16.1. select — Waiting for I/O completion

This module provides access to the select() and poll() functions available in most operating systems, epoll() available on Linux 2.5+ and kqueue() 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.

The module defines the following:

exception select.error

The exception raised when an error occurs. The accompanying value is a pair containing the numeric error code from errno and the corresponding string, as would be printed by the C function perror().


(Only supported on Linux 2.5.44 and newer.) Returns an edge polling object, which can be used as Edge or Level Triggered interface for I/O events; see section Edge and Level Trigger Polling (epoll) Objects below for the methods supported by epolling objects.

New in version 2.6.


(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 Polling Objects below for the methods supported by polling objects.


(Only supported on BSD.) Returns a kernel queue object; see section Kqueue Objects below for the methods supported by kqueue objects.

New in version 2.6.

select.kevent(ident, filter=KQ_FILTER_READ, flags=KQ_EV_ADD, fflags=0, data=0, udata=0)

(Only supported on BSD.) Returns a kernel event object; see section Kevent Objects below for the methods supported by kevent objects.

New in version 2.6.

select.select(rlist, wlist, xlist[, timeout])

This is a straightforward interface to the Unix select() system call. The first three arguments are sequences of ‘waitable objects’: either integers representing file descriptors or objects with a parameterless method named fileno() returning such an integer:

  • rlist: wait until ready for reading
  • wlist: wait until ready for writing
  • xlist: wait for an “exceptional condition” (see the manual page for what your system considers such a condition)

Empty sequences are allowed, but acceptance of three empty sequences is platform-dependent. (It is known to work on Unix but not on Windows.) The optional timeout argument specifies a time-out as a floating point number in seconds. When the timeout 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.

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.

Among the acceptable object types in the sequences are Python file objects (e.g. sys.stdin, or objects returned by open() or os.popen()), socket objects returned by socket.socket(). You may also define a wrapper class yourself, as long as it has an appropriate fileno() method (that really returns a file descriptor, not just a random integer).


File objects on Windows are not acceptable, but sockets are. On Windows, the underlying select() function is provided by the WinSock library, and does not handle file descriptors that don’t originate from WinSock.


Files reported as ready for writing by select(), poll() or similar interfaces in this module are guaranteed to not block on a write of up to PIPE_BUF bytes. This value is guaranteed by POSIX to be at least 512. Availability: Unix.

New in version 2.7.

16.1.1. Edge and Level Trigger Polling (epoll) Objects



Constant Meaning
EPOLLIN Available for read
EPOLLOUT Available for write
EPOLLPRI Urgent data for read
EPOLLERR Error condition happened on the assoc. fd
EPOLLHUP Hang up happened on the assoc. fd
EPOLLET Set Edge Trigger behavior, the default is Level Trigger behavior
EPOLLONESHOT Set one-shot behavior. After one event is pulled out, the fd is internally disabled
EPOLLRDBAND Priority data band can be read.
EPOLLWRBAND Priority data may be written.

Close the control file descriptor of the epoll object.


Return the file descriptor number of the control fd.


Create an epoll object from a given file descriptor.

epoll.register(fd[, eventmask])

Register a fd descriptor with the epoll object.


Registering a file descriptor that’s already registered raises an IOError – contrary to Polling Objects‘s register.

epoll.modify(fd, eventmask)

Modify a register file descriptor.


Remove a registered file descriptor from the epoll object.

epoll.poll([timeout=-1[, maxevents=-1]])

Wait for events. timeout in seconds (float)

16.1.2. Polling Objects

The poll() system call, supported on most Unix systems, provides better scalability for network servers that service many, many clients at the same time. poll() scales better because the system call only requires listing the file descriptors of interest, while select() builds a bitmap, turns on bits for the fds of interest, and then afterward the whole bitmap has to be linearly scanned again. select() is O(highest file descriptor), while poll() is O(number of file descriptors).

poll.register(fd[, eventmask])

Register a file descriptor with the polling object. Future calls to the poll() method will then check whether the file descriptor has any pending I/O events. fd can be either an integer, or an object with a fileno() method that returns an integer. File objects implement fileno(), so they can also be used as the argument.

eventmask is an optional bitmask describing the type of events you want to check for, and can be a combination of the constants POLLIN, POLLPRI, and POLLOUT, described in the table below. If not specified, the default value used will check for all 3 types of events.

