python-peps/pep-0492.txt

PEP: 492
Title: Coroutines with async and await syntax
Version: $Revision$
Last-Modified: $Date$
Author: Yury Selivanov <yselivanov@sprymix.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 09-Apr-2015
Python-Version: 3.5


Abstract
========

This PEP introduces new syntax for coroutines, asynchronous ``with``
statements and ``for`` loops.  The main motivation behind this proposal is to
streamline writing and maintaining asynchronous code, as well as to simplify
previously hard to implement code patterns.


Rationale and Goals
===================

Current Python supports implementing coroutines via generators (PEP 342),
further enhanced by the ``yield from`` syntax introduced in PEP 380.
This approach has a number of shortcomings:

* it is easy to confuse coroutines with regular generators, since they share
  the same syntax; async libraries often attempt to alleviate this by using
  decorators (e.g. ``@asyncio.coroutine`` [1]_);

* it is not possible to natively define a coroutine which has no ``yield``
  or  ``yield from`` statements, again requiring the use of decorators to
  fix potential refactoring issues;

* support for asynchronous calls is limited to expressions where ``yield`` is
  allowed syntactically, limiting the usefulness of syntactic features, such
  as ``with`` and ``for`` statements.

This proposal makes coroutines a native Python language feature, and clearly
separates them from generators.  This removes generator/coroutine ambiguity,
and makes it possible to reliably define coroutines without reliance on a
specific library.  This also enables linters and IDEs to improve static code
analysis and refactoring.

Native coroutines and the associated new syntax features make it possible
to define context manager and iteration protocols in asynchronous terms.
As shown later in this proposal, the new ``async with`` statement lets Python
programs perform asynchronous calls when entering and exiting a runtime
context, and the new ``async for`` statement makes it possible to perform
asynchronous calls in iterators.


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

This proposal introduces new syntax and semantics to enhance coroutine support
in Python, it does not change the internal implementation of coroutines, which
are still based on generators.

It is strongly suggested that the reader understands how coroutines are
implemented in Python (PEP 342 and PEP 380).  It is also recommended to read
PEP 3156 (asyncio framework).

From this point in this document we use the word *coroutine* to refer to
functions declared using the new syntax.  *generator-based coroutine* is used
where necessary to refer to coroutines that are based on generator syntax.


New Coroutine Declaration Syntax
--------------------------------

The following new syntax is used to declare a coroutine::

    async def read_data(db):
        pass

Key properties of coroutines:

* ``async def`` functions are always coroutines, even if they do not contain
  ``await`` expressions.

* It is a ``SyntaxError`` to have ``yield`` or ``yield from`` expressions in
  an ``async`` function.

* Internally, a new code object flag - ``CO_ASYNC`` - is introduced to enable
  runtime detection of coroutines (and migrating existing code).
  All coroutines have both ``CO_ASYNC`` and ``CO_GENERATOR`` flags set.

* Regular generators, when called, return a *generator object*; similarly,
  coroutines return a *coroutine object*.

* ``StopIteration`` exceptions are not propagated out of coroutines, and are
  replaced with a ``RuntimeError``.  For regular generators such behavior
  requires a future import (see PEP 479).


types.async_def()
-----------------

A new function ``async_def(gen)`` is added to the ``types`` module.  It
applies ``CO_ASYNC`` flag to the passed generator's code object, so that it
returns a *coroutine object* when called.

This feature enables an easy upgrade path for existing libraries.


Await Expression
----------------

The following new ``await`` expression is used to obtain a result of coroutine
execution::

    async def read_data(db):
        data = await db.fetch('SELECT ...')
        ...

``await``, similarly to ``yield from``, suspends execution of ``read_data``
coroutine until ``db.fetch`` *awaitable* completes and returns the result
data.

It uses the ``yield from`` implementation with an extra step of validating its
argument.  ``await`` only accepts an *awaitable*, which can be one of:

* A *coroutine object* returned from a coroutine or a generator decorated with
  ``types.async_def()``.

