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
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().
- (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.
||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
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.
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