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+ <span id="select-waiting-for-i-o-completion"></span><h1>select — Waiting for I/O completion</h1>  <p>This module provides access to the <code>select()</code> and <code>poll()</code> functions available in most operating systems, <code>devpoll()</code> available on Solaris and derivatives, <code>epoll()</code> available on Linux 2.5+ and <code>kqueue()</code> available on most BSD. 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> <div class="admonition note"> <p class="admonition-title">Note</p> <p>The <a class="reference internal" href="selectors#module-selectors" title="selectors: High-level I/O multiplexing."><code>selectors</code></a> module allows high-level and efficient I/O multiplexing, built upon the <a class="reference internal" href="#module-select" title="select: Wait for I/O completion on multiple streams."><code>select</code></a> module primitives. Users are encouraged to use the <a class="reference internal" href="selectors#module-selectors" title="selectors: High-level I/O multiplexing."><code>selectors</code></a> module instead, unless they want precise control over the OS-level primitives used.</p> </div> <div class="availability docutils container"> <p><a class="reference internal" href="https://docs.python.org/3.12/library/intro.html#availability"><span class="std std-ref">Availability</span></a>: not Emscripten, not WASI.</p> <p>This module does not work or is not available on WebAssembly platforms <code>wasm32-emscripten</code> and <code>wasm32-wasi</code>. See <a class="reference internal" href="https://docs.python.org/3.12/library/intro.html#wasm-availability"><span class="std std-ref">WebAssembly platforms</span></a> for more information.</p> </div> <p>The module defines the following:</p> <dl class="py exception"> <dt class="sig sig-object py" id="select.error">
+<code>exception select.error</code> </dt> <dd>
+<p>A deprecated alias of <a class="reference internal" href="exceptions#OSError" title="OSError"><code>OSError</code></a>.</p> <div class="versionchanged"> <p><span class="versionmodified changed">Changed in version 3.3: </span>Following <span class="target" id="index-0"></span><a class="pep reference external" href="https://peps.python.org/pep-3151/"><strong>PEP 3151</strong></a>, this class was made an alias of <a class="reference internal" href="exceptions#OSError" title="OSError"><code>OSError</code></a>.</p> </div> </dd>
+</dl> <dl class="py function"> <dt class="sig sig-object py" id="select.devpoll">
+<code>select.devpoll()</code> </dt> <dd>
+<p>(Only supported on Solaris and derivatives.) Returns a <code>/dev/poll</code> polling object; see section <a class="reference internal" href="#devpoll-objects"><span class="std std-ref">/dev/poll Polling Objects</span></a> below for the methods supported by devpoll objects.</p> <p><code>devpoll()</code> objects are linked to the number of file descriptors allowed at the time of instantiation. If your program reduces this value, <code>devpoll()</code> will fail. If your program increases this value, <code>devpoll()</code> may return an incomplete list of active file descriptors.</p> <p>The new file descriptor is <a class="reference internal" href="os#fd-inheritance"><span class="std std-ref">non-inheritable</span></a>.</p> <div class="versionadded"> <p><span class="versionmodified added">New in version 3.3.</span></p> </div> <div class="versionchanged"> <p><span class="versionmodified changed">Changed in version 3.4: </span>The new file descriptor is now non-inheritable.</p> </div> </dd>
+</dl> <dl class="py function"> <dt class="sig sig-object py" id="select.epoll">
+<code>select.epoll(sizehint=- 1, flags=0)</code> </dt> <dd>