* An object with an ``__await__`` method returning an iterator.

  Any ``yield from`` chain of calls ends with a ``yield``.  This is a
  fundamental mechanism of how *Futures* are implemented.  Since, internally,
  coroutines are a special kind of generators, every ``await`` is suspended by
  a ``yield`` somewhere down the chain of ``await`` calls (please refer to PEP
  3156 for a detailed explanation.)

  To enable this behavior for coroutines, a new magic method called
  ``__await__`` is added.  In asyncio, for instance, to enable Future objects
  in ``await`` statements, the only change is to add ``__await__ = __iter__``
  line to ``asyncio.Future`` class.

  Objects with ``__await__`` method are called *Future-like* objects in the
  rest of this PEP.

  Also, please note that ``__aiter__`` method (see its definition below) cannot
  be used for this purpose.  It is a different protocol, and would be like
  using ``__iter__`` instead of ``__call__`` for regular callables.

It is a ``SyntaxError`` to use ``await`` outside of a coroutine.


Asynchronous Context Managers and "async with"
----------------------------------------------

An *asynchronous context manager* is a context manager that is able to suspend
execution in its *enter* and *exit* methods.

To make this possible, a new protocol for asynchronous context managers is
proposed.  Two new magic methods are added: ``__aenter__`` and ``__aexit__``.
Both must return an *awaitable*.

An example of an asynchronous context manager::

    class AsyncContextManager:
        async def __aenter__(self):
            await log('entering context')

        async def __aexit__(self, exc_type, exc, tb):
            await log('exiting context')


New Syntax
''''''''''

A new statement for asynchronous context managers is proposed::

    async with EXPR as VAR:
        BLOCK


which is semantically equivalent to::

    mgr = (EXPR)
    aexit = type(mgr).__aexit__
    aenter = type(mgr).__aenter__(mgr)
    exc = True

    try:
        try:
            VAR = await aenter
            BLOCK
        except:
            exc = False
            exit_res = await aexit(mgr, *sys.exc_info())
            if not exit_res:
                raise

    finally:
        if exc:
            await aexit(mgr, None, None, None)


As with regular ``with`` statements, it is possible to specify multiple context
managers in a single ``async with`` statement.

It is an error to pass a regular context manager without ``__aenter__`` and
``__aexit__`` methods to ``async with``.  It is a ``SyntaxError`` to use
``async with`` outside of a coroutine.


Example
'''''''

With asynchronous context managers it is easy to implement proper database
transaction managers for coroutines::

    async def commit(session, data):
        ...

        async with session.transaction():
            ...
            await session.update(data)
            ...

Code that needs locking also looks lighter::

    async with lock:
        ...

instead of::

    with (yield from lock):
        ...


Asynchronous Iterators and "async for"
--------------------------------------

An *asynchronous iterable* is able to call asynchronous code in its *iter*
implementation, and *asynchronous iterator* can call asynchronous code in its
*next* method.  To support asynchronous iteration:

1. An object must implement an  ``__aiter__`` method returning an *awaitable*
   resulting in an *asynchronous iterator object*.

2. An *asynchronous iterator object* must implement an ``__anext__`` method
   returning an *awaitable*.

3. To stop iteration```__anext__`` must raise a ``StopAsyncIteration``
   exception.

An example of asynchronous iterable::

    class AsyncIterable:
        async def __aiter__(self):
            return self

        async def __anext__(self):
            data = await self.fetch_data()
            if data:
                return data
            else:
                raise StopAsyncIteration

        async def fetch_data(self):
            ...


New Syntax
''''''''''

A new statement for iterating through asynchronous iterators is proposed::

    async for TARGET in ITER:
        BLOCK
    else:
        BLOCK2

which is semantically equivalent to::

    iter = (ITER)
    iter = await type(iter).__aiter__(iter)
    running = True
    while running:
        try:
            TARGET = await type(iter).__anext__(iter)
        except StopAsyncIteration:
            running = False
        else:
            BLOCK
    else:
        BLOCK2


It is an error to pass a regular iterable without ``__aiter__`` method to
``async for``.  It is a ``SyntaxError`` to use ``async for`` outside of a
coroutine.

