Python win32com demo outlookaddin.py control button Caption not working

coming straight to the point, I tried using the Python win32com demo outlookAddin.py and it was working fine with Outlook 2010.

I upgraded to Outlook 2016 and since then the outlookAddin.py is still working however the addin msoControlButton that it is adding in the Add-ins ribbon has no caption.

Can anyone please help me understand how to resolve this. The code is by default available to anyone installed win32com but I am still giving it below for ready reference.

from win32com import universal
from win32com.server.exception import COMException
from win32com.client import gencache, DispatchWithEvents
import winerror
import pythoncom
from win32com.client import constants
import sys

# Support for COM objects we use.
gencache.EnsureModule('{00062FFF-0000-0000-C000-000000000046}', 0, 9, 0, bForDemand=True) # Outlook 9
gencache.EnsureModule('{2DF8D04C-5BFA-101B-BDE5-00AA0044DE52}', 0, 2, 1, bForDemand=True) # Office 9

# The TLB defining the interfaces we implement
universal.RegisterInterfaces('{AC0714F2-3D04-11D1-AE7D-00A0C90F26F4}', 0, 1, 0, ["_IDTExtensibility2"])

class ButtonEvent:
    def OnClick(self, button, cancel):
        import win32ui # Possible, but not necessary, to use a Pythonwin GUI
        win32ui.MessageBox("Hello from Python")
        return cancel

class FolderEvent:
    def OnItemAdd(self, item):
        try:
            print("An item was added to the inbox with subject:", item.Subject)
        except AttributeError:
            print("An item was added to the inbox, but it has no subject! - ", repr(item))



class OutlookAddin:
    _com_interfaces_ = ['_IDTExtensibility2']
    _public_methods_ = []
    _reg_clsctx_ = pythoncom.CLSCTX_INPROC_SERVER
    _reg_clsid_ = "{0F47D9F3-598B-4d24-B7E3-92AC15ED27E2}"
    _reg_progid_ = "Python.Test.OutlookAddin"
    _reg_policy_spec_ = "win32com.server.policy.EventHandlerPolicy"
    def OnConnection(self, application, connectMode, addin, custom):
        print("OnConnection", application, connectMode, addin, custom)
        # ActiveExplorer may be none when started without a UI (eg, WinCE synchronisation)
        activeExplorer = application.ActiveExplorer()
        if activeExplorer is not None:
            bars = activeExplorer.CommandBars
            toolbar = bars.Item("Standard")
            item = toolbar.Controls.Add(Type=constants.msoControlButton, Temporary=True)
            # Hook events for the item
            item = self.toolbarButton = DispatchWithEvents(item, ButtonEvent)
            item.Caption = "Python"
            item.TooltipText = "Click for Python"
            item.Enabled = True


        # And now, for the sake of demonstration, setup a hook for all new messages
        inbox = application.Session.GetDefaultFolder(constants.olFolderInbox)
        self.inboxItems = DispatchWithEvents(inbox.Items, FolderEvent)

    def OnDisconnection(self, mode, custom):
        print("OnDisconnection")
    def OnAddInsUpdate(self, custom):
        print("OnAddInsUpdate", custom)
    def OnStartupComplete(self, custom):
        print("OnStartupComplete", custom)
    def OnBeginShutdown(self, custom):
        print("OnBeginShutdown", custom)

def RegisterAddin(klass):
    import winreg
    key = winreg.CreateKey(winreg.HKEY_CURRENT_USER, "Software\\Microsoft\\Office\\Outlook\\Addins")
    subkey = winreg.CreateKey(key, klass._reg_progid_)
    winreg.SetValueEx(subkey, "CommandLineSafe", 0, winreg.REG_DWORD, 0)
    winreg.SetValueEx(subkey, "LoadBehavior", 0, winreg.REG_DWORD, 3)
    winreg.SetValueEx(subkey, "Description", 0, winreg.REG_SZ, klass._reg_progid_)
    winreg.SetValueEx(subkey, "FriendlyName", 0, winreg.REG_SZ, klass._reg_progid_)

def UnregisterAddin(klass):
    import winreg
    try:
        winreg.DeleteKey(winreg.HKEY_CURRENT_USER, "Software\\Microsoft\\Office\\Outlook\\Addins\\" + klass._reg_progid_)
    except WindowsError:
        pass

if __name__ == '__main__':
    import win32com.server.register
    win32com.server.register.UseCommandLine(OutlookAddin)
    if "--unregister" in sys.argv:
        UnregisterAddin(OutlookAddin)
    else:
        RegisterAddin(OutlookAddin)

Solution

CommandBars were deprecated. Office 2010 was the first version where command bars were not used for the UI. You can read more about the Fluent UI (aka Ribbon UI) in the following series of articles:

The recommended way for customizing the Office UI is using the Fluent UI. Command Bars remains in-place for backward compatibility such as executing buttons programmatically and etc. You shouldn't rely on command bars for creating a custom UI now.