python-peps/pep-3156.txt

PEP: 3156
Title: Asynchronous IO Support Rebooted
Version: $Revision$
Last-Modified: $Date$
Author: Guido van Rossum <guido@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 12-Dec-2012
Post-History: TBD

Abstract
========

This is a proposal for asynchronous I/O in Python 3, starting with
Python 3.3.  Consider this the concrete proposal that is missing from
PEP 3153.  The proposal includes a pluggable event loop API, transport
and protocol abstractions similar to those in Twisted, and a
higher-level scheduler based on yield-from (PEP 380).  A reference
implementation is in the works under the code name tulip.


Introduction
============

The event loop is the place where most interoperability occurs.  It
should be easy for (Python 3.3 ports of) frameworks like Twisted,
Tornado, or ZeroMQ to either adapt the default event loop
implementation to their needs using a lightweight wrapper or proxy, or
to replace the default event loop implementation with an adaptation of
their own event loop implementation.  (Some frameworks, like Twisted,
have multiple event loop implementations.  This should not be a
problem since these all have the same interface.)

It should even be possible for two different third-party frameworks to
interoperate, either by sharing the default event loop implementation
(each using its own adapter), or by sharing the event loop
implementation of either framework.  In the latter case two levels of
adaptation would occur (from framework A's event loop to the standard
event loop interface, and from there to framework B's event loop).
Which event loop implementation is used should be under control of the
main program (though a default policy for event loop selection is
provided).

Thus, two separate APIs are defined:

- getting and setting the current event loop object
- the interface of a conforming event loop and its minimum guarantees

An event loop implementation may provide additional methods and
guarantees.

The event loop interface does not depend on yield-from.  Rather, it
uses a combination of callbacks, additional interfaces (transports and
protocols), and Futures.  The latter are similar to those defined in
PEP 3148, but have a different implementation and are not tied to
threads.  In particular, they have no wait() method; the user is
expected to use callbacks.

For users (like myself) who don't like using callbacks, a scheduler is
provided for writing asynchronous I/O code as coroutines using the PEP
380 yield-from expressions.  The scheduler is not pluggable;
pluggability occurs at the event loop level, and the scheduler should
work with any conforming event loop implementation.

For interoperability between code written using coroutines and other
async frameworks, the scheduler has a Task class that behaves like a
Future.  A framework that interoperates at the event loop level can
wait for a Future to complete by adding a callback to the Future.
Likewise, the scheduler offers an operation to suspend a coroutine
until a callback is called.

Limited interoperability with threads is provided by the event loop
interface; there is an API to submit a function to an executor (see
PEP 3148) which returns a Future that is compatible with the event
loop.


Non-goals
=========

Interoperability with systems like Stackless Python or
greenlets/gevent is not a goal of this PEP.


Specification
=============

Dependencies
------------

Python 3.3 is required.  No new language or standard library features
beyond Python 3.3 are required.  No third-party modules or packages
are required.

Module Namespace
----------------

The specification here will live in a new toplevel package.  Different
components will live in separate submodules of that package.  The
package will import common APIs from their respective submodules and
make them available as package attributes (similar to the way the
email package works).

The name of the toplevel package is currently unspecified.  The
reference implementation uses the name 'tulip', but the name will
change to something more boring if and when the implementation is
moved into the standard library (hopefully for Python 3.4).

Until the boring name is chosen, this PEP will use 'tulip' as the
toplevel package name.  Classes and functions given without a module
name are assumed to be accessed via the toplevel package.

Event Loop Policy: Getting and Setting the Event Loop
-----------------------------------------------------

To get the current event loop, use ``get_event_loop()``.  This returns
an instance of the ``EventLoop`` class defined below or an equivalent
object.  It is possible that ``get_event_loop()`` returns a different
object depending on the current thread, or depending on some other
notion of context.

To set the current event loop, use ``set_event_loop(eventloop)``,
where ``eventloop`` is an instance of the ``EventLoop`` class or
equivalent.  This uses the same notion of context as
``get_event_loop()``.

To change the way ``get_event_loop()`` and ``set_event_loop()`` work
(including their notion of context), call
``set_event_loop_policy(policy)``, where ``policy`` is an event loop
policy object.  The policy object can be any object that has methods
``get_event_loop()`` and ``set_event_loop(eventloop)`` behaving like
the functions described above.  The default event loop policy is an
instance of the class ``DefaultEventLoopPolicy``.  The current event loop
policy object can be retrieved by calling ``get_event_loop_policy()``.

An event loop policy may but does not have to enforce that there is
only one event loop in existence.  The default event loop policy does
not enforce this, but it does enforce that there is only one event
loop per thread.

Event Loop Interface
--------------------

A conforming event loop object has the following methods:

..
  Look for a better way to format method docs.  PEP 12 doesn't
  seem to have one.  PEP 418 uses ^^^, which makes sub-headings.

- ``run()``.  Runs the event loop until there is nothing left to do.
  This means, in particular:
  
  - No more calls scheduled with ``call_later()`` (except for canceled
    calls).

  - No more registered file descriptors.  It is up to the registering
    party to unregister a file descriptor when it is closed.

- ``call_later(when, callback, *args)``.  Arrange for
  ``callback(*args)`` to be called approximately ``when`` seconds in
  the future, once, unless canceled.  As usual in Python, ``when`` may
  be a floating point number to represent smaller intervals.  Returns
  a ``DelayedCall`` object representing the callback, whose
  ``cancel()`` method can be used to cancel the callback.

