PEP: 492
Title: Coroutines with async and await syntax
Author: Yury Selivanov <yury@edgedb.com>
Discussions-To: python-dev@python.org
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 09-Apr-2015
Python-Version: 3.5
Post-History: 17-Apr-2015, 21-Apr-2015, 27-Apr-2015, 29-Apr-2015, 05-May-2015


Abstract
========

The growth of Internet and general connectivity has triggered the
proportionate need for responsive and scalable code.  This proposal
aims to answer that need by making writing explicitly asynchronous,
concurrent Python code easier and more Pythonic.

It is proposed to make *coroutines* a proper standalone concept in
Python, and introduce new supporting syntax.  The ultimate goal
is to help establish a common, easily approachable, mental
model of asynchronous programming in Python and make it as close to
synchronous programming as possible.

This PEP assumes that the asynchronous tasks are scheduled and
coordinated by an Event Loop similar to that of stdlib module
``asyncio.events.AbstractEventLoop``.  While the PEP is not tied to any
specific Event Loop implementation, it is relevant only to the kind of
coroutine that uses ``yield`` as a signal to the scheduler, indicating
that the coroutine will be waiting until an event (such as IO) is
completed.

We believe that the changes proposed here will help keep Python
relevant and competitive in a quickly growing area of asynchronous
programming, as many other languages have adopted, or are planning to
adopt, similar features: [2]_, [5]_, [6]_, [7]_, [8]_, [10]_.


API Design and Implementation Revisions
=======================================

1. Feedback on the initial beta release of Python 3.5 resulted in a
   redesign of the object model supporting this PEP to more clearly
   separate native coroutines from generators - rather than being a
   new kind of generator, native coroutines are now their own
   completely distinct type (implemented in [17]_).

   This change was implemented based primarily due to problems
   encountered attempting to integrate support for native coroutines
   into the Tornado web server (reported in [18]_).

2. In CPython 3.5.2, the ``__aiter__`` protocol was updated.

   Before 3.5.2, ``__aiter__`` was expected to return an *awaitable*
   resolving to an *asynchronous iterator*.  Starting with 3.5.2,
   ``__aiter__`` should return asynchronous iterators directly.

   If the old protocol is used in 3.5.2, Python will raise a
   ``PendingDeprecationWarning``.

   In CPython 3.6, the old ``__aiter__`` protocol will still be
   supported with a ``DeprecationWarning`` being raised.

   In CPython 3.7, the old ``__aiter__`` protocol will no longer be
   supported: a ``RuntimeError`` will be raised if ``__aiter__``
   returns anything but an asynchronous iterator.

   See [19]_ and [20]_ for more details.


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; this is especially true for new developers.

* Whether or not a function is a coroutine is determined by a presence
  of ``yield``  or ``yield from`` statements in its *body*, which can
  lead to unobvious errors when such statements appear in or disappear
  from function body during refactoring.

* 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.

This specification presumes knowledge of the implementation of
coroutines in Python (:pep:`342` and :pep:`380`).  Motivation for the syntax
changes proposed here comes from the asyncio framework (:pep:`3156`) and
the "Cofunctions" proposal (:pep:`3152`, now rejected in favor of this
specification).

From this point in this document we use the word *native 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.  *coroutine* is used in contexts where both
definitions are applicable.


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

The following new syntax is used to declare a *native 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, two new code object flags were introduced:

  - ``CO_COROUTINE`` is used to mark *native coroutines*
    (defined with new syntax).

  - ``CO_ITERABLE_COROUTINE`` is used to make *generator-based
    coroutines* compatible with *native coroutines* (set by
    `types.coroutine()`_ function).

* 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`).

* When a *native coroutine* is garbage collected, a ``RuntimeWarning``
  is raised if it was never awaited on (see also
  `Debugging Features`_).

* See also `Coroutine objects`_ section.


types.coroutine()
-----------------

A new function ``coroutine(fn)`` is added to the ``types`` module.  It
allows interoperability between existing *generator-based coroutines*
in asyncio and *native coroutines* introduced by this PEP::

    @types.coroutine
    def process_data(db):
        data = yield from read_data(db)
        ...

