# `select` – 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).

## Functions

select.poll()

  Create an instance of the Poll class.

select.select(rlist, wlist, xlist[, timeout])

  Wait for activity on a set of objects.

  This function is provided by some MicroPython ports for compatibility and is not efficient. Usage of `Poll` is recommended instead.

## class `Poll`

### Methods

poll.register(obj[, eventmask])

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

  - `select.POLLIN` - data available for reading

  - `select.POLLOUT` - more data can be written

  Note that flags like `select.POLLHUP` and `select.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 `select.POLLIN | select.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()`).

poll.unregister(obj)

  Unregister obj from polling.

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 `select.POLL*` constants described above. Note that flags `select.POLLHUP` and `select.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 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 MicroPython extension.

Language and External Links

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External links
  openmv.io
  forums.openmv.io
  github.com/openmv/openmv
  micropython.org
  forum.micropython.org
  github.com/micropython/micropython