[Tkinter-discuss] How to bind to wheel mouse event?

[jepler at unpythonic.net]

On Unix, the events are called <Button-4> and <Button-5>.
On Windows, there is a single event called <MouseWheel>, and on its
event object there is a 'delta' field which gives the direction.

Here's a snippet from the Tk manpage for 'bind':
    MouseWheel
        Some mice on the Windows platform support a mouse wheel which is used
        for scrolling documents without using the scrollbars.  By rolling the
        wheel, the system will generate MouseWheel events that the application
        can use to scroll.  Like Key events the event is always routed to the
        window that currently has focus.  When the event is received you can
        use the %D substitution to get the delta field for the event which is a
        integer value of motion that the mouse wheel has moved.  The smallest
        value for which the system will report is defined by the OS.   On
        Windows 95 & 98 machines this value is at least 120 before it is
        reported.  How ever, higher resolution devices may be available in the
        future.  The sign of the value determines which direction your widget
        should scroll.  Positive values should scroll up and negative values
        should scroll down.

Jeff