bu creates a Button widget:
button = g.bu(text='Press me.')
The return value from
bu is a Button object. The button that appears in the Frame is a graphical representation of this object; you can control the button by invoking methods on it.
bu takes up to 32 parameters that control the appearance and function of the button. These parameters are called options. Instead of providing values for all 32 options, you can use keyword arguments, like
text='Press me.', to specify only the options you need and use the default values for the rest.
When you add a widget to the Frame, it gets “shrink-wrapped;” that is, the Frame shrinks to the size of the Button. If you add more widgets, the Frame grows to accommodate them.
la creates a Label widget:
label = g.la(text='Press the button.')
By default, Tkinter stacks the widgets top-to-bottom and centers them. We’ll see how to override that behavior soon.
If you press the button, you will see that it doesn’t do much. That’s because you haven’t “wired it up;” that is, you haven’t told it what to do!
The option that controls the behavior of a button is
command. The value of
command is a function that gets executed when the button is pressed. For example, here is a function that creates a new Label:
def make_label(): g.la(text='Thank you.')
Now we can create a button with this function as its command:
button2 = g.bu(text='No, press me!', command=make_label)
When you press this button, it should execute
make_label and a new label should appear.
The value of the
command option is a function object, which is known as a callback because after you call
bu to create the button, the flow of execution “calls back” when the user presses the button.
This kind of flow is characteristic of event-driven programming. User actions, like button presses and key strokes, are called events. In event-driven programming, the flow of execution is determined by user actions rather than by the programmer.
The challenge of event-driven programming is to construct a set of widgets and callbacks that work correctly (or at least generate appropriate error messages) for any sequence of user actions.
Write a program that creates a GUI with a single button. When the button is pressed it should create a second button. When that button is pressed, it should create a label that says, “Nice job!”.
What happens if you press the buttons more than once?