Constant Meaning
POLLIN There is data to read
POLLPRI There is urgent data to read
POLLOUT Ready for output: writing will not block
POLLERR Error condition of some sort
POLLNVAL Invalid request: descriptor not open

Registering a file descriptor that’s already registered is not an error, and has the same effect as registering the descriptor exactly once.

poll.modify(fd, eventmask)

Modifies an already registered fd. This has the same effect as register(fd, eventmask). Attempting to modify a file descriptor that was never registered causes an IOError exception with errno ENOENT to be raised.

New in version 2.6.


Remove a file descriptor being tracked by a polling object. Just like the register() method, fd can be an integer or an object with a fileno() method that returns an integer.

Attempting to remove a file descriptor that was never registered causes a KeyError exception to be raised.


Polls the set of registered file descriptors, and returns a possibly-empty list containing (fd, event) 2-tuples for the descriptors that have events or errors to report. fd is the file descriptor, and event is a bitmask with bits set for the reported events for that descriptor — POLLIN for waiting input, POLLOUT 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 timeout is given, it specifies the length of time in milliseconds which the system will wait for events before returning. If timeout is omitted, negative, or None, the call will block until there is an event for this poll object.

16.1.3. Kqueue Objects


Close the control file descriptor of the kqueue object.


Return the file descriptor number of the control fd.


Create a kqueue object from a given file descriptor.

kqueue.control(changelist, max_events[, timeout=None]) → eventlist

Low level interface to kevent

  • changelist must be an iterable of kevent object or None
  • max_events must be 0 or a positive integer
  • timeout in seconds (floats possible)

16.1.4. Kevent Objects



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 fileno() function. kevent stores the integer internally.


Name of the kernel filter.

Constant Meaning
KQ_FILTER_READ Takes a descriptor and returns whenever there is data available to read
KQ_FILTER_WRITE Takes a descriptor and returns whenever there is data available to write
KQ_FILTER_VNODE Returns when one or more of the requested events watched in fflag occurs
KQ_FILTER_PROC Watch for events on a process id
KQ_FILTER_NETDEV Watch for events on a network device [not available on Mac OS X]
KQ_FILTER_SIGNAL Returns whenever the watched signal is delivered to the process
KQ_FILTER_TIMER Establishes an arbitrary timer

Filter action.

Constant Meaning
KQ_EV_ADD Adds or modifies an event
KQ_EV_DELETE Removes an event from the queue
KQ_EV_ENABLE Permitscontrol() to returns the event
KQ_EV_DISABLE Disablesevent
KQ_EV_ONESHOT Removes event after first occurrence
KQ_EV_CLEAR Reset the state after an event is retrieved
KQ_EV_SYSFLAGS internal event
KQ_EV_FLAG1 internal event
KQ_EV_EOF Filter specific EOF condition
KQ_EV_ERROR See return values

Filter specific flags.


Constant Meaning
KQ_NOTE_LOWAT low water mark of a socket buffer

KQ_FILTER_VNODE filter flags:

Constant Meaning
KQ_NOTE_DELETE unlink() was called
KQ_NOTE_WRITE a write occurred
KQ_NOTE_EXTEND the file was extended
KQ_NOTE_ATTRIB an attribute was changed
KQ_NOTE_LINK the link count has changed
KQ_NOTE_RENAME the file was renamed
KQ_NOTE_REVOKE access to the file was revoked

KQ_FILTER_PROC filter flags:

Constant Meaning
KQ_NOTE_EXIT the process has exited
KQ_NOTE_FORK the process has called fork()
KQ_NOTE_EXEC the process has executed a new process
KQ_NOTE_PCTRLMASK internal filter flag
KQ_NOTE_PDATAMASK internal filter flag
KQ_NOTE_TRACK follow a process across fork()
KQ_NOTE_CHILD returned on the child process for NOTE_TRACK
KQ_NOTE_TRACKERR unable to attach to a child

KQ_FILTER_NETDEV filter flags (not available on Mac OS X):

Constant Meaning
KQ_NOTE_LINKUP link is up
KQ_NOTE_LINKDOWN link is down
KQ_NOTE_LINKINV link state is invalid

Filter specific data.


User defined value.