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<H1><A NAME="SECTION009300000000000000000">
7.3 <tt class="module">select</tt> --
Waiting for I/O completion</A>
</H1>
<P>
<P>
This module provides access to the <tt class="cfunction">select()</tt>
and <tt class="cfunction">poll()</tt> functions
available in most operating systems. Note that on Windows, it only
works for sockets; on other operating systems, it also works for other
file types (in particular, on Unix, it works on pipes). It cannot
be used on regular files to determine whether a file has grown since
it was last read.
<P>
The module defines the following:
<P>
<dl><dt><b><span class="typelabel">exception</span> <a name="l2h-1916"><tt class="exception">error</tt></a></b>
<dd>
The exception raised when an error occurs. The accompanying value is
a pair containing the numeric error code from <tt class="cdata">errno</tt> and the
corresponding string, as would be printed by the C function
<tt class="cfunction">perror()</tt>.
</dl>
<P>
<dl><dt><b><a name="l2h-1917"><tt class="function">poll</tt></a></b>()
<dd>
(Not supported by all operating systems.) Returns a polling object,
which supports registering and unregistering file descriptors, and
then polling them for I/O events;
see section <A href="poll-objects.html#poll-objects">7.3.1</A> below for the methods supported by
polling objects.
</dl>
<P>
<dl><dt><b><a name="l2h-1918"><tt class="function">select</tt></a></b>(<var>iwtd, owtd, ewtd</var><big>[</big><var>, timeout</var><big>]</big>)
<dd>
This is a straightforward interface to the Unix <tt class="cfunction">select()</tt>
system call. The first three arguments are lists of `waitable
objects': either integers representing file descriptors or
objects with a parameterless method named <tt class="method">fileno()</tt> returning
such an integer. The three lists of waitable objects are for input,
output and `exceptional conditions', respectively. Empty lists are
allowed, but acceptance of three empty lists is platform-dependent.
(It is known to work on Unix but not on Windows.) The optional
<var>timeout</var> argument specifies a time-out as a floating point number
in seconds. When the <var>timeout</var> argument is omitted the function
blocks until at least one file descriptor is ready. A time-out value
of zero specifies a poll and never blocks.
<P>
The return value is a triple of lists of objects that are ready:
subsets of the first three arguments. When the time-out is reached
without a file descriptor becoming ready, three empty lists are
returned.
<P>
Amongst the acceptable object types in the lists are Python file
objects (e.g. <code>sys.stdin</code>, or objects returned by
<tt class="function">open()</tt> or <tt class="function">os.popen()</tt>), socket objects
returned by <tt class="function">socket.socket()</tt>,<a name="l2h-1920"> </a><a name="l2h-1922"> </a>.
You may also define a <i class="dfn">wrapper</i> class yourself, as long as it has
an appropriate <tt class="method">fileno()</tt> method (that really returns a file
descriptor, not just a random integer).
<span class="note"><b class="label">Note:</b>
File objects on Windows are not acceptable, but sockets
are.<a name="l2h-1923"> </a> On Windows, the underlying <tt class="cfunction">select()</tt>
function is provided by the WinSock library, and does not handle file
desciptors that don't originate from WinSock.</span>
</dl>
<P>
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<span class="release-info">Release 2.2, documentation updated on December 21, 2001.</span>
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