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.
register(obj[, eventmask[, userdata]])¶
streamobj 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.POLLERRare 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.
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.
Unregister obj from polling. If obj was not previously registered,
KeyErrorexception will be raised.
As a Pycopy extension, if second parameter is False, exception will not be raised.
Modify the eventmask for obj. If obj is not registered,
OSErroris raised with error of ENOENT.
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 (
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
eventelement specifies which events happened with a stream and is a combination of
uselect.POLL*constants described above. Note that flags
uselect.POLLERRcan 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.
ipoll(timeout=-1, flags=0, /)¶
If flags is 1, one-shot behaviour 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 behaviour 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
websocket objects, etc. Some
Pycopy ports may allow to pass such objects to
However the overall API contract for them is slightly relaxed: if such
a wrapper was returned as suitable forreading (
reading it may still lead to the EAGAIN underlying error (and e.g.
None returned from .read() method). Similarly for
That’s unlike native stream objects, for which it’s guaranteed that
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).