blender-syntax-panels

Quick Reference

Critical Warnings

NEVER modify scene data inside Panel.draw() — draw callbacks are read-only. Modifying properties triggers infinite redraw loops.

NEVER call bpy.ops.* inside draw() — operators MUST be exposed as UI buttons, not called directly during drawing.

NEVER omit bl_space_type or bl_region_type — registration fails without both.

NEVER use bl_idname values without the correct type tag (_PT_ for panels, _MT_ for menus, _UL_ for UILists).

ALWAYS use layout.panel() for collapsible sections in Blender 4.1+ instead of registering separate sub-panel classes.

ALWAYS match bl_space_type, bl_region_type, and bl_category between parent and child panels.

Panel Naming Convention

{ADDON_PREFIX}_{TYPE_TAG}_{name}

Type tags:
  _PT_  = Panel
  _MT_  = Menu
  _UL_  = UIList
  _HT_  = Header

Examples:
  MY_PT_main_panel
  MY_MT_object_menu
  MY_UL_item_list

Decision Tree: Which Layout Container?

Need horizontal arrangement?  → layout.row(align=False)
Need vertical arrangement?    → layout.column(align=False)
Need bordered section?        → layout.box()
Need percentage split?        → layout.split(factor=0.5)
Need grid arrangement?        → layout.grid_flow(row_major=True, columns=0)
Need collapsible section?
  Blender 4.1+?              → layout.panel("ID", text="Header")
  Blender < 4.1?             → Register a sub-panel with bl_parent_id
Need separator line?          → layout.separator()
Need header spacing?          → layout.separator_spacer()

Decision Tree: Panel Location

Sidebar (N-panel)?
  → bl_space_type = 'VIEW_3D', bl_region_type = 'UI'

Properties Editor?
  → bl_space_type = 'PROPERTIES', bl_region_type = 'WINDOW'
  → bl_context = 'object' | 'scene' | 'render' | 'data' | ...

Tool Shelf?
  → bl_space_type = 'VIEW_3D', bl_region_type = 'TOOLS'

Node Editor sidebar?
  → bl_space_type = 'NODE_EDITOR', bl_region_type = 'UI'

Image Editor sidebar?
  → bl_space_type = 'IMAGE_EDITOR', bl_region_type = 'UI'

---

Essential Patterns

Pattern 1: Minimal Panel

# Blender 3.x/4.x/5.x — minimum viable panel
import bpy

class MY_PT_example(bpy.types.Panel):
    bl_label = "My Panel"           # Header text (REQUIRED)
    bl_idname = "MY_PT_example"     # Unique ID (REQUIRED)
    bl_space_type = 'VIEW_3D'      # Editor type (REQUIRED)
    bl_region_type = 'UI'          # Region type (REQUIRED)
    bl_category = "My Tab"         # Sidebar tab name (REQUIRED for UI region)

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

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

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

Pattern 2: Panel with poll() and draw_header()

# Blender 3.x/4.x/5.x — conditional visibility + header checkbox
class MY_PT_conditional(bpy.types.Panel):
    bl_label = "Object Info"
    bl_idname = "MY_PT_conditional"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_category = "My Tab"
    bl_options = {'DEFAULT_CLOSED'}  # Start collapsed

    @classmethod
    def poll(cls, context):
        """Panel is visible ONLY when a mesh object is active."""
        return context.active_object is not None and context.active_object.type == 'MESH'

    def draw_header(self, context):
        """Draw a checkbox in the panel header."""
        self.layout.prop(context.scene.my_settings, "enabled", text="")

    def draw(self, context):
        layout = self.layout
        layout.active = context.scene.my_settings.enabled  # Grey out if disabled
        obj = context.active_object
        layout.prop(obj, "name")
        layout.prop(obj, "location")

Pattern 3: Sub-Panels (Parent-Child Hierarchy)

# Blender 3.x/4.x/5.x — sub-panel via bl_parent_id
class MY_PT_parent(bpy.types.Panel):
    bl_label = "Main Settings"
    bl_idname = "MY_PT_parent"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_category = "My Tab"

    def draw(self, context):
        self.layout.label(text="Parent content")

class MY_PT_child_a(bpy.types.Panel):
    bl_label = "Transform"
    bl_idname = "MY_PT_child_a"
    bl_space_type = 'VIEW_3D'       # MUST match parent
    bl_region_type = 'UI'           # MUST match parent
    bl_category = "My Tab"          # MUST match parent
    bl_parent_id = "MY_PT_parent"   # Links to parent
    bl_options = {'DEFAULT_CLOSED'}

    def draw(self, context):
        self.layout.prop(context.active_object, "location")

Pattern 4: Collapsible Sections with layout.panel() (Blender 4.1+)

