select — Waiting for I/O completion¶
This module provides access to the select() and poll() functions available in most operating systems, devpoll() available on Solaris and derivatives, 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 selectors module allows high-level and efficient I/O multiplexing, built upon the select module primitives. Users are encouraged to use the selectors module instead, unless they want precise control over the OS-level primitives used.
This module does not work or is not available on WebAssembly platforms wasm32-emscripten and wasm32-wasi . See WebAssembly platforms for more information.
The module defines the following:
Changed in version 3.3: Following PEP 3151, this class was made an alias of OSError .
(Only supported on Solaris and derivatives.) Returns a /dev/poll polling object; see section /dev/poll Polling Objects below for the methods supported by devpoll objects.
devpoll() objects are linked to the number of file descriptors allowed at the time of instantiation. If your program reduces this value, devpoll() will fail. If your program increases this value, devpoll() may return an incomplete list of active file descriptors.
The new file descriptor is non-inheritable .
Changed in version 3.4: The new file descriptor is now non-inheritable.
(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.
sizehint informs epoll about the expected number of events to be registered. It must be positive, or -1 to use the default. It is only used on older systems where epoll_create1() is not available; otherwise it has no effect (though its value is still checked).
flags is deprecated and completely ignored. However, when supplied, its value must be 0 or select.EPOLL_CLOEXEC , otherwise OSError is raised.
See the Edge and Level Trigger Polling (epoll) Objects section below for the methods supported by epolling objects.
epoll objects support the context management protocol: when used in a with statement, the new file descriptor is automatically closed at the end of the block.
The new file descriptor is non-inheritable .
Changed in version 3.3: Added the flags parameter.
Changed in version 3.4: Support for the with statement was added. The new file descriptor is now non-inheritable.
Deprecated since version 3.4: The flags parameter. select.EPOLL_CLOEXEC is used by default now. Use os.set_inheritable() to make the file descriptor inheritable.
(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.
The new file descriptor is non-inheritable .
Changed in version 3.4: The new file descriptor is now non-inheritable.
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.
select. select ( rlist , wlist , xlist [ , timeout ] ) ¶
This is a straightforward interface to the Unix select() system call. The first three arguments are iterables 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 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 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 iterables 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.
Changed in version 3.5: The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see PEP 475 for the rationale), instead of raising InterruptedError .
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 select() , poll() or another interface in this module. This doesn’t apply to other kind of file-like objects such as sockets.
This value is guaranteed by POSIX to be at least 512.
/dev/poll Polling Objects¶
Solaris and derivatives have /dev/poll . While select() is O(highest file descriptor) and poll() is O(number of file descriptors), /dev/poll is O(active file descriptors).
/dev/poll behaviour is very close to the standard poll() object.
Close the file descriptor of the polling object.
True if the polling object is closed.
Return the file descriptor number of the polling object.
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. The constants are the same that with poll() object. The default value is a combination of the constants POLLIN , POLLPRI , and POLLOUT .
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 poll() .
This method does an unregister() followed by a register() . It is (a bit) more efficient that doing the same explicitly.
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 is safely ignored.
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, -1, or None , the call will block until there is an event for this poll object.
Changed in version 3.5: The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see PEP 475 for the rationale), instead of raising InterruptedError .
Edge and Level Trigger Polling (epoll) Objects¶
https://linux.die.net/man/4/epoll
eventmask
| 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 |
| EPOLLEXCLUSIVE | 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. |
| EPOLLRDHUP | Stream socket peer closed connection or shut down writing half of connection. |
| EPOLLRDNORM | Equivalent to EPOLLIN |
| EPOLLRDBAND | Priority data band can be read. |
| EPOLLWRNORM | Equivalent to EPOLLOUT |
| EPOLLWRBAND | Priority data may be written. |
| EPOLLMSG | Ignored. |
New in version 3.6: EPOLLEXCLUSIVE was added. It’s only supported by Linux Kernel 4.5 or later.
Close the control file descriptor of the epoll object.
True if the epoll object is closed.
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.
epoll. modify ( fd , eventmask ) ¶
Modify a registered file descriptor.
Remove a registered file descriptor from the epoll object.
Changed in version 3.9: The method no longer ignores the EBADF error.
Wait for events. timeout in seconds (float)
Changed in version 3.5: The function is now retried with a recomputed timeout when interrupted by a signal, except if the signal handler raises an exception (see PEP 475 for the rationale), instead of raising InterruptedError .
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.
Что такое select?
Наткнулся на select в Python но информации мало про то что это такое. Я так понял это что то связанное с сокетами. Кто-нибудь расскажите, что это вообще такое?
Представьте себе, что вы пишите web сервер. И к нашему серверу подключено 100 юзеров. И это, к примеру, чатик.
Так вот, наши юзеры — это живые люди, и они не могут физически загрузить весь канал информацией, а значит, что каждое соединение с пользователем большую часть времени. будет просто висеть.
Предположим что у нас есть массив дескрипторов сокетов (или просто массив сокетов для упрощения) и мы тупо в бесконечном цикле обходим каждый и спрашиваем «есть че? нет? Ну ладно, следующий». В этой ситуации у нас 90% времени цикл будет пускаться в холостую и просто бернить процессорное время. А этим процессорным временем мог между прочим воспользоваться другой процесс, который тоже что-то важное делает.
Так вот, вместо того что бы бернить попросту процессорное время, мы можем загнать массив дескрипторов в функцию select, и она предоставит нам список сокетов, в которых мы можем отправлять данные, в которых есть что-то новенькое для чтения, и те, в которых произошли ошибки.
Более того, эта функция ничто иное как просто системный вызов. Она реализована не в python даже а в операционной системе, а вы просто работаете с оберткой. И если мы делали все тупым перебором, то в современных операционках за это отвечают весьма хитрые и оптимизированные алгоритмы.
Ну короч как-то так. Если вам хочется больше информации, не привязывайтесь к python, эта штука есть в любом языке программирования и работает одинаково. Так же помимо select есть еще poll и epoll.