As for with regular ``for`` statement, ``async for`` has an optional ``else``
clause.


Example 1
'''''''''

With asynchronous iteration protocol it is possible to asynchronously buffer
data during iteration::

    async for data in cursor:
        ...

Where ``cursor`` is an asynchronous iterator that prefetches ``N`` rows
of data from a database after every ``N`` iterations.

The following code illustrates new asynchronous iteration protocol::

    class Cursor:
        def __init__(self):
            self.buffer = collections.deque()

        def _prefetch(self):
            ...

        async def __aiter__(self):
            return self

        async def __anext__(self):
            if not self.buffer:
                self.buffer = await self._prefetch()
                if not self.buffer:
                    raise StopAsyncIteration
            return self.buffer.popleft()

then the ``Cursor`` class can be used as follows::

    async for row in Cursor():
        print(row)

which would be equivalent to the following code::

    i = await Cursor().__aiter__()
    while True:
        try:
            row = await i.__anext__()
        except StopAsyncIteration:
            break
        else:
            print(row)


Example 2
'''''''''

The following is a utility class that transforms a regular iterable to an
asynchronous one.  While this is not a very useful thing to do, the code
illustrates the relationship between regular and asynchronous iterators.

::

    class AsyncIteratorWrapper:
        def __init__(self, obj):
            self._it = iter(obj)

        async def __aiter__(self):
            return self

        async def __anext__(self):
            try:
                value = next(self._it)
            except StopIteration:
                raise StopAsyncIteration
            return value

    async for item in AsyncIteratorWrapper("abc"):
        print(item)


Why StopAsyncIteration?
'''''''''''''''''''''''

Coroutines are still based on generators internally.  So, before PEP 479, there
was no fundamental difference between

::

    def g1():
        yield from fut
        return 'spam'

and

::

    def g2():
        yield from fut
        raise StopIteration('spam')

And since PEP 479 is accepted and enabled by default for coroutines, the
following example will have its ``StopIteration`` wrapped into a
``RuntimeError``

::

    async def a1():
        await fut
        raise StopIteration('spam')

The only way to tell the outside code that the iteration has ended is to raise
something other than ``StopIteration``.  Therefore, a new built-in exception
class ``StopAsyncIteration`` was added.

Moreover, with semantics from PEP 479, all ``StopIteration`` exceptions raised
in coroutines are wrapped in ``RuntimeError``.


Debugging Features
------------------

One of the most frequent mistakes that people make when using generators as
coroutines is forgetting to use ``yield from``::

    @asyncio.coroutine
    def useful():
        asyncio.sleep(1) # this will do noting without 'yield from'

For debugging this kind of mistakes there is a special debug mode in asyncio,
in which ``@coroutine`` decorator wraps all functions with a special object
with a destructor logging a warning.  Whenever a wrapped generator gets garbage
collected, a detailed logging message is generated with information about where
exactly the decorator function was defined, stack trace of where it was
collected, etc.  Wrapper object also provides a convenient ``__repr__``
function with detailed information about the generator.

The only problem is how to enable these debug capabilities.  Since debug
facilities should be a no-op in production mode, ``@coroutine`` decorator makes
the decision of whether to wrap or not to wrap based on an OS environment
variable ``PYTHONASYNCIODEBUG``.  This way it is possible to run asyncio
programs with asyncio's own functions instrumented.  ``EventLoop.set_debug``, a
different debug facility, has no impact on ``@coroutine`` decorator's behavior.

With this proposal, coroutines is a native, distinct from generators,
concept.  A new method ``set_coroutine_wrapper`` is added to the ``sys`` module,
with which frameworks can provide advanced debugging facilities.

It is also important to make coroutines as fast and efficient as possible,
therefore there are no debug features enabled by default.