+<p>(Only supported on Linux 2.5.44 and newer.) Return an edge polling object, which can be used as Edge or Level Triggered interface for I/O events.</p> <p><em>sizehint</em> informs epoll about the expected number of events to be registered. It must be positive, or <code>-1</code> to use the default. It is only used on older systems where <code>epoll_create1()</code> is not available; otherwise it has no effect (though its value is still checked).</p> <p><em>flags</em> is deprecated and completely ignored. However, when supplied, its value must be <code>0</code> or <code>select.EPOLL_CLOEXEC</code>, otherwise <code>OSError</code> is raised.</p> <p>See the <a class="reference internal" href="#epoll-objects"><span class="std std-ref">Edge and Level Trigger Polling (epoll) Objects</span></a> section below for the methods supported by epolling objects.</p> <p><code>epoll</code> objects support the context management protocol: when used in a <a class="reference internal" href="../reference/compound_stmts#with"><code>with</code></a> statement, the new file descriptor is automatically closed at the end of the block.</p> <p>The new file descriptor is <a class="reference internal" href="os#fd-inheritance"><span class="std std-ref">non-inheritable</span></a>.</p> <div class="versionchanged"> <p><span class="versionmodified changed">Changed in version 3.3: </span>Added the <em>flags</em> parameter.</p> </div> <div class="versionchanged"> <p><span class="versionmodified changed">Changed in version 3.4: </span>Support for the <a class="reference internal" href="../reference/compound_stmts#with"><code>with</code></a> statement was added. The new file descriptor is now non-inheritable.</p> </div> <div class="deprecated"> <p><span class="versionmodified deprecated">Deprecated since version 3.4: </span>The <em>flags</em> parameter. <code>select.EPOLL_CLOEXEC</code> is used by default now. Use <a class="reference internal" href="os#os.set_inheritable" title="os.set_inheritable"><code>os.set_inheritable()</code></a> to make the file descriptor inheritable.</p> </div> </dd>
+</dl> <dl class="py function"> <dt class="sig sig-object py" id="select.poll">
+<code>select.poll()</code> </dt> <dd>
+<p>(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 class="reference internal" href="#poll-objects"><span class="std std-ref">Polling Objects</span></a> below for the methods supported by polling objects.</p> </dd>
+</dl> <dl class="py function"> <dt class="sig sig-object py" id="select.kqueue">
+<code>select.kqueue()</code> </dt> <dd>
+<p>(Only supported on BSD.) Returns a kernel queue object; see section <a class="reference internal" href="#kqueue-objects"><span class="std std-ref">Kqueue Objects</span></a> below for the methods supported by kqueue objects.</p> <p>The new file descriptor is <a class="reference internal" href="os#fd-inheritance"><span class="std std-ref">non-inheritable</span></a>.</p> <div class="versionchanged"> <p><span class="versionmodified changed">Changed in version 3.4: </span>The new file descriptor is now non-inheritable.</p> </div> </dd>
+</dl> <dl class="py function"> <dt class="sig sig-object py" id="select.kevent">
+<code>select.kevent(ident, filter=KQ_FILTER_READ, flags=KQ_EV_ADD, fflags=0, data=0, udata=0)</code> </dt> <dd>
+<p>(Only supported on BSD.) Returns a kernel event object; see section <a class="reference internal" href="#kevent-objects"><span class="std std-ref">Kevent Objects</span></a> below for the methods supported by kevent objects.</p> </dd>
+</dl> <dl class="py function"> <dt class="sig sig-object py" id="select.select">
+<code>select.select(rlist, wlist, xlist[, timeout])</code> </dt> <dd>
+<p>This is a straightforward interface to the Unix <code>select()</code> system call. The first three arguments are iterables of ‘waitable objects’: either integers representing file descriptors or objects with a parameterless method named <a class="reference internal" href="io#io.IOBase.fileno" title="io.IOBase.fileno"><code>fileno()</code></a> returning such an integer:</p> <ul class="simple"> <li>
+<em>rlist</em>: wait until ready for reading</li> <li>
+<em>wlist</em>: wait until ready for writing</li> <li>
+<em>xlist</em>: wait for an “exceptional condition” (see the manual page for what your system considers such a condition)</li> </ul> <p>Empty iterables are allowed, but acceptance of three empty iterables is platform-dependent. (It is known to work on Unix but not on Windows.) The optional <em>timeout</em> argument specifies a time-out as a floating point number in seconds. When the <em>timeout</em> 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> <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> <p id="index-1">Among the acceptable object types in the iterables are Python <a class="reference internal" href="../glossary#term-file-object"><span class="xref std std-term">file objects</span></a> (e.g. <code>sys.stdin</code>, or objects returned by <a class="reference internal" href="functions#open" title="open"><code>open()</code></a> or <a class="reference internal" href="os#os.popen" title="os.popen"><code>os.popen()</code></a>), socket objects returned by <a class="reference internal" href="socket#socket.socket" title="socket.socket"><code>socket.socket()</code></a>. You may also define a <em class="dfn">wrapper</em> class yourself, as long as it has an appropriate <a class="reference internal" href="io#io.IOBase.fileno" title="io.IOBase.fileno"><code>fileno()</code></a> method (that really returns a file descriptor, not just a random integer).