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
(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
- 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
Among the acceptable object types in the lists are Python file
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
desciptors that don't originate from WinSock.
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