Example::

    async def debug_me():
        await asyncio.sleep(1)

    def async_debug_wrap(generator):
        return asyncio.AsyncDebugWrapper(generator)

    sys.set_coroutine_wrapper(async_debug_wrap)

    debug_me()  # <- this line will likely GC the coroutine object and
                # trigger AsyncDebugWrapper's code.

    assert isinstance(debug_me(), AsyncDebugWrapper)

    sys.set_coroutine_wrapper(None) # <- this unsets any
                                    #    previously set wrapper
    assert not isinstance(debug_me(), AsyncDebugWrapper)

If ``sys.set_coroutine_wrapper()`` is called twice, the new wrapper replaces the
previous wrapper.  ``sys.set_coroutine_wrapper(None)`` unsets the wrapper.


Glossary
========

:Coroutine:
    A coroutine function, or just "coroutine", is declared with ``async def``.
    It uses ``await`` and ``return value``; see `New Coroutine Declaration
    Syntax`_ for details.

:Coroutine object:
    Returned from a coroutine function. See `Await Expression`_ for details.

:Future-like object:
    An object with an ``__await__`` method.  It is consumed by ``await`` in a
    coroutine. A coroutine waiting for a Future-like object is suspended until
    the Future-like object's ``__await__`` completes.  ``await`` returns the
    result of the Future-like object.  See `Await Expression`_ for details.

:Awaitable:
    A *future-like* object or a *coroutine object*.  See `Await Expression`_
    for details.

:Generator-based coroutine:
    Coroutines based in generator syntax.  Most common example is
    ``@asyncio.coroutine``.

:Asynchronous context manager:
   An asynchronous context manager has ``__aenter__`` and ``__aexit__`` methods
   and can be used with ``async with``.  See
   `Asynchronous Context Managers and "async with"`_ for details.

:Asynchronous iterable:
    An object with an ``__aiter__`` method, which must return an *asynchronous
    iterator* object.  Can be used with ``async for``.  See
    `Asynchronous Iterators and "async for"`_ for details.

:Asynchronous iterator:
    An asynchronous iterator has an ``__anext__`` method.  See
    `Asynchronous Iterators and "async for"`_ for details.


List of functions and methods
=============================

================= =======================================  =================
Method            Can contain                              Can't contain
================= =======================================  =================
async def func    await, return value                      yield, yield from
async def __a*__  await, return value                      yield, yield from
def __a*__        return Future-like                       await
def __await__     yield, yield from, return iterable       await
generator         yield, yield from, return value          await
================= =======================================  =================

Where:

* "async def func": coroutine;

* "async def __a*__": ``__aiter__``, ``__anext__``, ``__aenter__``,
  ``__aexit__`` defined with the ``async`` keyword;

* "def __a*__": ``__aiter__``, ``__anext__``, ``__aenter__``, ``__aexit__``
  defined without the ``async`` keyword, must return an *awaitable*;

* "def __await__": ``__await__`` method to implement *Future-like* objects;

* generator: a "regular" generator, function defined with ``def`` and which
  contains a least one ``yield`` or ``yield from`` expression.


Transition Plan
===============

To avoid backwards compatibility issues with ``async`` and ``await`` keywords,
it was decided to modify ``tokenizer.c`` in such a way, that it:

* recognizes ``async def`` name tokens combination (start of a coroutine);

* keeps track of regular functions and coroutines;

* replaces ``'async'`` token with ``ASYNC`` and ``'await'`` token with
  ``AWAIT`` when in the process of yielding tokens for coroutines.

This approach allows for seamless combination of new syntax features (all of
them available only in ``async`` functions) with any existing code.

An example of having "async def" and "async" attribute in one piece of code::

    class Spam:
        async = 42

    async def ham():
        print(getattr(Spam, 'async'))

    # The coroutine can be executed and will print '42'


Backwards Compatibility
-----------------------

The only backwards incompatible change is an extra argument ``is_async`` to
``FunctionDef`` AST node.  But since it is a documented fact that the structure
of AST nodes is an implementation detail and subject to change, this should not
be considered a serious issue.


