uselect – wait for events on a set of streams

This module implements a subset of the corresponding CPython module, as described below. For more information, refer to the original CPython documentation: select.

This module provides functions to efficiently wait for events on multiple streams (select streams which are ready for operations).



Create an instance of the Poll class., wlist, xlist[, timeout])

This function is inefficient and not supported by Pycopy. Use poll() instead. Full select module with this function is available in pycopy-lib.

class Poll


poll.register(obj[, eventmask[, userdata]])

Register stream obj for polling. eventmask is logical OR of:

  • uselect.POLLIN - data available for reading
  • uselect.POLLOUT - more data can be written

Note that flags like uselect.POLLHUP and uselect.POLLERR are not valid as input eventmask (these are unsolicited events which will be returned from poll() regardless of whether they are asked for). This semantics is per POSIX.

eventmask defaults to uselect.POLLIN | uselect.POLLOUT.

As a Pycopy extension, an arbitrary userdata can be passed, which will be associated with the added stream and which will be returned from ipoll() method (but NOT from poll() method).

It is OK to call this function multiple times for the same obj. Successive calls will update obj’s eventmask to the value of eventmask (i.e. will behave as modify()), and, if userdata is specified, it will be updated too.

poll.unregister(obj, throw=True)

Unregister obj from polling. If obj was not previously registered, KeyError exception will be raised.

As a Pycopy extension, if second parameter is False, exception will not be raised.

poll.modify(obj, eventmask)

Modify the eventmask for obj. If obj is not registered, OSError is raised with error of ENOENT.

poll.poll(timeout=-1, /)

Wait for at least one of the registered objects to become ready or have an exceptional condition, with optional timeout in milliseconds (if timeout arg is not specified or -1, there is no timeout).

Returns list of (obj, event, …) tuples. There may be other elements in tuple, depending on a platform and version, so don’t assume that its size is 2. The event element specifies which events happened with a stream and is a combination of uselect.POLL* constants described above. Note that flags uselect.POLLHUP and uselect.POLLERR can be returned at any time (even if were not asked for), and must be acted on accordingly (the corresponding stream unregistered from poll and likely closed), because otherwise all further invocations of poll() may return immediately with these flags set for this stream again.

In case of timeout, an empty list is returned.

Difference to CPython

Tuples returned may contain more than 2 elements as described above.

poll.ipoll(timeout=-1, flags=0, /)

Like poll.poll(), but instead returns an iterator which yields a callee-owned tuple. This function provides an efficient, allocation-free way to poll on streams.

If flags is 1, one-shot behavior for events is employed: streams for which events happened will have their event masks automatically reset (equivalent to poll.modify(obj, 0)), so new events for such a stream won’t be processed until new mask is set with poll.modify(). This behavior is useful for asynchronous I/O schedulers.

Difference to CPython

This function is a Pycopy extension.

Polling stream wrapper objects

Pycopy supports a concept of “stream wrapper objects”, where an original stream object (like a file or socket) is wrapped with an object which provides stream API, but also some additional functionality. Examples include ussl objects, websocket objects, etc. Some Pycopy ports may allow to pass such objects to poll.register. However the overall API contract for them is slightly relaxed: if such a wrapper was returned as suitable forreading (uselect.POLLIN), reading it may still lead to the EAGAIN underlying error (and e.g. None returned from .read() method). Similarly for uselect.POLLOUT. That’s unlike native stream objects, for which it’s guaranteed that after uselect.POLLIN is signalled, the .read() call will return some data (but that can be as small as 1 byte). This happens because a wrapper object may buffer some input data and/or process it internally (e.g. part of TLS/websocket framing and not user data transferred via them). Applications which may accept both native and wrapper streams should be prepared to deal with that.

Applications which are interested in the highest performance and larger portability may instead separate concepts of “polled stream” and “I/O stream”. A polled stream is always the original stream object, before any wrappers applied to it. An I/O stream is a top-level wrapper. An application would keep a pair of polled and I/O streams, and use the former to pass to uselect functions, while the latter - to read/write (still being ready to receive EAGAIN/None as described above).