</p> <div class="admonition note"> <p class="admonition-title">Note</p> <p id="index-2">File objects on Windows are not acceptable, but sockets are. On Windows, the underlying <code>select()</code> function is provided by the WinSock library, and does not handle file descriptors that don’t originate from WinSock.</p> </div> <div class="versionchanged"> <p><span class="versionmodified changed">Changed in version 3.5: </span>The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see <span class="target" id="index-3"></span><a class="pep reference external" href="https://peps.python.org/pep-0475/"><strong>PEP 475</strong></a> for the rationale), instead of raising <a class="reference internal" href="exceptions#InterruptedError" title="InterruptedError"><code>InterruptedError</code></a>.</p> </div> </dd>
+</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="select.PIPE_BUF">
+<code>select.PIPE_BUF</code> </dt> <dd>
+<p>The minimum number of bytes which can be written without blocking to a pipe when the pipe has been reported as ready for writing by <a class="reference internal" href="#select.select" title="select.select"><code>select()</code></a>, <code>poll()</code> or another interface in this module. This doesn’t apply to other kind of file-like objects such as sockets.</p> <p>This value is guaranteed by POSIX to be at least 512.</p> <div class="availability docutils container"> <p><a class="reference internal" href="https://docs.python.org/3.12/library/intro.html#availability"><span class="std std-ref">Availability</span></a>: Unix</p> </div> <div class="versionadded"> <p><span class="versionmodified added">New in version 3.2.</span></p> </div> </dd>
+</dl> <section id="dev-poll-polling-objects"> <span id="devpoll-objects"></span><h2>
+<code>/dev/poll</code> Polling Objects</h2> <p>Solaris and derivatives have <code>/dev/poll</code>. While <code>select()</code> is O(highest file descriptor) and <code>poll()</code> is O(number of file descriptors), <code>/dev/poll</code> is O(active file descriptors).</p> <p><code>/dev/poll</code> behaviour is very close to the standard <code>poll()</code> object.</p> <dl class="py method"> <dt class="sig sig-object py" id="select.devpoll.close">
+<code>devpoll.close()</code> </dt> <dd>
+<p>Close the file descriptor of the polling object.</p> <div class="versionadded"> <p><span class="versionmodified added">New in version 3.4.</span></p> </div> </dd>
+</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="select.devpoll.closed">
+<code>devpoll.closed</code> </dt> <dd>
+<p><code>True</code> if the polling object is closed.</p> <div class="versionadded"> <p><span class="versionmodified added">New in version 3.4.</span></p> </div> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.devpoll.fileno">
+<code>devpoll.fileno()</code> </dt> <dd>
+<p>Return the file descriptor number of the polling object.</p> <div class="versionadded"> <p><span class="versionmodified added">New in version 3.4.</span></p> </div> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.devpoll.register">
+<code>devpoll.register(fd[, eventmask])</code> </dt> <dd>