Grammar Updates
---------------

Grammar changes are also fairly minimal::

    await_expr: AWAIT test
    await_stmt: await_expr

    decorated: decorators (classdef | funcdef | async_funcdef)
    async_funcdef: ASYNC funcdef

    async_stmt: ASYNC (funcdef | with_stmt | for_stmt)

    compound_stmt: (if_stmt | while_stmt | for_stmt | try_stmt | with_stmt
                   | funcdef | classdef | decorated | async_stmt)

    atom: ('(' [yield_expr|await_expr|testlist_comp] ')' |
          '[' [testlist_comp] ']' |
          '{' [dictorsetmaker] '}' |
          NAME | NUMBER | STRING+ | '...' | 'None' | 'True' | 'False’)

    expr_stmt: testlist_star_expr (augassign (yield_expr|await_expr|testlist) |
                        ('=' (yield_expr|await_expr|testlist_star_expr))*)


Transition Period Shortcomings
------------------------------

There is just one.

Until ``async`` and ``await`` are not proper keywords, it is not possible (or
at least very hard) to fix ``tokenizer.c`` to recognize them on the **same
line** with ``def`` keyword::

    # async and await will always be parsed as variables

    async def outer():                             # 1
        def nested(a=(await fut)):
            pass

    async def foo(): return (await fut)            # 2

Since ``await`` and ``async`` in such cases are parsed as ``NAME`` tokens, a
``SyntaxError`` will be raised.

To workaround these issues, the above examples can be easily rewritten to a
more readable form::

    async def outer():                             # 1
        a_default = await fut
        def nested(a=a_default):
            pass

    async def foo():                               # 2
        return (await fut)

This limitation will go away as soon as ``async`` and ``await`` ate proper
keywords.  Or if it's decided to use a future import for this PEP.


Deprecation Plans
-----------------

``async`` and ``await`` names will be softly deprecated in CPython 3.5 and 3.6.
In 3.7 we will transform them to proper keywords.  Making ``async`` and
``await`` proper keywords before 3.7 might make it harder for people to port
their code to Python 3.


asyncio
-------

``asyncio`` module was adapted and tested to work with coroutines and new
statements.  Backwards compatibility is 100% preserved.

The required changes are mainly:

1. Modify ``@asyncio.coroutine`` decorator to use new ``types.async_def()``
   function.

2. Add ``__await__ = __iter__`` line to ``asyncio.Future`` class.

3. Add ``ensure_task()`` as an alias for ``async()`` function. Deprecate
   ``async()`` function.


Design Considerations
=====================

No implicit wrapping in Futures
-------------------------------

There is a proposal to add similar mechanism to ECMAScript 7 [2]_.  A key
difference is that JavaScript "async functions" always return a Promise. While
this approach has some advantages, it also implies that a new Promise object is
created on each "async function" invocation.

We could implement a similar functionality in Python, by wrapping all
coroutines in a Future object, but this has the following disadvantages:

1. Performance.  A new Future object would be instantiated on each coroutine
   call.  Moreover, this makes implementation of ``await`` expressions slower
   (disabling optimizations of ``yield from``).

2. A new built-in ``Future`` object would need to be added.

3. Coming up with a generic ``Future`` interface that is usable for any use
   case in any framework is a very hard to solve problem.

4. It is not a feature that is used frequently, when most of the code is
   coroutines.


Why "async" and "await" keywords
--------------------------------

async/await is not a new concept in programming languages:

* C# has it since long time ago [5]_;

* proposal to add async/await in ECMAScript 7 [2]_;
  see also Traceur project [9]_;

* Facebook's Hack/HHVM [6]_;

* Google's Dart language [7]_;

* Scala [8]_;

* proposal to add async/await to C++ [10]_;

* and many other less popular languages.

This is a huge benefit, as some users already have experience with async/await,
and because it makes working with many languages in one project easier (Python
with ECMAScript 7 for instance).


