Python Library Reference

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7.3 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 7.3.1 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 lists of `waitable objects': either integers representing file descriptors or objects with a parameterless method named fileno() returning such an integer. The three lists of waitable objects are for input, output and `exceptional conditions', respectively. Empty lists are allowed, but acceptance of three empty lists 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 lists 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 desciptors that don't originate from WinSock.

• 7.3.1 Polling Objects

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Python Library Reference

Previous: 7.2.3 Example Up: 7. Optional Operating System Next: 7.3.1 Polling Objects

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Release 2.2.3, documentation updated on 30 May 2003.