select --- 等待 I/O 完成

---

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.

備註

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.

可用性: not WASI.

此模組在 WebAssembly 平台上不起作用或無法使用。更多資訊請參閱 WebAssembly 平台。

The module defines the following:

exception select.error

A deprecated alias of OSError.

在 3.3 版的變更: Following PEP 3151, this class was made an alias of OSError.

select.devpoll()

(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.

在 3.3 版被加入.

在 3.4 版的變更: The new file descriptor is now non-inheritable.

select.epoll(sizehint=-1, flags=0)

(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.

在 3.3 版的變更: 新增 flags 參數。

在 3.4 版的變更: Support for the with statement was added. The new file descriptor is now non-inheritable.

在 3.4 版之後被棄用: The flags parameter. select.EPOLL_CLOEXEC is used by default now. Use os.set_inheritable() to make the file descriptor inheritable.

select.poll()

(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.

select.kqueue()

(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.

在 3.4 版的變更: 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=None)

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 or None, 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).

備註

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.

在 3.5 版的變更: 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.

select.PIPE_BUF

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 kinds of file-like objects such as sockets.

This value is guaranteed by POSIX to be at least 512.

可用性: Unix

在 3.2 版被加入.

/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.

devpoll.close()

Close the file descriptor of the polling object.

在 3.4 版被加入.

devpoll.closed

True if the polling object is closed.

在 3.4 版被加入.

devpoll.fileno()

Return the file descriptor number of the polling object.

在 3.4 版被加入.

devpoll.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. The constants are the same as with poll() object. The default value is a combination of the constants POLLIN, POLLPRI, and POLLOUT.

警告

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().

devpoll.modify(fd[, eventmask])

This method does an unregister() followed by a register(). It is (a bit) more efficient than doing the same explicitly.

devpoll.unregister(fd)

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.

devpoll.poll([timeout])

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.

在 3.5 版的變更: 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

常數	含義
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