File: //usr/share/doc/python-docs-2.4.3/html/lib/node726.html
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<link rel="STYLESHEET" href="lib.css" type='text/css' />
<link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" />
<link rel='start' href='../index.html' title='Python Documentation Index' />
<link rel="first" href="lib.html" title='Python Library Reference' />
<link rel='contents' href='contents.html' title="Contents" />
<link rel='index' href='genindex.html' title='Index' />
<link rel='last' href='about.html' title='About this document...' />
<link rel='help' href='about.html' title='About this document...' />
<link rel="next" href="node727.html" />
<link rel="prev" href="node725.html" />
<link rel="parent" href="node722.html" />
<link rel="next" href="node727.html" />
<meta name='aesop' content='information' />
<title>16.1.6.4 Coupling Widget Variables</title>
</head>
<body>
<DIV CLASS="navigation">
<div id='top-navigation-panel' xml:id='top-navigation-panel'>
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="16.1.6.3 Packer Options"
href="node725.html"><img src='../icons/previous.png'
border='0' height='32' alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="16.1.6 Handy Reference"
href="node722.html"><img src='../icons/up.png'
border='0' height='32' alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="16.1.6.5 The Window Manager"
href="node727.html"><img src='../icons/next.png'
border='0' height='32' alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
href="contents.html"><img src='../icons/contents.png'
border='0' height='32' alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
border='0' height='32' alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
href="genindex.html"><img src='../icons/index.png'
border='0' height='32' alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="node725.html">16.1.6.3 Packer Options</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="node722.html">16.1.6 Handy Reference</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="node727.html">16.1.6.5 The Window Manager</A>
</div>
<hr /></div>
</DIV>
<!--End of Navigation Panel-->
<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:
<div class="verbatim"><pre>
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()
</pre></div>
<P>
<DIV CLASS="navigation">
<div class='online-navigation'>
<p></p><hr />
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="16.1.6.3 Packer Options"
href="node725.html"><img src='../icons/previous.png'
border='0' height='32' alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="16.1.6 Handy Reference"
href="node722.html"><img src='../icons/up.png'
border='0' height='32' alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="16.1.6.5 The Window Manager"
href="node727.html"><img src='../icons/next.png'
border='0' height='32' alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
href="contents.html"><img src='../icons/contents.png'
border='0' height='32' alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
border='0' height='32' alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
href="genindex.html"><img src='../icons/index.png'
border='0' height='32' alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="node725.html">16.1.6.3 Packer Options</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="node722.html">16.1.6 Handy Reference</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="node727.html">16.1.6.5 The Window Manager</A>
</div>
</div>
<hr />
<span class="release-info">Release 2.4.3, documentation updated on 29 March 2006.</span>
</DIV>
<!--End of Navigation Panel-->
<ADDRESS>
See <i><a href="about.html">About this document...</a></i> for information on suggesting changes.
</ADDRESS>
</BODY>
</HTML>