+<p>Register a file descriptor with the polling object. Future calls to the <a class="reference internal" href="#select.poll" title="select.poll"><code>poll()</code></a> method will then check whether the file descriptor has any pending I/O events. <em>fd</em> can be either an integer, or an object with a <a class="reference internal" href="io#io.IOBase.fileno" title="io.IOBase.fileno"><code>fileno()</code></a> method that returns an integer. File objects implement <code>fileno()</code>, so they can also be used as the argument.</p> <p><em>eventmask</em> is an optional bitmask describing the type of events you want to check for. The constants are the same that with <code>poll()</code> object. The default value is a combination of the constants <code>POLLIN</code>, <code>POLLPRI</code>, and <code>POLLOUT</code>.</p> <div class="admonition warning"> <p class="admonition-title">Warning</p> <p>Registering a file descriptor that’s already registered is not an error, but the result is undefined. The appropriate action is to unregister or modify it first. This is an important difference compared with <code>poll()</code>.</p> </div> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.devpoll.modify">
+<code>devpoll.modify(fd[, eventmask])</code> </dt> <dd>
+<p>This method does an <a class="reference internal" href="#select.devpoll.unregister" title="select.devpoll.unregister"><code>unregister()</code></a> followed by a <a class="reference internal" href="#select.devpoll.register" title="select.devpoll.register"><code>register()</code></a>. It is (a bit) more efficient that doing the same explicitly.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.devpoll.unregister">
+<code>devpoll.unregister(fd)</code> </dt> <dd>
+<p>Remove a file descriptor being tracked by a polling object. Just like the <a class="reference internal" href="#select.devpoll.register" title="select.devpoll.register"><code>register()</code></a> method, <em>fd</em> can be an integer or an object with a <a class="reference internal" href="io#io.IOBase.fileno" title="io.IOBase.fileno"><code>fileno()</code></a> method that returns an integer.</p> <p>Attempting to remove a file descriptor that was never registered is safely ignored.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.devpoll.poll">
+<code>devpoll.poll([timeout])</code> </dt> <dd>
+<p>Polls the set of registered file descriptors, and returns a possibly empty list containing <code>(fd, event)</code> 2-tuples for the descriptors that have events or errors to report. <em>fd</em> is the file descriptor, and <em>event</em> is a bitmask with bits set for the reported events for that descriptor — <code>POLLIN</code> for waiting input, <code>POLLOUT</code> to indicate that the descriptor can be written to, and so forth. An empty list indicates that the call timed out and no file descriptors had any events to report. If <em>timeout</em> is given, it specifies the length of time in milliseconds which the system will wait for events before returning. If <em>timeout</em> is omitted, -1, or <a class="reference internal" href="constants#None" title="None"><code>None</code></a>, the call will block until there is an event for this poll object.</p> <div class="versionchanged"> <p><span class="versionmodified changed">Changed in version 3.5: </span>The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see <span class="target" id="index-4"></span><a class="pep reference external" href="https://peps.python.org/pep-0475/"><strong>PEP 475</strong></a> for the rationale), instead of raising <a class="reference internal" href="exceptions#InterruptedError" title="InterruptedError"><code>InterruptedError</code></a>.</p> </div> </dd>
+</dl> </section> <section id="edge-and-level-trigger-polling-epoll-objects"> <span id="epoll-objects"></span><h2>Edge and Level Trigger Polling (epoll) Objects</h2>  <p><a class="reference external" href="https://linux.die.net/man/4/epoll">https://linux.die.net/man/4/epoll</a></p> <p><em>eventmask</em></p> <table class="docutils align-default">  <thead> <tr>
+<th class="head"><p>Constant</p></th> <th class="head"><p>Meaning</p></th> </tr> </thead>  <tr>
+<td><p><code>EPOLLIN</code></p></td> <td><p>Available for read</p></td> </tr> <tr>
+<td><p><code>EPOLLOUT</code></p></td> <td><p>Available for write</p></td> </tr> <tr>
+<td><p><code>EPOLLPRI</code></p></td> <td><p>Urgent data for read</p></td> </tr> <tr>
+<td><p><code>EPOLLERR</code></p></td> <td><p>Error condition happened on the assoc. fd</p></td> </tr> <tr>
+<td><p><code>EPOLLHUP</code></p></td> <td><p>Hang up happened on the assoc. fd</p></td> </tr> <tr>
+<td><p><code>EPOLLET</code></p></td> <td><p>Set Edge Trigger behavior, the default is Level Trigger behavior</p></td> </tr> <tr>
+<td><p><code>EPOLLONESHOT</code></p></td> <td><p>Set one-shot behavior. After one event is pulled out, the fd is internally disabled</p></td> </tr> <tr>
