I would like to set various parameters when I open a panel. By parameter I mean toggling the bounds for an object or changing object.display_type etc. Is there a python way of calling some code (method, script, operator, handle etc.) when I open or close a panel?

  • 3
    $\begingroup$ I imagine it would be useful to be more specific about the circumstances you’re interested in as “a panel” is quite generic. Do you mean a tool panel? A panel in the tool shelf? A panel in the properties? A panel from an operator (either a custom one (your own or someone else’s?) or a pre-built one). Providing as much detail as possible as to the specific area you’re interested in will drastically improve the chances of someone being able to pick this up and provide an answer. $\endgroup$ Commented Jan 13, 2021 at 11:50
  • $\begingroup$ Thanks for the comment. I didn't really know there are much difference in the panels. I used to put my panels in the properties space, but I changed them to the 3D view tool-tab. They are part of a custom bigger addon I am working on. $\endgroup$
    – DrDress
    Commented Jan 13, 2021 at 12:42

2 Answers 2


Spit data from draw method to a draw call back.

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When playing around with draw callbacks noticed that can modify objects, whereas you cannot from a panel draw method.

Here is an example Is automatic "Reload from disk" confirmation in Text Editor possible?

A panel can be registered as default closed. In this case only the poll and draw header method will be called. When it is expanded we see the content of the draw method. Explained here.

For convenience the drawing class is added to the driver namespace. As explained in How do you remove a draw handler after it's been added? See also answer to operator error when open a new file re setting up a on the window manager.

Test script: defines a draw callback class and a panel (default closed). The drawing class has a list being fed from the poll and draw methods of the panel. The list is kept to 5 members. If the panel is being drawn it sees this and sets the active object to 'TEXTURED' else to 'WIRE'.

import bpy
import blf

class DrawingClass:
    def __init__(self, context, prop):
        self.status = [prop]
        self.prop = prop
        self.handle = bpy.types.SpaceView3D.draw_handler_add(
                   'WINDOW', 'POST_PIXEL')

    def draw_text_callback(self, context):
        font_id = 0  # XXX, need to find out how best to get this.

        # draw some text
        blf.position(font_id, 15, 50, 0)
        blf.size(font_id, 20, 72)
        blf.draw(font_id, "%s %s" % (context.scene.name, ",".join(self.status)))
        ob = context.object
        if ob:
            ob.display_type = 'TEXTURED' if "Open" in self.status else 'WIRE'

    def remove_handle(self):
        bpy.types.SpaceView3D.draw_handler_remove(self.handle, 'WINDOW')         

class HelloWorldPanel(bpy.types.Panel):
    """Creates a Panel in the Object properties window"""
    bl_label = "Hello World Panel"
    bl_idname = "OBJECT_PT_hello"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_options = {'DEFAULT_CLOSED'}

    def __init__(self):

    def poll(self, context):

        dc = dns.get("dc")
        if dc:
            while len(dc.status) > 4:
            dc.status.insert(0, "Closed")

        return True    
    def draw(self, context):
        layout = self.layout
        dc = dns.get("dc")
        if dc:
            dc.status.insert(0, "Open")

def register():

def unregister():

if __name__ == "__main__":

    context = bpy.context             
    dns = bpy.app.driver_namespace
    dc = dns.get("dc")
    if not dc:
        dns["dc"] = DrawingClass(context, "Closed")

Have used similar for a panel "heat" addon, where a counter is added to the panel class for every draw call. After a while "cold" panels, are closed, moved or unregistered.


Notice: You must save the script to __init__.py and put it in a file to zip it.
The file name you can change it for zip.
If you install it without zip, it doesn't work.

import bpy
from bpy.app.handlers import persistent
bl_info = {
    "name" : "Test Call in start",
    "author" : "Public",
    "version" : (1, 0),
    "blender" : (2, 90, 0),
    "location" : "View3d > Tool",
    "category" : "3D View",

def get_user_prefs(context):
    if hasattr(context, "user_preferences"):
        return context.user_preferences

    return context.preferences

def load_handler(dummy): # you can do something when blender open
    P = get_user_prefs(bpy.context).addons[__package__].preferences

class MY_Preferences(bpy.types.AddonPreferences):
    bl_idname = __package__

    def update_panel_category(self, context):
        has_panel = hasattr(bpy.types, My_Panel.bl_idname)
        if has_panel:
        My_Panel.bl_category = self.my_category

    my_float: bpy.props.FloatProperty(name = "My Float", default = 1.0,)
    my_category: bpy.props.StringProperty(name = "Category", default = "Item",
        update = update_panel_category,
    my_message: bpy.props.StringProperty(name = "message", description = "message", default = '')

class My_Panel(bpy.types.Panel):
    bl_idname = "MY_PT_Panel"
    bl_label = "My Panel"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_category = "Item"

    def draw(self, context):
        layout = self.layout
        P = get_user_prefs(bpy.context).addons[__package__].preferences

        layout.row().operator("message.messagebox", text = "")
        layout.row().prop(P, "my_category")

class MessageBox(bpy.types.Operator):
    bl_idname = "message.messagebox"
    bl_label = ""

    def execute(self, context):
        self.report({'INFO'}, "do something when you press 'ok'")
        return {'FINISHED'}
    def invoke(self, context, event):
        return context.window_manager.invoke_props_dialog(self, width = 400)

    def draw(self, context):
        P = get_user_prefs(bpy.context).addons[__package__].preferences
        layout = self.layout
        layout.label(text="my float")
        layout.prop(P, 'my_float', text="")

classes = (MY_Preferences, My_Panel, MessageBox,)
def register():
    for c in classes:
def unregister():
    for c in classes:
if __name__ == "__main__":
  • 1
    $\begingroup$ Can also look at Panel.is_registered see blender.stackexchange.com/a/202896/15543 For a single python file addon would instead call it test_addon.py (or some such) rather than going thru the rigmarole of zipping and making a zip test_addon.zip to make a folder named test_addon to put a single __init__.py file into. Do not ever place an addon named __init__.py directly into any blender addons folder. $\endgroup$
    – batFINGER
    Commented Jan 15, 2021 at 7:09
  • 1
    $\begingroup$ btw not sure that this fits the question remit in that (I assume) OP wants to do something when the panel is expanded as opposed to registered. $\endgroup$
    – batFINGER
    Commented Jan 15, 2021 at 7:20
  • $\begingroup$ @ batFINGER. Exactly right. Both multifile addons and running code on startup are questions I asked priviously. But is not what I meant in this post. $\endgroup$
    – DrDress
    Commented Jan 15, 2021 at 9:00
  • $\begingroup$ Hmm. I am told that the bounty was auto awarded. I thought that it was given when I accepted the answer. Which I did. Thanks batFINGER! $\endgroup$
    – DrDress
    Commented Jan 20, 2021 at 9:15

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