The function applies ``CO_ITERABLE_COROUTINE`` flag to
generator-function's code object, making it return a *coroutine* object.

If ``fn`` is not a *generator function*, it is wrapped.  If it returns
a *generator*, it will be wrapped in an *awaitable* proxy object
(see below the definition of awaitable objects).

Note, that the ``CO_COROUTINE`` flag is not applied by
``types.coroutine()`` to make it possible to separate *native
coroutines* defined with new syntax, from *generator-based coroutines*.


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 *native coroutine* object returned from a *native coroutine
  function*.

* A *generator-based coroutine* object returned from a function
  decorated with ``types.coroutine()``.

* 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.

  It is a ``TypeError`` if ``__await__`` returns anything but an
  iterator.

* Objects defined with CPython C API with a ``tp_as_async.am_await``
  function, returning an *iterator* (similar to ``__await__`` method).

It is a ``SyntaxError`` to use ``await`` outside of an ``async def``
function (like it is a ``SyntaxError`` to use ``yield`` outside of
``def`` function).

It is a ``TypeError`` to pass anything other than an *awaitable* object
to an ``await`` expression.


Updated operator precedence table
'''''''''''''''''''''''''''''''''

``await`` keyword is defined as follows::

    power ::=  await ["**" u_expr]
    await ::=  ["await"] primary

where "primary" represents the most tightly bound operations of the
language.  Its syntax is::

    primary ::=  atom | attributeref | subscription | slicing | call

See Python Documentation [12]_ and `Grammar Updates`_ section of this
proposal for details.

The key ``await`` difference from ``yield`` and ``yield from``
operators is that *await expressions* do not require parentheses around
them most of the times.

Also, ``yield from`` allows any expression as its argument, including
expressions like ``yield from a() + b()``, that would be parsed as
``yield from (a() + b())``, which is almost always a bug.  In general,
the result of any arithmetic operation is not an *awaitable* object.
To avoid this kind of mistakes, it was decided to make ``await``
precedence lower than ``[]``, ``()``, and ``.``, but higher than ``**``
operators.

+------------------------------+-----------------------------------+
| Operator                     | Description                       |
+==============================+===================================+
| ``yield`` ``x``,             | Yield expression                  |
| ``yield from`` ``x``         |                                   |
+------------------------------+-----------------------------------+
| ``lambda``                   | Lambda expression                 |
+------------------------------+-----------------------------------+
| ``if`` -- ``else``           | Conditional expression            |
+------------------------------+-----------------------------------+
| ``or``                       | Boolean OR                        |
+------------------------------+-----------------------------------+
| ``and``                      | Boolean AND                       |
+------------------------------+-----------------------------------+
| ``not`` ``x``                | Boolean NOT                       |
+------------------------------+-----------------------------------+
| ``in``, ``not in``,          | Comparisons, including membership |
| ``is``, ``is not``, ``<``,   | tests and identity tests          |
| ``<=``, ``>``, ``>=``,       |                                   |
| ``!=``, ``==``               |                                   |
+------------------------------+-----------------------------------+
| ``|``                        | Bitwise OR                        |
+------------------------------+-----------------------------------+
| ``^``                        | Bitwise XOR                       |
+------------------------------+-----------------------------------+
| ``&``                        | Bitwise AND                       |
+------------------------------+-----------------------------------+
| ``<<``, ``>>``               | Shifts                            |
+------------------------------+-----------------------------------+
| ``+``, ``-``                 | Addition and subtraction          |
+------------------------------+-----------------------------------+
| ``*``, ``@``, ``/``, ``//``, | Multiplication, matrix            |
| ``%``                        | multiplication, division,         |
|                              | remainder                         |
+------------------------------+-----------------------------------+
| ``+x``, ``-x``, ``~x``       | Positive, negative, bitwise NOT   |
+------------------------------+-----------------------------------+
| ``**``                       | Exponentiation                    |
+------------------------------+-----------------------------------+
| ``await`` ``x``              | Await expression                  |
+------------------------------+-----------------------------------+
| ``x[index]``,                | Subscription, slicing,            |
| ``x[index:index]``,          | call, attribute reference         |
| ``x(arguments...)``,         |                                   |
| ``x.attribute``              |                                   |
+------------------------------+-----------------------------------+
| ``(expressions...)``,        | Binding or tuple display,         |
| ``[expressions...]``,        | list display,                     |
| ``{key: value...}``,         | dictionary display,               |
| ``{expressions...}``         | set display                       |
+------------------------------+-----------------------------------+


