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lets say i have a panel and i want to collapse a part if a bool is False

import bpy


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 = 'PROPERTIES'
 bl_region_type = 'WINDOW'
 bl_context = "object"

 def draw(self, context):
 layout = self.layout
 # where should i define this bool ? 
 layout.prop(place, "bool")
 if bool is True:
 obj = context.object

 row = layout.row()
 row.label(text="Hello world!", icon='WORLD_DATA')

 row = layout.row()
 row.label(text="Active object is: " + obj.name)



def register():
 bpy.utils.register_class(HelloWorldPanel)


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


if __name__ == "__main__":
 register()

Blender 2.8 has sub panels.

As of blender 2.8 can if bl_parent_id is set in a panel it will be appended to the parent panel. Use the usual 'DEFAULT_CLOSED' in the bl_options set to start with panel closed.

This avoids needing to set up some boolean property to open close UI altogether.

Here is a sample file I've been testing for ordering panels. First panel is defined as a child of the object transform panel. Second panel is a child of first. Can unregister and re-register with different parent ids to sort sub-panels, which is what I was testing

The two demo sub panels appended to object transform

import bpy
from bl_ui.properties_object import ObjectButtonsPanel, OBJECT_PT_transform

class LayoutDemoPanel(bpy.types.Panel, ObjectButtonsPanel):
 bl_label = "Layout Demo"
 bl_idname = "SCENE_PT_layout"
 bl_parent_id = 'OBJECT_PT_transform'
 bl_space_type = 'PROPERTIES'
 bl_region_type = 'WINDOW'

 def draw(self, context):
 layout = self.layout

 scene = context.scene

 # Create a simple row.
 layout.label(text=" Simple Row:")

 row = layout.row()
 row.prop(scene, "frame_start")

class LayoutDemoPanel2(bpy.types.Panel, ObjectButtonsPanel):
 bl_label = "Layout Demo 2"
 bl_idname = "SCENE_PT_layout2"
 bl_parent_id = 'SCENE_PT_layout'
 bl_space_type = 'PROPERTIES'
 bl_region_type = 'WINDOW'

 def draw(self, context):
 layout = self.layout

 scene = context.scene

 # Create a simple row.
 layout.label(text=" Simple Row:")

 row = layout.row()
 row.prop(scene, "frame_start")


def register():
 bpy.utils.register_class(LayoutDemoPanel)
 bpy.utils.register_class(LayoutDemoPanel2)

def unregister():
 bpy.utils.unregister_class(LayoutDemoPanel2)
 bpy.utils.unregister_class(LayoutDemoPanel1)


if __name__ == "__main__":
 register()

If you have some logical setting that when set needs more UI How to dynamically show/hide panel elements using python?