# 15.1 `select` -- Waiting for I/O completion

This module provides access to the `select()` and `poll()` functions available in most operating systems. 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 `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()`.

`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 15.1.1 below for the methods supported by polling objects.

`select`( | iwtd, owtd, ewtd[, 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. The three sequences of waitable objects are for input, output and `exceptional conditions', respectively. 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). Note: 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.