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<H3><A NAME="SECTION0018167000000000000000">
16.1.6.7 Bindings and Events</A>
</H3>
<P>
<a id='l2h-4840' xml:id='l2h-4840'></a>
<P>
The bind method from the widget command allows you to watch for
certain events and to have a callback function trigger when that event
type occurs. The form of the bind method is:
<P>
<div class="verbatim"><pre>
def bind(self, sequence, func, add=''):
</pre></div>
where:
<P>
<DL>
<DT><STRONG>sequence</STRONG></DT>
<DD>is a string that denotes the target kind of event. (See the bind
man page and page 201 of John Ousterhout's book for details).
<P>
</DD>
<DT><STRONG>func</STRONG></DT>
<DD>is a Python function, taking one argument, to be invoked when the
event occurs. An Event instance will be passed as the argument.
(Functions deployed this way are commonly known as <var>callbacks</var>.)
<P>
</DD>
<DT><STRONG>add</STRONG></DT>
<DD>is optional, either "<tt class="samp"></tt>" or "<tt class="samp">+</tt>". Passing an empty string
denotes that this binding is to replace any other bindings that this
event is associated with. Preceeding with a "<tt class="samp">+</tt>" means that this
function is to be added to the list of functions bound to this event type.
</DD>
</DL>
<P>
For example:
<div class="verbatim"><pre>
def turnRed(self, event):
event.widget["activeforeground"] = "red"
self.button.bind("<Enter>", self.turnRed)
</pre></div>
<P>
Notice how the widget field of the event is being accessed in the
<tt class="method">turnRed()</tt> callback. This field contains the widget that
caught the X event. The following table lists the other event fields
you can access, and how they are denoted in Tk, which can be useful
when referring to the Tk man pages.
<P>
<div class="verbatim"><pre>
Tk Tkinter Event Field Tk Tkinter Event Field
-- ------------------- -- -------------------
%f focus %A char
%h height %E send_event
%k keycode %K keysym
%s state %N keysym_num
%t time %T type
%w width %W widget
%x x %X x_root
%y y %Y y_root
</pre></div>
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<span class="release-info">Release 2.4.3, documentation updated on 29 March 2006.</span>
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