer 1

Although i am not the add-on-Pro....i think it is because you are showing the global variable field too. So it takes the value from that field. If you comment out

#        col.prop(context.scene, "globalString", text="Global")

it works.

But i am sure batfinger can explain this better ;)

Answer 2

It is because localUpdate is an instance method. localString is a class annotation adding a property to all instances but at the time is defined, instance methods are not yet registered in the class. (At least that's my guess)

So you have to move localUpdate out of the class definition :

This works :

import bpy
from bpy.types import Operator, Panel
from bpy.props import StringProperty


bl_info = {
    "name" : "Op Test",
    "description" : "a test",
    "author" : "Marty",
    "version" : (0, 0, 1),
    "blender" : (2, 80, 0),
    "location" : "View3D",
    "support" : "COMMUNITY",
    "category" : "3D View"
}


def localUpdate(self, context):
    print("localUpdate called")
    print(self.localString)
    print(f'before {bpy.context.scene.globalString}')
    context.scene.globalString = self.localString
    print(f' after {bpy.context.scene.globalString}')


class TLA_OT_message(Operator):
    bl_idname = "tla.message"
    bl_label = "An Op"
    bl_options = {"REGISTER", "UNDO"}

    localString : bpy.props.StringProperty(
        name="localString",
        default="local",
        update=localUpdate)

    def execute(self, context):
        self.report({"INFO"}, f"local string is {self.localString}")
        return {"FINISHED"}

 
class TL_PT_panel(Panel):
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"
    bl_category = "tla"
    bl_label = "Panel"
    bl_options = set()

    def draw(self, context):
        col = self.layout.column()
        col.operator(TLA_OT_message.bl_idname, text = "push me")
        col.prop(context.scene, "globalString", text="Global")


def anUpdater(self, context):
    print("an Updater called")


def register():
    bpy.utils.register_class(TLA_OT_message)
    bpy.utils.register_class(TL_PT_panel)
    bpy.types.Scene.globalString = bpy.props.StringProperty(
        name="globalString",
        default = "global",
        update=anUpdater,
    )


def unregister():
    del bpy.types.Scene.globalString
    bpy.utils.unregister_class(TL_PT_panel)
    bpy.utils.unregister_class(TLA_OT_message)


if __name__ == "__main__":
    register()

Answer 3

In the end, the answer is "You can't get there from here". More precisely, when you add an update function to a property in a class, Blender massages that property at registration time, in such a way that the context argument to the update function becomes the window manager, and not the context you need to look up the property.

This is why the info window gets the message bpy.data.window_managers["WinMan"].(null) = "other" in my example.

It doesn't matter whether the update function is defined within the class or not, it just doesn't work. For now, I have a workaround that works for presets, although it does require a bit of excess cleverness.

import bpy
from bpy.types import Operator, Panel
from bpy.props import StringProperty

bl_info = {
    "name" : "Op Test",
    "description" : "a test",
    "author" : "Marty",
    "version" : (0, 0, 1),
    "blender" : (2, 80, 0),
    "location" : "View3D",
    "support" : "COMMUNITY",
    "category" : "3D View"
}

class TLA_OT_message(Operator):
    bl_idname = "tla.message"
    bl_label = "An Op"
    bl_options = {"REGISTER", "UNDO", "PRESET"}
    
    need_update = False
    
    localString : bpy.props.StringProperty(
        name="localString",
        default="local",
    )
            
    def updateGlobal(self, context):
        context.scene.globalString = self.localString
        
    @classmethod
    def requireUpdate(cls):
        cls.need_update = True

    def execute(self, context):
        if TLA_OT_message.need_update:
            self.localString = context.scene.globalString
        self.report({"INFO"}, f"local string is {self.localString}")
        self.updateGlobal(context)
        # The call to updateGlobal will cause the global's anUpdater
        # function to be called, causing need_update to be set to True
        # but we don't need an update so we override that change before
        # leaving the function.
        TLA_OT_message.need_update = False
        return {"FINISHED"}
        
class TLA_PT_panel(Panel):
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"
    bl_category = "tla"
    bl_label = "Panel"
    bl_options = set()
    
    def draw(self, context):
        col = self.layout.column()
        col.operator(TLA_OT_message.bl_idname, text = "push me")
        col.prop(context.scene, "globalString", text="Global")

def anUpdater(self, context):
    TLA_OT_message.requireUpdate()

def register():
    bpy.utils.register_class(TLA_OT_message)
    bpy.utils.register_class(TLA_PT_panel)
    bpy.types.Scene.globalString = bpy.props.StringProperty(
        name="globalString",
        default = "global",
        update=anUpdater,
    )
    
def unregister():
    del bpy.types.Scene.globalString
    bpy.utils.unregister_class(TLA_PT_panel)
    bpy.utils.unregister_class(TLA_OT_message)
    
if __name__ == "__main__":
    register()
```