+<td><p><code>EPOLLEXCLUSIVE</code></p></td> <td><p>Wake only one epoll object when the associated fd has an event. The default (if this flag is not set) is to wake all epoll objects polling on a fd.</p></td> </tr> <tr>
+<td><p><code>EPOLLRDHUP</code></p></td> <td><p>Stream socket peer closed connection or shut down writing half of connection.</p></td> </tr> <tr>
+<td><p><code>EPOLLRDNORM</code></p></td> <td><p>Equivalent to <code>EPOLLIN</code></p></td> </tr> <tr>
+<td><p><code>EPOLLRDBAND</code></p></td> <td><p>Priority data band can be read.</p></td> </tr> <tr>
+<td><p><code>EPOLLWRNORM</code></p></td> <td><p>Equivalent to <code>EPOLLOUT</code></p></td> </tr> <tr>
+<td><p><code>EPOLLWRBAND</code></p></td> <td><p>Priority data may be written.</p></td> </tr> <tr>
+<td><p><code>EPOLLMSG</code></p></td> <td><p>Ignored.</p></td> </tr>  </table> <div class="versionadded"> <p><span class="versionmodified added">New in version 3.6: </span><code>EPOLLEXCLUSIVE</code> was added. It’s only supported by Linux Kernel 4.5 or later.</p> </div>  <dl class="py method"> <dt class="sig sig-object py" id="select.epoll.close">
+<code>epoll.close()</code> </dt> <dd>
+<p>Close the control file descriptor of the epoll object.</p> </dd>
+</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="select.epoll.closed">
+<code>epoll.closed</code> </dt> <dd>
+<p><code>True</code> if the epoll object is closed.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.epoll.fileno">
+<code>epoll.fileno()</code> </dt> <dd>
+<p>Return the file descriptor number of the control fd.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.epoll.fromfd">
+<code>epoll.fromfd(fd)</code> </dt> <dd>
+<p>Create an epoll object from a given file descriptor.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.epoll.register">
+<code>epoll.register(fd[, eventmask])</code> </dt> <dd>
+<p>Register a fd descriptor with the epoll object.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.epoll.modify">
+<code>epoll.modify(fd, eventmask)</code> </dt> <dd>
+<p>Modify a registered file descriptor.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.epoll.unregister">
+<code>epoll.unregister(fd)</code> </dt> <dd>
+<p>Remove a registered file descriptor from the epoll object.</p> <div class="versionchanged"> <p><span class="versionmodified changed">Changed in version 3.9: </span>The method no longer ignores the <a class="reference internal" href="errno#errno.EBADF" title="errno.EBADF"><code>EBADF</code></a> error.</p> </div> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.epoll.poll">
+<code>epoll.poll(timeout=None, maxevents=- 1)</code> </dt> <dd>
+<p>Wait for events. timeout in seconds (float)</p> <div class="versionchanged"> <p><span class="versionmodified changed">Changed in version 3.5: </span>The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see <span class="target" id="index-5"></span><a class="pep reference external" href="https://peps.python.org/pep-0475/"><strong>PEP 475</strong></a> for the rationale), instead of raising <a class="reference internal" href="exceptions#InterruptedError" title="InterruptedError"><code>InterruptedError</code></a>.</p> </div> </dd>
+</dl> </section> <section id="polling-objects"> <span id="poll-objects"></span><h2>Polling Objects</h2> <p>The <code>poll()</code> system call, supported on most Unix systems, provides better scalability for network servers that service many, many clients at the same time. <code>poll()</code> scales better because the system call only requires listing the file descriptors of interest, while <code>select()</code> builds a bitmap, turns on bits for the fds of interest, and then afterward the whole bitmap has to be linearly scanned again. <code>select()</code> is O(highest file descriptor), while <code>poll()</code> is O(number of file descriptors).</p> <dl class="py method"> <dt class="sig sig-object py" id="select.poll.register">
+<code>poll.register(fd[, eventmask])</code> </dt> <dd>
