How to use Application.OnUndo in the xlwings API way

The Application.OnUndo method in Excel’s object model is a powerful feature that allows developers to customize the text displayed on the Undo button in the Quick Access Toolbar and specify a macro to run when that Undo command is executed. This is particularly useful for creating custom undo sequences for complex operations that involve multiple steps or external data changes, going beyond Excel’s built-in undo stack. In xlwings, which provides a Pythonic interface to automate Excel, you can access this functionality through the Application object’s api property, which exposes the underlying COM object, enabling you to call VBA-compatible methods directly.

Functionality:
The primary function of OnUndo is to assign a custom undo procedure. When a user clicks the Undo button after your code has set this property, Excel will run the specified macro instead of performing a standard undo. This allows for tailored reversal of actions that might not be captured by Excel’s native undo history, such as modifications to external databases, specific formatting sequences, or multi-sheet operations. It essentially overrides the default undo behavior for the next undo action only.

Syntax in xlwings:
In xlwings, you interact with the OnUndo method via the COM API. The general syntax is:

app.api.OnUndo(Text, Procedure)

• Text: A required String argument. This is the text that will appear on the Undo button (e.g., “Undo Custom Import”). It should clearly describe the action to be reversed.
• Procedure: A required String argument. This is the name of the macro (a VBA subroutine) that Excel will execute when the Undo button is clicked. The macro must be stored in a code module of the workbook.

Important Notes on Parameters:

1. The Procedure must be a macro accessible in the workbook. In an xlwings context, you can write UDFs (User Defined Functions) or macros in VBA modules that are called from Python, but the OnUndo method itself calls VBA code. Therefore, you typically need a VBA macro in place.
2. The custom undo text remains active only for the next undo operation. After the user clicks Undo or performs another action, Excel reverts to its default undo text and behavior.
3. This method does not work for undoing events that occur after the workbook is closed; it is session-specific.

Example Usage with xlwings:
Suppose you have a Python script using xlwings that imports data and performs a complex transformation. You want to provide an undo option that reverts this import. First, ensure you have a VBA macro named UndoCustomImport in a module of your workbook. This macro might clear the imported range or restore original values.

Here is a sample xlwings code snippet:

import xlwings as xw

# Connect to the active Excel instance or create one
app = xw.apps.active

# Connect to the specific workbook
wb = app.books['MyWorkbook.xlsm']

# Run your custom data import and processing code
# ... (e.g., clear a range, write new data from a DataFrame)

# Set the custom Undo text and procedure
app.api.OnUndo("Undo Data Import", "UndoCustomImport")

# Inform the user
print("Data import completed. You can undo this action using 'Undo Data Import' in Excel.")