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| 55 | <H3><A NAME="SECTION0018164000000000000000"> |
| 56 | 16.1.6.4 Coupling Widget Variables</A> |
| 57 | </H3> |
| 58 | <P> |
| 59 | The current-value setting of some widgets (like text entry widgets) |
| 60 | can be connected directly to application variables by using special |
| 61 | options. These options are <code>variable</code>, <code>textvariable</code>, |
| 62 | <code>onvalue</code>, <code>offvalue</code>, and <code>value</code>. This |
| 63 | connection works both ways: if the variable changes for any reason, |
| 64 | the widget it's connected to will be updated to reflect the new value. |
| 65 | |
| 66 | <P> |
| 67 | Unfortunately, in the current implementation of <tt class="module"><a href="module-Tkinter.html">Tkinter</a></tt> it is |
| 68 | not possible to hand over an arbitrary Python variable to a widget |
| 69 | through a <code>variable</code> or <code>textvariable</code> option. The only |
| 70 | kinds of variables for which this works are variables that are |
| 71 | subclassed from a class called Variable, defined in the |
| 72 | <tt class="module"><a href="module-Tkinter.html">Tkinter</a></tt> module. |
| 73 | |
| 74 | <P> |
| 75 | There are many useful subclasses of Variable already defined: |
| 76 | <tt class="class">StringVar</tt>, <tt class="class">IntVar</tt>, <tt class="class">DoubleVar</tt>, and |
| 77 | <tt class="class">BooleanVar</tt>. To read the current value of such a variable, |
| 78 | call the <tt class="method">get()</tt> method on |
| 79 | it, and to change its value you call the <tt class="method">set()</tt> method. If |
| 80 | you follow this protocol, the widget will always track the value of |
| 81 | the variable, with no further intervention on your part. |
| 82 | |
| 83 | <P> |
| 84 | For example: |
| 85 | <div class="verbatim"><pre> |
| 86 | class App(Frame): |
| 87 | def __init__(self, master=None): |
| 88 | Frame.__init__(self, master) |
| 89 | self.pack() |
| 90 | |
| 91 | self.entrythingy = Entry() |
| 92 | self.entrythingy.pack() |
| 93 | |
| 94 | # here is the application variable |
| 95 | self.contents = StringVar() |
| 96 | # set it to some value |
| 97 | self.contents.set("this is a variable") |
| 98 | # tell the entry widget to watch this variable |
| 99 | self.entrythingy["textvariable"] = self.contents |
| 100 | |
| 101 | # and here we get a callback when the user hits return. |
| 102 | # we will have the program print out the value of the |
| 103 | # application variable when the user hits return |
| 104 | self.entrythingy.bind('<Key-Return>', |
| 105 | self.print_contents) |
| 106 | |
| 107 | def print_contents(self, event): |
| 108 | print "hi. contents of entry is now ---->", \ |
| 109 | self.contents.get() |
| 110 | </pre></div> |
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