+<p>Register a file descriptor with the polling object. Future calls to the <a class="reference internal" href="#select.poll" title="select.poll"><code>poll()</code></a> method will then check whether the file descriptor has any pending I/O events. <em>fd</em> can be either an integer, or an object with a <a class="reference internal" href="io#io.IOBase.fileno" title="io.IOBase.fileno"><code>fileno()</code></a> method that returns an integer. File objects implement <code>fileno()</code>, so they can also be used as the argument.</p> <p><em>eventmask</em> is an optional bitmask describing the type of events you want to check for, and can be a combination of the constants <code>POLLIN</code>, <code>POLLPRI</code>, and <code>POLLOUT</code>, described in the table below. If not specified, the default value used will check for all 3 types of events.</p> <table class="docutils align-default">  <thead> <tr>
+<th class="head"><p>Constant</p></th> <th class="head"><p>Meaning</p></th> </tr> </thead>  <tr>
+<td><p><code>POLLIN</code></p></td> <td><p>There is data to read</p></td> </tr> <tr>
+<td><p><code>POLLPRI</code></p></td> <td><p>There is urgent data to read</p></td> </tr> <tr>
+<td><p><code>POLLOUT</code></p></td> <td><p>Ready for output: writing will not block</p></td> </tr> <tr>
+<td><p><code>POLLERR</code></p></td> <td><p>Error condition of some sort</p></td> </tr> <tr>
+<td><p><code>POLLHUP</code></p></td> <td><p>Hung up</p></td> </tr> <tr>
+<td><p><code>POLLRDHUP</code></p></td> <td><p>Stream socket peer closed connection, or shut down writing half of connection</p></td> </tr> <tr>
+<td><p><code>POLLNVAL</code></p></td> <td><p>Invalid request: descriptor not open</p></td> </tr>  </table> <p>Registering a file descriptor that’s already registered is not an error, and has the same effect as registering the descriptor exactly once.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.poll.modify">
+<code>poll.modify(fd, eventmask)</code> </dt> <dd>
+<p>Modifies an already registered fd. This has the same effect as <code>register(fd, eventmask)</code>. Attempting to modify a file descriptor that was never registered causes an <a class="reference internal" href="exceptions#OSError" title="OSError"><code>OSError</code></a> exception with errno <code>ENOENT</code> to be raised.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.poll.unregister">
+<code>poll.unregister(fd)</code> </dt> <dd>
+<p>Remove a file descriptor being tracked by a polling object. Just like the <a class="reference internal" href="#select.poll.register" title="select.poll.register"><code>register()</code></a> method, <em>fd</em> can be an integer or an object with a <a class="reference internal" href="io#io.IOBase.fileno" title="io.IOBase.fileno"><code>fileno()</code></a> method that returns an integer.</p> <p>Attempting to remove a file descriptor that was never registered causes a <a class="reference internal" href="exceptions#KeyError" title="KeyError"><code>KeyError</code></a> exception to be raised.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.poll.poll">
+<code>poll.poll([timeout])</code> </dt> <dd>
+<p>Polls the set of registered file descriptors, and returns a possibly empty list containing <code>(fd, event)</code> 2-tuples for the descriptors that have events or errors to report. <em>fd</em> is the file descriptor, and <em>event</em> is a bitmask with bits set for the reported events for that descriptor — <code>POLLIN</code> for waiting input, <code>POLLOUT</code> to indicate that the descriptor can be written to, and so forth. An empty list indicates that the call timed out and no file descriptors had any events to report. If <em>timeout</em> is given, it specifies the length of time in milliseconds which the system will wait for events before returning. If <em>timeout</em> is omitted, negative, or <a class="reference internal" href="constants#None" title="None"><code>None</code></a>, the call will block until there is an event for this poll object.</p> <div class="versionchanged"> <p><span class="versionmodified changed">Changed in version 3.5: </span>The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see <span class="target" id="index-6"></span><a class="pep reference external" href="https://peps.python.org/pep-0475/"><strong>PEP 475</strong></a> for the rationale), instead of raising <a class="reference internal" href="exceptions#InterruptedError" title="InterruptedError"><code>InterruptedError</code></a>.</p> </div> </dd>
+</dl> </section> <section id="kqueue-objects"> <span id="id1"></span><h2>Kqueue Objects</h2> <dl class="py method"> <dt class="sig sig-object py" id="select.kqueue.close">
