How to edit a custom property in a python script?

Question

I am creating a custom property on an Empty in a script, and then editing the property's data such as the name, limits, and Tooltip description. I am using the commands echo'ed from the info panel when I perform these operations in the Blender UI. While the object is selected and active I get these commands...

bpy.ops.wm.properties_add(data_path="object")
bpy.ops.wm.properties_edit(data_path="object", property="Scale", value="1.0", default="1.0", min=0, max=10, use_soft_limits=False, is_overridable_library=False, soft_min=0, soft_max=10, description="")

When using these commands in the script or Python console, the first "add" command succeeds, but the edit command fails. I get this error message...

Traceback (most recent call last):
  File "<blender_console>", line 1, in <module>
  File "/private/var/folders/z4/pm8ygsf16f1_kw3dsfpttthm0000gn/T/AppTranslocation/66464DE9-7039-455C-8E08-0581D47CFD84/d/Blender.app/Contents/Resources/2.80/scripts/modules/bpy/ops.py", line 201, in __call__
    ret = op_call(self.idname_py(), None, kw)
RuntimeError: Error: Direct execution not supported

Is it possible to edit a custom property from a script, and what is the proper command, please. Thanks.

Answer 1

_RNA_UI dictionary

As descibed here https://blender.stackexchange.com/a/43786/15543

import bpy

context = bpy.context
obj = context.object

if not obj.get('_RNA_UI'):
    obj['_RNA_UI'] = {}

# set it
obj["scale_factor"] = 1.0

# property attributes.for UI 
obj['_RNA_UI']["scale_factor"] = {"description":"Scale Factor",
                  "default": 1.0,
                  "min":0.0,
                  "max":10.0,
                  "soft_min":0.0,
                  "soft_max":10.0,
                  "is_overridable_library":0,
                  }

# test UI in text editor footer

def draw(self, context):
    ob = context.object
    self.layout.prop(ob, '["scale_factor"]')

bpy.types.TEXT_HT_footer.append(draw)

Answer 2

Since Blender 3.0 (relevant commit)

The UI for a custom property is described by a IDPropertyUIManager that can be retrieved by the id_properties_ui method.

import bpy

context = bpy.context
obj = context.object

# create the property, set it the initial value
if "scale_factor" not in obj:
    obj["scale_factor"] = 1.0

# get or create the UI object for the property
ui = obj.id_properties_ui("scale_factor")
ui.update(description = "Scale factor")
ui.update(default = 1.0)
ui.update(min=0.0, soft_min=0.0)
ui.update(max=10.0, soft_max=10.0)

# test UI in text editor footer

def draw(self, context):
    ob = context.object
    self.layout.prop(ob, '["scale_factor"]')

bpy.types.TEXT_HT_footer.append(draw)

Before Blender 3.0

The UI for a custom property is described by a dict object, stored in a special/hidden _RNA_UI property.

The internal blender addons tend not to access _RNA_UI directly, but use the rna_prop_ui.rna_idprop_ui_prop_get function to get or create the relevent "ui" object.

import bpy

context = bpy.context
obj = context.object

from rna_prop_ui import rna_idprop_ui_prop_get

# create the property, set it the initial value
obj["scale_factor"] = 1.0

# get or create the UI object for the property
ui = rna_idprop_ui_prop_get(obj, "scale_factor", create=True)
ui['description'] = "Scale factor"
ui['default'] = 1.0
ui['min'] = ui['soft_min'] = 0.0
ui['max'] = ui['soft_max'] = 10.0

# test UI in text editor footer

def draw(self, context):
    ob = context.object
    self.layout.prop(ob, '["scale_factor"]')

bpy.types.TEXT_HT_footer.append(draw)
```

Answer 3

So I added some tag_redraw code straight from Blender's API documentation, with changes to focus on the Properties window. It does the job of redrawing the UI, but that has no affect on the outcome of the above code. The code that makes it work is the repeated obj.get just above the creation of the property.

import bpy

context = bpy.context
obj = context.object

rna_ui = obj.get('_RNA_UI')
if rna_ui is None:
    rna_ui = obj['_RNA_UI'] = {}

rna_ui = obj.get('_RNA_UI')

# set it
obj["Scale_Factor"] = 1.0

# property attributes.for UI 
rna_ui["Scale_Factor"] = {"description":"Multiplier for Scale",
                  "default": 1.0,
                  "min":0.0,
                  "max":10.0,
                  "soft_min":0.0,
                  "soft_max":10.0,
                  "is_overridable_library":False
                  }
# redraw Properties panel
for window in bpy.context.window_manager.windows:
    screen = window.screen

    for area in screen.areas:
        if area.type == 'PROPERTIES':
            area.tag_redraw()
            break