- ``call_soon(callback, *args)``.  Equivalent to ``call_later(0,
  callback, *args)``.

- ``call_soon_threadsafe(callback, *args)``.  Like
  ``call_soon(callback, *args)``, but when called from another thread
  while the event loop is blocked waiting for I/O, unblocks the event
  loop.  This is the _only_ method that is safe to call from another
  thread or from a signal handler.  (To schedule a callback for a
  later time in a threadsafe manner, you can use
  ``ev.call_soon_threadsafe(ev.call_later, when, callback, *args)``.)

The following methods for registering callbacks for file descriptors
are optional.  The default implementation provides them but the user
normally doesn't use these directly -- they are used by the transport
implementations exclusively:

- ``add_reader(fd, callback, *args)``.  Arrange for
  ``callback(*args)`` to be called whenever file descriptor ``fd`` is
  ready for reading.  Returns a ``DelayedCall`` object which can be
  used to cancel the callback.  Note that, unlike ``call_later()``,
  the callback may be called many times.  Calling ``add_reader()``
  again for the same file descriptor implicitly cancels the previous
  callback for that file descriptor.

- ``add_writer(fd, callback, *args)``.  Like ``add_reader()``,
  but registers the callback for writing instead of for reading.

- ``remove_reader(fd)``.  Cancels the current read callback for file
  descriptor ``fd``, if one is set.  A no-op if no callback is
  currently set for the file descriptor.  (The reason for providing
  this alternate interface is that it is often more convenient to
  remember the file descriptor than to remember the ``DelayedCall``
  object.)

- ``remove_writer(fd)``.  This is to ``add_writer()`` as
  ``remove_reader()`` is to ``add_reader()``.

- TBD: A method to submit a call to a PEP 3148 executor.  Or a method
  to wait for a PEP 3148 Future.  Or both.

- TBD: Methods that return ``Futures``, in particular to make a
  connection and to set up a listener.

Callback Sequencing
-------------------

When two callbacks are scheduled for the same time, they are run
in the order in which they are registered.  For example::

  ev.call_soon(foo)
  ev.call_soon(bar)

guarantees that ``foo()`` is called before ``bar()``.

If ``call_soon()`` is used, this guarantee is even true if the system
clock were to run backwards.  This is also the case for
``call_later(0, callback, *args)``.  However, if ``call_later()`` is
used with a nonzero ``when`` argument, all bets are off if the system
clock were to runs backwards.  (A good event loop implementation
should use ``time.monotonic()`` to avoid problems when the clock runs
backward.  See PEP 418.)

Context
-------

All event loops have a notion of context.  For the default event loop
implementation, the context is a thread.  An event loop implementation
should run all callbacks in the same context.  An event loop
implementation should run only one callback at a time, so callbacks
can assume automatic mutual exclusion with other callbacks scheduled
in the same event loop.

The DelayedCall Class
---------------------

TBD.  (Only one method, ``cancel()``, and a read-only property,
``canceled``.  Perhaps also ``callback`` and ``args`` properties.)

Futures
-------

TBD.

Transports
----------

TBD.

Protocols
---------

TBD.

Coroutines and the Scheduler
----------------------------

TBD.

Callback Style
--------------

Most interfaces taking a callback also take positional arguments.  For
instance, to arrange for ``foo("abc", 42)`` to be called soon, you
call ``ev.call_soon(foo, "abc", 42)``.  To schedule the call
``foo()``, use ``ev.call_soon(foo)``.  This convention greatly reduces
the number of small lambdas required in typical callback programming.

This convention specifically does _not_ support keyword arguments.
Keyword arguments are used to pass optional extra information about
the callback.  This allows graceful evolution of the API without
having to worry about whether a keyword might be significant to a
callee somewhere.  If you have a callback that _must_ be called with a
keyword argument, you can use a lambda or ``functools.partial``.  For
example::

  ev.call_soon(functools.partial(foo, "abc", repeat=42))

Choosing an Event Loop Implementation
-------------------------------------

TBD.  (This is about the choice to use e.g. select vs. poll vs. epoll,
and how to override the choice.  Probably belongs in the event loop
policy.)


Open Issues
===========

- What have I missed that hasn't been marked with TBD yet?

- A better name for ``DelayedCall`` (I really don't like adjectives. :-)

- Do we need an API for stopping the event loop, given that we have
  the termination condition?  Is the termination condition compatible
  with other frameworks?

- Do we need an API to run the event loop for a little while?

- Should we have ``future.add_callback(callback, *args)``, using the
  convention from the section "Callback Style" above, or should we
  stick with the PEP 3148 specification of
  ``future.add_done_callback(callback)`` which calls
  ``callback(future)``?  (Glyph suggested using a different method
  name since add_done_callback() does not guarantee that the callback
  will be called in the right context.)

- Do we need introspection APIs?  E.g. asking for the read callback
  given a file descriptor.


Acknowledgments
===============

Apart from PEP 3153, influences include PEP 380 and Greg Ewing's
tutorial for yield-from, Twisted, Tornado, ZeroMQ, pyftpdlib, tulip
(the author's attempts at synthesis of all these), wattle (Steve
Dower's counter-proposal), numerous discussions on python-ideas from
September through December 2012, a Skype session with Steve Dower and
Dino Viehland, email exchanges with Ben Darnell, an audience with
Niels Provos (original author of libevent), and two in-person meetings
with several Twisted developers, including Glyph, Brian Warner, David
Reid, and Duncan McGreggor.


Copyright
=========

This document has been placed in the public domain.



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