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<H3><A NAME="SECTION0018164000000000000000">
16.1.6.4 Coupling Widget Variables</A>
</H3>
<P>
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 <code>variable</code>, <code>textvariable</code>,
<code>onvalue</code>, <code>offvalue</code>, and <code>value</code>. 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.
<P>
Unfortunately, in the current implementation of <tt class="module"><a href="module-Tkinter.html">Tkinter</a></tt> it is
not possible to hand over an arbitrary Python variable to a widget
through a <code>variable</code> or <code>textvariable</code> option. The only
kinds of variables for which this works are variables that are
subclassed from a class called Variable, defined in the
<tt class="module"><a href="module-Tkinter.html">Tkinter</a></tt> module.
<P>
There are many useful subclasses of Variable already defined:
<tt class="class">StringVar</tt>, <tt class="class">IntVar</tt>, <tt class="class">DoubleVar</tt>, and
<tt class="class">BooleanVar</tt>. To read the current value of such a variable,
call the <tt class="method">get()</tt> method on
it, and to change its value you call the <tt class="method">set()</tt> method. If
you follow this protocol, the widget will always track the value of
the variable, with no further intervention on your part.
<P>
For example:
<dl><dd><pre class="verbatim">
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()
</pre></dl>
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<span class="release-info">Release 2.2, documentation updated on December 21, 2001.</span>
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