+<code>kqueue.close()</code> </dt> <dd>
+<p>Close the control file descriptor of the kqueue object.</p> </dd>
+</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="select.kqueue.closed">
+<code>kqueue.closed</code> </dt> <dd>
+<p><code>True</code> if the kqueue object is closed.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.kqueue.fileno">
+<code>kqueue.fileno()</code> </dt> <dd>
+<p>Return the file descriptor number of the control fd.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.kqueue.fromfd">
+<code>kqueue.fromfd(fd)</code> </dt> <dd>
+<p>Create a kqueue object from a given file descriptor.</p> </dd>
+</dl> <dl class="py method"> <dt class="sig sig-object py" id="select.kqueue.control">
+<code>kqueue.control(changelist, max_events[, timeout]) → eventlist</code> </dt> <dd>
+<p>Low level interface to kevent</p> <ul class="simple"> <li>changelist must be an iterable of kevent objects or <code>None</code>
+</li> <li>max_events must be 0 or a positive integer</li> <li>timeout in seconds (floats possible); the default is <code>None</code>, to wait forever</li> </ul> <div class="versionchanged"> <p><span class="versionmodified changed">Changed in version 3.5: </span>The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see <span class="target" id="index-7"></span><a class="pep reference external" href="https://peps.python.org/pep-0475/"><strong>PEP 475</strong></a> for the rationale), instead of raising <a class="reference internal" href="exceptions#InterruptedError" title="InterruptedError"><code>InterruptedError</code></a>.</p> </div> </dd>
+</dl> </section> <section id="kevent-objects"> <span id="id2"></span><h2>Kevent Objects</h2> <p><a class="reference external" href="https://man.freebsd.org/cgi/man.cgi?query=kqueue&amp;sektion=2">https://man.freebsd.org/cgi/man.cgi?query=kqueue&amp;sektion=2</a></p> <dl class="py attribute"> <dt class="sig sig-object py" id="select.kevent.ident">
+<code>kevent.ident</code> </dt> <dd>
+<p>Value used to identify the event. The interpretation depends on the filter but it’s usually the file descriptor. In the constructor ident can either be an int or an object with a <a class="reference internal" href="io#io.IOBase.fileno" title="io.IOBase.fileno"><code>fileno()</code></a> method. kevent stores the integer internally.</p> </dd>
+</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="select.kevent.filter">
+<code>kevent.filter</code> </dt> <dd>
+<p>Name of the kernel filter.</p> <table class="docutils align-default">  <thead> <tr>
+<th class="head"><p>Constant</p></th> <th class="head"><p>Meaning</p></th> </tr> </thead>  <tr>
+<td><p><code>KQ_FILTER_READ</code></p></td> <td><p>Takes a descriptor and returns whenever there is data available to read</p></td> </tr> <tr>
+<td><p><code>KQ_FILTER_WRITE</code></p></td> <td><p>Takes a descriptor and returns whenever there is data available to write</p></td> </tr> <tr>
+<td><p><code>KQ_FILTER_AIO</code></p></td> <td><p>AIO requests</p></td> </tr> <tr>
+<td><p><code>KQ_FILTER_VNODE</code></p></td> <td><p>Returns when one or more of the requested events watched in <em>fflag</em> occurs</p></td> </tr> <tr>
+<td><p><code>KQ_FILTER_PROC</code></p></td> <td><p>Watch for events on a process id</p></td> </tr> <tr>
+<td><p><code>KQ_FILTER_NETDEV</code></p></td> <td><p>Watch for events on a network device [not available on macOS]</p></td> </tr> <tr>
+<td><p><code>KQ_FILTER_SIGNAL</code></p></td> <td><p>Returns whenever the watched signal is delivered to the process</p></td> </tr> <tr>
+<td><p><code>KQ_FILTER_TIMER</code></p></td> <td><p>Establishes an arbitrary timer</p></td> </tr>  </table> </dd>
+</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="select.kevent.flags">
+<code>kevent.flags</code> </dt> <dd>
+<p>Filter action.</p> <table class="docutils align-default">  <thead> <tr>
+<th class="head"><p>Constant</p></th> <th class="head"><p>Meaning</p></th> </tr> </thead>  <tr>
+<td><p><code>KQ_EV_ADD</code></p></td> <td><p>Adds or modifies an event</p></td> </tr> <tr>
+<td><p><code>KQ_EV_DELETE</code></p></td> <td><p>Removes an event from the queue</p></td> </tr> <tr>
+<td><p><code>KQ_EV_ENABLE</code></p></td> <td><p>Permitscontrol() to returns the event</p></td> </tr> <tr>
+<td><p><code>KQ_EV_DISABLE</code></p></td> <td><p>Disablesevent</p></td> </tr> <tr>
