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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<html>

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<head>

<title>The Python X Library: 5.1 Getting Events</title>

<!--

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</head>

<td valign="middle" align="left">[<a href="python-xlib_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>

<td valign="middle" align="left">[Index]</td>

</tr></table>

<hr>

<h2 class="section">5.1 Getting Events</h2>

<p>Events can be sent at any time, not necessarily when the client is ready

to receive an event. Therefore they must be stored temporarily from that

they are read from the network until the client is ready to handle them.

Read but unhandled events are stored on an event queue in the Display

object. There are two functions to access this queue:

</p>

<dl>

<dt><a name="index-next_005fevent-on-Display"></a>Method on Display: <strong>next_event</strong><em> ( )</em></dt>

<dd>

<p>Return the next event in the event queue. If the event queue is empty,

block until an event is read from the network, and return that one.

</p>

</dd></dl>

<dl>

<dt><a name="index-pending_005fevents-on-Display"></a>Method on Display: <strong>pending_events</strong><em> ( )</em></dt>

<dd>

<p>Return the number of events which can be returned without blocking.

</p>

</dd></dl>

<p>A trivial event loop would simply loop infinitely, waiting for an

event and then handling it. It could look like this:

</p>

<pre class="example">while 1:

event = disp.next_event()

handle_event(event)

</pre></div>

<p>However, most applications need more control, e.g. to simultaneously

handle a network connection or at regular intervals schedule timeouts.

The module <code>select</code> is often used for this. <code>Display</code> objects

can be used with <code>select</code>, since they have the required

<code>fileno()</code> method. When <code>select</code> indicates that a

<code>Display</code> object is ready for reading, that means that the server

has sent some data to the client. That alone doesn’t guarantee that an

entire event has arrived, so one must first use <code>pending_events()</code>

to make sure that <code>next_event()</code> will return without blocking. A

simple event loop which waits for events or a one-second timeout looks

like this:

</p>

<pre class="example">while 1:

# Wait for display to send something, or a timeout of one second

readable, w, e = select.select([disp], [], [], 1)

# if no files are ready to be read, it's an timeout

if not readable:

handle_timeout()

# if display is readable, handle as many events as have been received

elif disp in readable:

i = disp.pending_events()

while i > 0:

event = disp.next_event()

handle_event(event)

i = i - 1

# loop around to wait for more things to happen

</pre></div>

<hr>

<td valign="middle" align="left">[<a href="python-xlib_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>

<td valign="middle" align="left">[Index]</td>

</tr></table>

<p>

This document was generated on <i>September 16, 2020</i> using <a href="http://www.nongnu.org/texi2html/"><i>texi2html 5.0</i></a>.

</font>

<br>

</p>

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