Create your first Blender add-on:Panel

This thread is to talk about a basic UI components - Panel. Offical Document about Panel https://docs.blender.org/api/current/bpy.types.Panel.html

First, lets see a simple example of building a Panel.

import bpy
class ADDONNAME_PT_hello_world_panel(bpy.types.Panel):
    "here is for description"
    bl_idname = "ADDONNAME_PT_hello_world"
    bl_label = "Hello World"
    bl_space_type = "PROPERTIES"
    bl_region_type = "WINDOW"
    bl_context = "object"
    bl_options = {"DEFAULT_CLOSED"}
    def draw(self, context):
        self.layout.label(text="Hello World Text")

bpy.utils.register_class(ADDONNAME_PT_hello_world_panel)

Line by line explanation.

import bpy

bpy is a module in Blender for creator to access the information/module inside.

ADDONNAME_PT_hello_world_panel:

This line is for setting up a Panel class.

class - Telling python interpreter a class object is being created

ADDONNAME_PT_hello_world_panel - The name of the class. In a python coding situation using class ADDONNAME_PT_hello_world_panel: is good enough to create a class. However, for coding in Blender, please follow the naming conversion in here. https://darkfallblender.blogspot.com/2020/07/blender-python-tutorial-class-naming.html

(bpy.types.Panel) - For creating a Blender Panel, we need to add this so Blender know it is a Panel object

"here is for description" - Normal users can't see this description with mouse hovering on the panel but it helps other creators to understand what this panel do.

bl_idname = "OBJECT_PT_hello_world" - custom ID of the panel. It will use the class name if not set.

bl_label = "Hello World" - The label of the Panel [Ref:https://i.imgur.com/4skGftj.png]

bl_space_type = "PROPERTIES" - This line indicate the Panel will be place inside the space of "PROPERTIES". Space type can refer to official document [1] or here [2]

bl_region_type = "WINDOW" - There are different regions under different spaces. However, there is no clear chart indicating the relation. For "PROPERTIES" space, "WINDOW" is the main region of it [ref: https://i.imgur.com/59XAFcu.png]

bl_context = "object" - It means the panel will be placed inside object properties. For other tabs, please refer to this [ref: https://i.imgur.com/S5L3c8F.png]

bl_options = {"DEFAULT_CLOSED"} - The official document is clearly explained [ref:https://docs.blender.org/api/current/bpy.types.Panel.html#bpy.types.Panel.bl_options] The only thing that need to aware of is the options need to be wrapped in {}

def draw(self, context):
    self.layout.label(text="Hello World Text")

Blender will look into the created panel class and search for some designated function names.

def - This is how python define a function

draw(self, context): - This is the function name to let Blender know the following lines are for drawing UI elements.

self.layout - self can be consider as the current class. layout is the function to start drawing the UI [ref: https://docs.blender.org/api/current/bpy.types.Panel.html#bpy.types.Panel.layout]

.label - an UI elements

(text="Hello World Text") - this is for defining the detail of the label element [ref: https://docs.blender.org/api/current/bpy.types.UILayout.html#bpy.types.UILayout.label]

bpy.utils.register_class(ADDONNAME_PT_hello_world_panel)

bpy.utils.register_class - This line is the reason why the library need to be imported at the beginning. It goes into the bpy module, then inside the utils there is a function register_class() to register the objects.

(ADDONNAME_PT_hello_world_panel) - inside the () is the name of the panel class so register_class function will register the panel into Blender