On-value-change type of event for widgets or use .trace_variable() technique?

Question

Is there an on-value-change type of event for data input widgets like Entry, Text, Spinner, Checkbutton, Radiobutton? By on-value-change, I mean the ability to detect when the value of a widget has changed due to keyboard input or cut/delete/paste (and Text edit_undo/edit_redo) activity? I see no such event described in the Tkinter event documentation [1].

Is the proper technique to link Tkinter variables to widget values I want to monitor and use these variables' .trace_variable( 'w', ... ) methods to bind to value changes? This seems like the right approach, but I haven't seen a lot of trace_variable() use in the Tkinter application source code that I've studied ... leading me to be cautious about using this approach.

[1] http://infohost.nmt.edu/tcc/help/pubs/tkinter/events.html

Answer 1

Different widgets call for different solutions. For example, check buttons and radio buttons have a command option, and with an entry widget you can use the built-in validation features.

For all the widgets that can be tied to a variable, doing a variable trace is a common solution. The text widget is one exception since you can't associate it with a variable without a lot of effort.

In the tcl/tk world I associate all my widgets to a single array (tcl's name for a hash map / dictionary) and then put a single trace on the array. Unfortunately tkinter doesn't directly support tcl arrays. However, support is somewhat easy to hack in. For more information see my response to this question: How to run a code whenever a Tkinter widget value changes?