# Blender 4.1+ ONLY — collapsible section WITHOUT separate class registration
class MY_PT_modern(bpy.types.Panel):
    bl_label = "Modern Panel"
    bl_idname = "MY_PT_modern"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_category = "My Tab"

    def draw(self, context):
        layout = self.layout
        layout.label(text="Always visible")

        # layout.panel() returns (header_layout, panel_layout) or (header_layout, None)
        header, body = layout.panel("MY_PT_modern_section", default_closed=True)
        header.label(text="Collapsible Section")
        if body is not None:
            body.prop(context.active_object, "name")
            body.prop(context.active_object, "location")

Pattern 5: UILayout API — Common Elements

# Blender 3.x/4.x/5.x — UILayout methods
def draw(self, context):
    layout = self.layout
    obj = context.active_object
    props = context.scene.my_settings

    # Property split (label left, widget right)
    layout.use_property_split = True
    layout.use_property_decorate = True  # Show keyframe dots

    # Row (horizontal)
    row = layout.row(align=True)
    row.prop(obj, "location", index=0, text="X")
    row.prop(obj, "location", index=1, text="Y")
    row.prop(obj, "location", index=2, text="Z")

    # Column (vertical)
    col = layout.column(align=True)
    col.prop(props, "count")
    col.prop(props, "scale")

    # Box (bordered container)
    box = layout.box()
    box.label(text="Section", icon='PREFERENCES')
    box.prop(props, "option_a")

    # Split (percentage-based columns)
    split = layout.split(factor=0.3)
    split.column().label(text="Label:")
    split.column().prop(props, "value", text="")

    # Grid flow
    grid = layout.grid_flow(row_major=True, columns=3, even_columns=True)
    for i in range(9):
        grid.label(text=f"Item {i}")

    # Separator
    layout.separator()

    # Operator button
    op = layout.operator("my.operator", text="Run", icon='PLAY')
    op.my_prop = 42  # Set operator property BEFORE execution

    # Conditional enable/disable
    row = layout.row()
    row.enabled = props.enabled
    row.operator("my.action")

    # Alert styling (red highlight)
    row = layout.row()
    row.alert = True
    row.label(text="Warning!", icon='ERROR')

Pattern 6: Menu Definition

# Blender 3.x/4.x/5.x — custom menu
class MY_MT_example(bpy.types.Menu):
    bl_label = "My Menu"
    bl_idname = "MY_MT_example"

    def draw(self, context):
        layout = self.layout
        layout.operator("my.action_a", text="Action A", icon='FILE')
        layout.operator("my.action_b", text="Action B")
        layout.separator()
        layout.menu("MY_MT_submenu")  # Nested submenu
        layout.operator("wm.call_menu", text="Open Other").name = "MY_MT_other"

    @classmethod
    def poll(cls, context):
        return context.active_object is not None

# Invoke from panel:
# layout.menu("MY_MT_example")

# Append to existing Blender menu:
def draw_my_menu_item(self, context):
    self.layout.menu("MY_MT_example")

bpy.types.VIEW3D_MT_object.append(draw_my_menu_item)

# Remove in unregister():
bpy.types.VIEW3D_MT_object.remove(draw_my_menu_item)

Pattern 7: UIList

# Blender 3.x/4.x/5.x — custom list widget
class MY_UL_items(bpy.types.UIList):
    bl_idname = "MY_UL_items"

    def draw_item(self, context, layout, data, item, icon, active_data,
                  active_propname, index):
        if self.layout_type in {'DEFAULT', 'COMPACT'}:
            row = layout.row(align=True)
            row.prop(item, "name", text="", emboss=False)
            row.prop(item, "enabled", text="")
        elif self.layout_type == 'GRID':
            layout.alignment = 'CENTER'
            layout.label(text=item.name, icon_value=icon)

    def filter_items(self, context, data, propname):
        items = getattr(data, propname)
        flt_flags = [self.bitflag_filter_item] * len(items)
        flt_neworder = list(range(len(items)))

        # Filter by name
        if self.filter_name:
            for i, item in enumerate(items):
                if self.filter_name.lower() not in item.name.lower():
                    flt_flags[i] = 0

        return flt_flags, flt_neworder

# Use in panel draw():
# layout.template_list("MY_UL_items", "", data, "items", data, "active_index")

Pattern 8: Properties Editor Panel

# Blender 3.x/4.x/5.x — panel in Properties editor
class MY_PT_object_props(bpy.types.Panel):
    bl_label = "Custom Properties"
    bl_idname = "MY_PT_object_props"
    bl_space_type = 'PROPERTIES'     # Properties editor
    bl_region_type = 'WINDOW'        # Main area (NOT 'UI')
    bl_context = "object"            # Object properties tab

    @classmethod
    def poll(cls, context):
        return context.active_object is not None

    def draw(self, context):
        self.layout.prop(context.active_object, "name")

