How to detect when an OptionMenu or Checkbutton change?

Many tkinter controls can be associated with a variable. For those you can put a trace on the variable so that some function gets called whenever the variable changes.

Example:

In the following example the callback will be called whenever the variable changes, regardless of how it is changed.

def callback(*args):
    print(f"the variable has changed to '{var.get()}'")

root = tk.Tk()
var = tk.StringVar(value="one")
var.trace("w", callback)

For more information about the arguments that are passed to the callback see this answer

To have an event fired when a selection is made set the command option for OptionMenu

ex.

def OptionMenu_SelectionEvent(event): # I'm not sure on the arguments here, it works though
    ## do something
    pass

var = StringVar()
var.set("one")
options = ["one", "two", "three"]
OptionMenu(frame, var, *(options), command = OptionMenu_SelectionEvent).pack()

If you are using a Tkinter Variable class like StringVar() for storing the variables in your Tkinter OptionMenu or Checkbutton, you can use its trace() method.

trace(), basically, monitors the variable when it is read from or written to.

The trace() method takes 2 arguments - mode and function callback.

trace(mode, callback)

• The mode argument is one of “r” (call observer when variable is read by someone), “w” (call when variable is written by someone), or “u” (undefine; call when the variable is deleted).
• The callback argument is the call you want to make to the function when the variable is changed.

This is how it is used -

def callback(*args):
    print("variable changed!")

var = StringVar()
var.trace("w", callback)
var.set("hello")

Source : http://effbot.org/tkinterbook/variable.htm