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model update

The radio buttons can share a model

The text fields you originally created each stored a single data item. For every
widget on the screen, there was a single piece of data. But that’s not true
for your radio buttons. The three radio buttons will be used by the user to
record just one thing: the depot that a delivery is sent to. The three radio
buttons needs to share a single piece of data. And that piece of data is called the
model.
I‛ll keep my eye on
the model until I‛m
clicked...

This is the model
hidden away in your
computer's memory. Click me, so I can
update the model...

I‛ve just been clicked,
so I‛ll SET the model
to my value.

So if the Seattle radio button is selected, it will update the model with
a new value.
You haven’t set values on the radio buttons yet; you’ve only set text
descriptions. You could set the values to be whatever you want, but it is
simplest to set them to the same thing used as the description of the field:

The text is the description This VALUE is the one that
that appears on the screen. will be used in the model.

RadioButton(app, text = "Cambridge, MA", value = "Cambridge, MA")
You can make the text different
from the value, but let's leave
them the same here.
So what happens after the model is updated?

276 Chapter 8