### 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()

              self.button.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()