The simplest way to add functionality to a button is called command binding, whereby a callback function is mentioned in the form of command = some_callback in the widget option. Note that the command option is available only for a few selected widgets.

Take a look at the following sample code:

def my_callback ():
   # do something when button is clicked

After defining the preceding callback, we can connect it to, say, a button with the command option referring to the callback, as follows:

tk.Button(root, text="Click me", command=my_callback)

A callback is a function memory reference ( my_callback in the preceding example) that is called by another function (which is Button in the preceding example) and that takes the first function as a parameter. Put simply, a callback is a function that you provide to another function so that it can calling it.

Note that my_callback is passed without parentheses, (), from within the widget command option, because when the callback functions are set it is necessary to pass a reference to a function rather than actually call it.

If you add parentheses, (), as you would for any normal function, it would be called as soon as the program runs. In contrast, the callback is called only when an event occurs (the pressing of a button in this case).