<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <html> <head> <title>7.5.4.1 Semaphore Example </title> <META NAME="description" CONTENT="7.5.4.1 Semaphore Example "> <META NAME="keywords" CONTENT="lib"> <META NAME="resource-type" CONTENT="document"> <META NAME="distribution" CONTENT="global"> <meta http-equiv="Content-Type" content="text/html; charset="> <link rel="STYLESHEET" href="lib.css"> <link rel="first" href="lib.html"> <link rel="contents" href="contents.html" title="Contents"> <link rel="index" href="genindex.html" title="Index"> <LINK REL="previous" href="semaphore-objects.html"> <LINK REL="up" href="semaphore-objects.html"> <LINK REL="next" href="event-objects.html"> </head> <body> <DIV CLASS="navigation"> <table align="center" width="100%" cellpadding="0" cellspacing="2"> <tr> <td><A href="semaphore-objects.html"><img src="../icons/previous.gif" border="0" height="32" alt="Previous Page" width="32"></A></td> <td><A href="semaphore-objects.html"><img src="../icons/up.gif" border="0" height="32" alt="Up One Level" width="32"></A></td> <td><A href="event-objects.html"><img src="../icons/next.gif" border="0" height="32" alt="Next Page" width="32"></A></td> <td align="center" width="100%">Python Library Reference</td> <td><A href="contents.html"><img src="../icons/contents.gif" border="0" height="32" alt="Contents" width="32"></A></td> <td><a href="modindex.html" title="Module Index"><img src="../icons/modules.gif" border="0" height="32" alt="Module Index" width="32"></a></td> <td><A href="genindex.html"><img src="../icons/index.gif" border="0" height="32" alt="Index" width="32"></A></td> </tr></table> <b class="navlabel">Previous:</b> <a class="sectref" href="semaphore-objects.html">7.5.4 Semaphore Objects</A> <b class="navlabel">Up:</b> <a class="sectref" href="semaphore-objects.html">7.5.4 Semaphore Objects</A> <b class="navlabel">Next:</b> <a class="sectref" href="event-objects.html">7.5.5 Event Objects</A> <br><hr> </DIV> <!--End of Navigation Panel--> <H3><A NAME="SECTION009541000000000000000"> </A> <BR> 7.5.4.1 <tt class="class">Semaphore</tt> Example </H3> <P> Semaphores are often used to guard resources with limited capacity, for example, a database server. In any situation where the size of the resource size is fixed, you should use a bounded semaphore. Before spawning any worker threads, your main thread would initialize the semaphore: <P> <dl><dd><pre class="verbatim"> maxconnections = 5 ... pool_sema = BoundedSemaphore(value=maxconnections) </pre></dl> <P> Once spawned, worker threads call the semaphore's acquire and release methods when they need to connect to the server: <P> <dl><dd><pre class="verbatim"> pool_sema.acquire() conn = connectdb() ... use connection ... conn.close() pool_sema.release() </pre></dl> <P> The use of a bounded semaphore reduces the chance that a programming error which causes the semaphore to be released more than it's acquired will go undetected. <P> <DIV CLASS="navigation"> <p><hr> <table align="center" width="100%" cellpadding="0" cellspacing="2"> <tr> <td><A href="semaphore-objects.html"><img src="../icons/previous.gif" border="0" height="32" alt="Previous Page" width="32"></A></td> <td><A href="semaphore-objects.html"><img src="../icons/up.gif" border="0" height="32" alt="Up One Level" width="32"></A></td> <td><A href="event-objects.html"><img src="../icons/next.gif" border="0" height="32" alt="Next Page" width="32"></A></td> <td align="center" width="100%">Python Library Reference</td> <td><A href="contents.html"><img src="../icons/contents.gif" border="0" height="32" alt="Contents" width="32"></A></td> <td><a href="modindex.html" title="Module Index"><img src="../icons/modules.gif" border="0" height="32" alt="Module Index" width="32"></a></td> <td><A href="genindex.html"><img src="../icons/index.gif" border="0" height="32" alt="Index" width="32"></A></td> </tr></table> <b class="navlabel">Previous:</b> <a class="sectref" href="semaphore-objects.html">7.5.4 Semaphore Objects</A> <b class="navlabel">Up:</b> <a class="sectref" href="semaphore-objects.html">7.5.4 Semaphore Objects</A> <b class="navlabel">Next:</b> <a class="sectref" href="event-objects.html">7.5.5 Event Objects</A> <hr> <span class="release-info">Release 2.2, documentation updated on December 21, 2001.</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>