+<td><p><code>KQ_EV_ONESHOT</code></p></td> <td><p>Removes event after first occurrence</p></td> </tr> <tr>
+<td><p><code>KQ_EV_CLEAR</code></p></td> <td><p>Reset the state after an event is retrieved</p></td> </tr> <tr>
+<td><p><code>KQ_EV_SYSFLAGS</code></p></td> <td><p>internal event</p></td> </tr> <tr>
+<td><p><code>KQ_EV_FLAG1</code></p></td> <td><p>internal event</p></td> </tr> <tr>
+<td><p><code>KQ_EV_EOF</code></p></td> <td><p>Filter specific EOF condition</p></td> </tr> <tr>
+<td><p><code>KQ_EV_ERROR</code></p></td> <td><p>See return values</p></td> </tr>  </table> </dd>
+</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="select.kevent.fflags">
+<code>kevent.fflags</code> </dt> <dd>
+<p>Filter specific flags.</p> <p><code>KQ_FILTER_READ</code> and <code>KQ_FILTER_WRITE</code> filter flags:</p> <table class="docutils align-default">  <thead> <tr>
+<th class="head"><p>Constant</p></th> <th class="head"><p>Meaning</p></th> </tr> </thead>  <tr>
+<td><p><code>KQ_NOTE_LOWAT</code></p></td> <td><p>low water mark of a socket buffer</p></td> </tr>  </table> <p><code>KQ_FILTER_VNODE</code> filter flags:</p> <table class="docutils align-default">  <thead> <tr>
+<th class="head"><p>Constant</p></th> <th class="head"><p>Meaning</p></th> </tr> </thead>  <tr>
+<td><p><code>KQ_NOTE_DELETE</code></p></td> <td><p><em>unlink()</em> was called</p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_WRITE</code></p></td> <td><p>a write occurred</p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_EXTEND</code></p></td> <td><p>the file was extended</p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_ATTRIB</code></p></td> <td><p>an attribute was changed</p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_LINK</code></p></td> <td><p>the link count has changed</p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_RENAME</code></p></td> <td><p>the file was renamed</p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_REVOKE</code></p></td> <td><p>access to the file was revoked</p></td> </tr>  </table> <p><code>KQ_FILTER_PROC</code> filter flags:</p> <table class="docutils align-default">  <thead> <tr>
+<th class="head"><p>Constant</p></th> <th class="head"><p>Meaning</p></th> </tr> </thead>  <tr>
+<td><p><code>KQ_NOTE_EXIT</code></p></td> <td><p>the process has exited</p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_FORK</code></p></td> <td><p>the process has called <em>fork()</em></p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_EXEC</code></p></td> <td><p>the process has executed a new process</p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_PCTRLMASK</code></p></td> <td><p>internal filter flag</p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_PDATAMASK</code></p></td> <td><p>internal filter flag</p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_TRACK</code></p></td> <td><p>follow a process across <em>fork()</em></p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_CHILD</code></p></td> <td><p>returned on the child process for <em>NOTE_TRACK</em></p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_TRACKERR</code></p></td> <td><p>unable to attach to a child</p></td> </tr>  </table> <p><code>KQ_FILTER_NETDEV</code> filter flags (not available on macOS):</p> <table class="docutils align-default">  <thead> <tr>
+<th class="head"><p>Constant</p></th> <th class="head"><p>Meaning</p></th> </tr> </thead>  <tr>
+<td><p><code>KQ_NOTE_LINKUP</code></p></td> <td><p>link is up</p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_LINKDOWN</code></p></td> <td><p>link is down</p></td> </tr> <tr>
+<td><p><code>KQ_NOTE_LINKINV</code></p></td> <td><p>link state is invalid</p></td> </tr>  </table> </dd>
+</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="select.kevent.data">
+<code>kevent.data</code> </dt> <dd>
+<p>Filter specific data.</p> </dd>
+</dl> <dl class="py attribute"> <dt class="sig sig-object py" id="select.kevent.udata">
+<code>kevent.udata</code> </dt> <dd>
+<p>User defined value.</p> </dd>
+</dl> </section> <div class="_attribution">
+  <p class="_attribution-p">
+    &copy; 2001&ndash;2023 Python Software Foundation<br>Licensed under the PSF License.<br>
+    <a href="https://docs.python.org/3.12/library/select.html" class="_attribution-link">https://docs.python.org/3.12/library/select.html</a>
+  </p>
+</div>