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I am creating a simple add-on in Blender. Initially I tested the code in Python console, then I wrapped it in an operator and added it to Object menu, it worked well:

class MyOperator(bpy.types.Operator):
 bl_idname = 'object.MyOperator'
 bl_label = 'MyOperator'
 bl_options = 'REGISTER', 'UNDO'

 def execute(self, context):
 # ...affairs...
 return 'FINISHED'


def task_func(self, context):
 self.layout.operator(MyOperator.bl_idname)


def register():
 bpy.utils.register_class(MyOperator)
 bpy.types.VIEW3D_MT_object.append(task_func)

But now I need to add several operators and a submenu. I tried this example, and but it is not working:

class MyMenu(bpy.types.Menu):
 bl_idname = 'object.MyMenu'
 bl_label = 'MyMenu'

 def draw(self, context):
 layout = self.layout
 layout.operator(MyMenu.bl_idname, text=MyMenu.bl_label)


def menu_func(self, context):
 # None of these is working :(
 self.layout.call_menu(name=MyMenu.bl_idname)
 self.layout.menu(MyMenu.bl_idname)


def register():
 bpy.utils.register_class(MyOperator)
 bpy.types.VIEW3D_MT_object.append(menu_func)

So, what's the problem about? How can I solve it?

There were a number of issues. From not registering menu class and having invalid bl_idname's (no capital letters)

Here is working code. In 2.8 it is a requirement that class names reflect the bl_idname.

import bpy

class OBJECT_OT_myoperator(bpy.types.Operator):
 bl_idname = 'object.myoperator'
 bl_label = 'MyOperator'
 bl_options = 'REGISTER', 'UNDO'

 def execute(self, context):
 # ...affairs...
 return 'FINISHED'

class OBJECT_MT_mymenu(bpy.types.Menu):
 bl_idname = 'object.mymenu'
 bl_label = 'MyMenu'

 def draw(self, context):
 layout = self.layout
 layout.operator(OBJECT_OT_myoperator.bl_idname)

def menu_func(self, context):
 self.layout.menu(OBJECT_MT_mymenu.bl_idname)

def register():
 bpy.utils.register_class(OBJECT_OT_myoperator)
 bpy.utils.register_class(OBJECT_MT_mymenu)
 bpy.types.VIEW3D_MT_object.append(menu_func)

def unregister():
 bpy.utils.unregister_class(OBJECT_OT_myooperator)
 bpy.utils.unregister_class(OBJECT_MT_mymenu)
 bpy.types.VIEW3D_MT_object.remove(menu_func)

if __name__ == "__main__":
 register()