---

bl_space_type Values

Value	Editor
'VIEW_3D'	3D Viewport
'PROPERTIES'	Properties editor
'OUTLINER'	Outliner
'NODE_EDITOR'	Node editor (shader, compositor, geometry)
'TEXT_EDITOR'	Text editor
'IMAGE_EDITOR'	UV/Image editor
'SEQUENCE_EDITOR'	Video Sequencer
'CLIP_EDITOR'	Movie Clip editor
'PREFERENCES'	Preferences window
'GRAPH_EDITOR'	Graph editor (FCurves)
'DOPESHEET_EDITOR'	Dope Sheet
'NLA_EDITOR'	NLA editor
'FILE_BROWSER'	File browser
'SPREADSHEET'	Spreadsheet (Blender 3.0+)

bl_region_type Values

Value	Location
'UI'	Sidebar (N-panel)
'TOOLS'	Tool shelf (T-panel)
'HEADER'	Header bar
'WINDOW'	Main area (Properties editor panels)
'TOOL_PROPS'	Active tool properties
'TOOL_HEADER'	Tool header (Blender 2.83+)
'EXECUTE'	Operator redo region
'FOOTER'	Footer bar
'NAVIGATION_BAR'	Navigation bar (Preferences)

bl_context Values (Properties editor only)

Value	Tab
"render"	Render properties
"output"	Output properties
"view_layer"	View Layer properties
"scene"	Scene properties
"world"	World properties
"object"	Object properties
"modifier"	Modifier properties
"particle"	Particle properties
"physics"	Physics properties
"constraint"	Constraint properties
"data"	Object data properties (mesh, curve, etc.)
"material"	Material properties
"texture"	Texture properties
"bone"	Bone properties
"bone_constraint"	Bone constraint properties

bl_options Values (Panel)

Value	Effect
'DEFAULT_CLOSED'	Panel starts collapsed
'HIDE_HEADER'	No header bar (cannot collapse)
'INSTANCED'	Panel can be used as template in multiple contexts
'HEADER_LAYOUT_EXPAND'	Header expands to fill available space

---

Common Operations

Adding Popover Panels

# Blender 3.x/4.x/5.x — popover panel (floating panel from header button)
class MY_PT_popover(bpy.types.Panel):
    bl_label = "Popover Settings"
    bl_idname = "MY_PT_popover"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'WINDOW'  # WINDOW for popover panels
    bl_options = {'INSTANCED'}  # Required for popover usage

    def draw(self, context):
        self.layout.prop(context.scene, "frame_current")

# In a header or panel draw():
# layout.popover("MY_PT_popover", text="Settings")

Pie Menus

# Blender 3.x/4.x/5.x — pie menu
class MY_MT_pie(bpy.types.Menu):
    bl_label = "My Pie Menu"
    bl_idname = "MY_MT_pie"

    def draw(self, context):
        pie = self.layout.menu_pie()
        pie.operator("transform.translate")   # West (left)
        pie.operator("transform.rotate")      # East (right)
        pie.operator("transform.resize")      # South (bottom)
        pie.operator("object.shade_smooth")   # North (top)
        # Additional items: NW, NE, SW, SE

# Invoke: bpy.ops.wm.call_menu_pie(name="MY_MT_pie")

Dynamic Panel Content

# Blender 3.x/4.x/5.x — panel content based on selection
def draw(self, context):
    layout = self.layout
    obj = context.active_object

    if obj is None:
        layout.label(text="No object selected", icon='INFO')
        return

    layout.prop(obj, "name")

    if obj.type == 'MESH':
        layout.label(text=f"Vertices: {len(obj.data.vertices)}")
        layout.label(text=f"Faces: {len(obj.data.polygons)}")
    elif obj.type == 'LIGHT':
        layout.prop(obj.data, "energy")
        layout.prop(obj.data, "color")
    elif obj.type == 'CAMERA':
        layout.prop(obj.data, "lens")

---

Version-Specific Notes

Feature	Version	Notes
layout.panel()	4.1+	Inline collapsible sections without class registration
bl_order	3.0+	Integer controlling panel sort order
SPREADSHEET space type	3.0+	New editor type
TOOL_HEADER region	2.83+	Separate tool header region
Sub-panel support	2.80+	bl_parent_id for panel hierarchy

---

Reference Links

• references/methods.md — Complete API signatures for Panel, UILayout, Menu, UIList
• references/examples.md — Working code examples for common panel patterns
• references/anti-patterns.md — What NOT to do when creating panels and UI

Official Sources

• https://docs.blender.org/api/current/bpy.types.Panel.html
• https://docs.blender.org/api/current/bpy.types.UILayout.html
• https://docs.blender.org/api/current/bpy.types.Menu.html
• https://docs.blender.org/api/current/bpy.types.UIList.html
• https://docs.blender.org/api/current/info_quickstart.html