Is it possible to disable menu items in custom menus?

Question

I want to disable and enable menu items (layout.operator) in my custom menu, the disabled ones should be displayed greyed. I managed to disable the whole menu with layout.enabled = False, but I failed to disable a single menu item.

Something similar what I want to achieve can be seen in the "Ui Menu" Python template with the entry "Hello world!", but this is a layout.label and not a layout.operator. Before I use this approach as workaround I'd like to know if there is a more easy way to dis- or enable single menu items.

Answer 1

As already said by pink vertex and Jerryno, a poll() classmethod should be added to the operator you add to the custom menu. The disabling (graying out) will be managed by Blender automatically:

import bpy

class SimpleOperator(bpy.types.Operator):
    bl_label = "Simple Operator"
    bl_idname = "wm.simple_operator"

    @classmethod
    def poll(cls, context):
        return len(context.selected_objects) % 2 == 0

    def execute(self, context):
        self.report({'INFO'}, "Simple Operator executed.")
        return {'FINISHED'}

class SimpleOperator2(bpy.types.Operator):
    bl_label = "Simple Operator 2"
    bl_idname = "wm.simple_operator2"

    @classmethod
    def poll(cls, context):
        return (context.object is not None and
                context.object.type == 'CAMERA')

    def execute(self, context):
        self.report({'INFO'}, "Simple Operator 2 executed.")
        return {'FINISHED'}


class SimpleCustomMenu(bpy.types.Menu):
    bl_label = "Simple Custom Menu"
    bl_idname = "OBJECT_MT_simple_custom_menu"

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

        layout.label("Enabled, if even number of objects selected", icon='INFO')
        layout.operator("wm.simple_operator")
        layout.separator()
        layout.label("Enabled, if active object is a camera", icon='INFO')
        layout.operator("wm.simple_operator2")


def register():
    bpy.utils.register_module(__name__)


def unregister():
    bpy.utils.unregister_module(__name__)

if __name__ == "__main__":
    register()

    # The menu can also be called from scripts
    bpy.ops.wm.call_menu(name=SimpleCustomMenu.bl_idname)

Answer 2

If you want specific GUI elements to be disabled, set corresponding row.enable or column.enable to False:

row = layout.row()
row.operator("something")
# maybe more row stuff here
row.enable = False

If you want this build in into the operator itself, include the poll method. This will grey it in the UI and also be safe when the user calls the operator from search (spacebar) for example!