Why "__aiter__" is a coroutine
------------------------------

In principle, ``__aiter__`` could be a regular function.  There are several
good reasons to make it a coroutine:

* as most of the ``__anext__``, ``__aenter__``, and ``__aexit__`` methods are
  coroutines, users would often make a mistake defining it as ``async``
  anyways;

* there might be a need to run some asynchronous operations in ``__aiter__``,
  for instance to prepare DB queries or do some file operation.


Importance of "async" keyword
-----------------------------

While it is possible to just implement ``await`` expression and treat all
functions with at least one ``await`` as coroutines, this approach makes
APIs design, code refactoring and its long time support harder.

Let's pretend that Python only has ``await`` keyword::

    def useful():
        ...
        await log(...)
        ...

    def important():
        await useful()

If ``useful()`` function is refactored and someone removes all ``await``
expressions from it, it would become a regular python function, and all code
that depends on it, including ``important()`` would be broken.  To mitigate this
issue a decorator similar to ``@asyncio.coroutine`` has to be introduced.


Why "async def"
---------------

For some people bare ``async name(): pass`` syntax might look more appealing
than ``async def name(): pass``.  It is certainly easier to type.  But on the
other hand, it breaks the symmetry between ``async def``, ``async with`` and
``async for``, where ``async`` is a modifier, stating that the statement is
asynchronous.  It is also more consistent with the existing grammar.


Why not a __future__ import
---------------------------

``__future__`` imports are inconvenient and easy to forget to add.  Also, they
are enabled for the whole source file.  Consider that there is a big project
with a popular module named "async.py".  With future imports it is required to
either import it using ``__import__()`` or ``importlib.import_module()`` calls,
or to rename the module.  The proposed approach makes it possible to continue
using old code and modules without a hassle, while coming up with a migration
plan for future python versions.


Why magic methods start with "a"
--------------------------------

New asynchronous magic methods ``__aiter__``, ``__anext__``, ``__aenter__``,
and ``__aexit__`` all start with the same prefix "a".  An alternative proposal
is to use "async" prefix, so that ``__aiter__`` becomes ``__async_iter__``.
However, to align new magic methods with the existing ones, such as
``__radd__`` and ``__iadd__`` it was decided to use a shorter version.


Why not reuse existing magic names
----------------------------------

An alternative idea about new asynchronous iterators and context managers was
to reuse existing magic methods, by adding an ``async`` keyword to their
declarations::

    class CM:
        async def __enter__(self): # instead of __aenter__
            ...

This approach has the following downsides:

* it would not be possible to create an object that works in both ``with`` and
  ``async with`` statements;

* it would look confusing and would require some implicit magic behind the
  scenes in the interpreter;

* one of the main points of this proposal is to make coroutines as simple
  and foolproof as possible.


Comprehensions
--------------

For the sake of restricting the broadness of this PEP there is no new syntax
for asynchronous comprehensions.  This should be considered in a separate PEP,
if there is a strong demand for this feature.


Performance
===========

Overall Impact
--------------

This proposal introduces no observable performance impact.  Here is an output
of python's official set of benchmarks [4]_:

::

    python perf.py -r -b default ../cpython/python.exe ../cpython-aw/python.exe

    [skipped]

    Report on Darwin ysmac 14.3.0 Darwin Kernel Version 14.3.0:
    Mon Mar 23 11:59:05 PDT 2015; root:xnu-2782.20.48~5/RELEASE_X86_64
    x86_64 i386

    Total CPU cores: 8

    ### etree_iterparse ###
    Min: 0.365359 -> 0.349168: 1.05x faster
    Avg: 0.396924 -> 0.379735: 1.05x faster
    Significant (t=9.71)
    Stddev: 0.01225 -> 0.01277: 1.0423x larger

    The following not significant results are hidden, use -v to show them:
    django_v2, 2to3, etree_generate, etree_parse, etree_process, fastpickle,
    fastunpickle, json_dump_v2, json_load, nbody, regex_v8, tornado_http.


