### 16.1.6.4 Coupling Widget Variables

The current-value setting of some widgets (like text entry widgets) can be connected directly to application variables by using special options. These options are `variable`, `textvariable`, `onvalue`, `offvalue`, and `value`. This connection works both ways: if the variable changes for any reason, the widget it's connected to will be updated to reflect the new value.

Unfortunately, in the current implementation of `Tkinter` it is not possible to hand over an arbitrary Python variable to a widget through a `variable` or `textvariable` option. The only kinds of variables for which this works are variables that are subclassed from a class called Variable, defined in the `Tkinter` module.

There are many useful subclasses of Variable already defined: `StringVar`, `IntVar`, `DoubleVar`, and `BooleanVar`. To read the current value of such a variable, call the `get()` method on it, and to change its value you call the `set()` method. If you follow this protocol, the widget will always track the value of the variable, with no further intervention on your part.

For example:

    class App(Frame):
        def __init__(self, master=None):
            Frame.__init__(self, master)
            self.pack()

            self.entrythingy = Entry()
            self.entrythingy.pack()

            # here is the application variable
            self.contents = StringVar()
            # set it to some value
            self.contents.set("this is a variable")
            # tell the entry widget to watch this variable
            self.entrythingy["textvariable"] = self.contents

            # and here we get a callback when the user hits return.
            # we will have the program print out the value of the
            # application variable when the user hits return
            self.entrythingy.bind('<Key-Return>',
                                  self.print_contents)

        def print_contents(self, event):
            print "hi. contents of entry is now ---->", \
                  self.contents.get()