At first, callback functions can seem to be a little bit intimidating. You call the function, passing it some arguments, and then the function tells you that it is really very busy and it will call you back!

You wonder: Will this function ever call me back? And how long do I have to wait? In Python, even callback functions are easy and, yes, they usually do call you back. They just have to complete their assigned task first (hey, it was you who coded them in the first place…).

Let's understand a little bit more about what happens when we code callbacks into our GUI. Our GUI is event-driven. After it has been created and displayed onscreen, it typically sits there waiting for an event to happen. It is basically waiting for an event to be sent to it. We can send an event to our GUI by clicking one of its action buttons. This creates an event and, in a sense, we called our GUI by sending it a message.

Now, what is supposed to happen after we send a message to our GUI? What happens after clicking the button depends on whether we created an event handler and associated it with this button. If we did not create an event handler, clicking the button will have no effect. The event handler is a callback function (or method, if we use classes). The callback method is also sitting there passively, like our GUI, waiting to be invoked. Once our GUI gets its button clicked, it will invoke the callback.

The callback often does some processing and, when done, it returns the result to our GUI.