Tokenizer modifications
-----------------------

There is no observable slowdown of parsing python files with the modified
tokenizer: parsing of one 12Mb file (``Lib/test/test_binop.py`` repeated 1000
times) takes the same amount of time.


async/await
-----------

The following micro-benchmark was used to determine performance difference
between "async" functions and generators::

    import sys
    import time

    def binary(n):
        if n <= 0:
            return 1
        l = yield from binary(n - 1)
        r = yield from binary(n - 1)
        return l + 1 + r

    async def abinary(n):
        if n <= 0:
            return 1
        l = await abinary(n - 1)
        r = await abinary(n - 1)
        return l + 1 + r

    def timeit(gen, depth, repeat):
        t0 = time.time()
        for _ in range(repeat):
            list(gen(depth))
        t1 = time.time()
        print('{}({}) * {}: total {:.3f}s'.format(
            gen.__name__, depth, repeat, t1-t0))

The result is that there is no observable performance difference.  Minimum
timing of 3 runs

::

    abinary(19) * 30: total 12.985s
    binary(19) * 30: total 12.953s

Note that depth of 19 means 1,048,575 calls.


Reference Implementation
========================

The reference implementation can be found here: [3]_.

List of high-level changes and new protocols
--------------------------------------------

1. New syntax for defining coroutines: ``async def`` and new ``await``
   keyword.

2. New ``__await__`` method for Future-like objects.

3. New syntax for asynchronous context managers: ``async with``.  And
   associated protocol with ``__aenter__`` and ``__aexit__`` methods.

4. New syntax for asynchronous iteration: ``async for``.  And associated
   protocol with ``__aiter__``, ``__aexit__`` and new built-in exception
   ``StopAsyncIteration``.

5. New AST nodes: ``AsyncFor``, ``AsyncWith``, ``Await``; ``FunctionDef`` AST
   node got a new argument ``is_async``.

6. New functions: ``sys.set_coroutine_wrapper(callback)`` and
   ``types.async_def(gen)``.

7. New ``CO_ASYNC`` bit flag for code objects.

While the list of changes and new things is not short, it is important to
understand, that most users will not use these features directly.  It is
intended to be used in frameworks and libraries to provide users with
convenient to use and unambiguous APIs with ``async def``, ``await``, ``async
for`` and ``async with`` syntax.


Working example
---------------

All concepts proposed in this PEP are implemented [3]_ and can be tested.

::

    import asyncio

    async def echo_server():
        print('Serving on localhost:8000')
        await asyncio.start_server(handle_connection, 'localhost', 8000)

    async def handle_connection(reader, writer):
        print('New connection...')

        while True:
            data = await reader.read(8192)

            if not data:
                break

            print('Sending {:.10}... back'.format(repr(data)))
            writer.write(data)

    loop = asyncio.get_event_loop()
    loop.run_until_complete(echo_server())
    try:
        loop.run_forever()
    finally:
        loop.close()


References
==========

.. [1] https://docs.python.org/3/library/asyncio-task.html#asyncio.coroutine

.. [2] http://wiki.ecmascript.org/doku.php?id=strawman:async_functions

.. [3] https://github.com/1st1/cpython/tree/await

.. [4] https://hg.python.org/benchmarks

.. [5] https://msdn.microsoft.com/en-us/library/hh191443.aspx

.. [6] http://docs.hhvm.com/manual/en/hack.async.php

.. [7] https://www.dartlang.org/articles/await-async/

.. [8] http://docs.scala-lang.org/sips/pending/async.html

.. [9] https://github.com/google/traceur-compiler/wiki/LanguageFeatures#async-functions-experimental

.. [10] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3722.pdf (PDF)


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

I thank Guido van Rossum, Victor Stinner, Elvis Pranskevichus, Andrew Svetlov,
and Łukasz Langa for their initial feedback.


Copyright
=========

This document has been placed in the public domain.

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