Examples of "await" expressions
'''''''''''''''''''''''''''''''

Valid syntax examples:

================================== ==================================
Expression                         Will be parsed as
================================== ==================================
``if await fut: pass``             ``if (await fut): pass``
``if await fut + 1: pass``         ``if (await fut) + 1: pass``
``pair = await fut, 'spam'``       ``pair = (await fut), 'spam'``
``with await fut, open(): pass``   ``with (await fut), open(): pass``
``await foo()['spam'].baz()()``    ``await ( foo()['spam'].baz()() )``
``return await coro()``            ``return ( await coro() )``
``res = await coro() ** 2``        ``res = (await coro()) ** 2``
``func(a1=await coro(), a2=0)``    ``func(a1=(await coro()), a2=0)``
``await foo() + await bar()``      ``(await foo()) + (await bar())``
``-await foo()``                   ``-(await foo())``
================================== ==================================

Invalid syntax examples:

================================== ==================================
Expression                         Should be written as
================================== ==================================
``await await coro()``             ``await (await coro())``
``await -coro()``                  ``await (-coro())``
================================== ==================================


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__

    VAR = await aenter(mgr)
    try:
        BLOCK
    except:
        if not await aexit(mgr, *sys.exc_info()):
            raise
    else:
        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 an ``async def`` function.


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 (or, if defined
   with CPython C API, ``tp_as_async.am_aiter`` slot) returning an
   *asynchronous iterator object*.

2. An *asynchronous iterator object* must implement an ``__anext__``
   method (or, if defined with CPython C API, ``tp_as_async.am_anext``
   slot) returning an *awaitable*.

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

An example of asynchronous iterable::

    class AsyncIterable:
        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 = 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 a ``TypeError`` to pass a regular iterable without ``__aiter__``
method to ``async for``.  It is a ``SyntaxError`` to use ``async for``
outside of an ``async def`` function.

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()

        async def _prefetch(self):
            ...

        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 = 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.

.. code:: python

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

        def __aiter__(self):
            return self

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

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


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

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

.. code:: python

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

and

.. code:: python

    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``

.. code:: python

    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``.


Coroutine objects
-----------------

Differences from generators
'''''''''''''''''''''''''''

This section applies only to *native coroutines* with ``CO_COROUTINE``
flag, i.e. defined with the new ``async def`` syntax.

**The behavior of existing *generator-based coroutines* in asyncio
remains unchanged.**

Great effort has been made to make sure that coroutines and
generators are treated as distinct concepts:

1. *Native coroutine* objects do not implement ``__iter__`` and
   ``__next__`` methods.  Therefore, they cannot be iterated over or
   passed to ``iter()``, ``list()``, ``tuple()`` and other built-ins.
   They also cannot be used in a ``for..in`` loop.

   An attempt to use ``__iter__`` or ``__next__`` on a *native
   coroutine* object will result in a ``TypeError``.

2. *Plain generators* cannot ``yield from`` *native coroutines*:
   doing so will result in a ``TypeError``.

3. *generator-based coroutines* (for asyncio code must be decorated
   with ``@asyncio.coroutine`` [1]_) can ``yield from`` *native coroutine
   objects*.

4. ``inspect.isgenerator()`` and ``inspect.isgeneratorfunction()``
   return ``False`` for *native coroutine* objects and *native
   coroutine functions*.


Coroutine object methods
''''''''''''''''''''''''

Coroutines are based on generators internally, thus they share the
implementation.  Similarly to generator objects, *coroutines* have
``throw()``, ``send()`` and ``close()`` methods.  ``StopIteration`` and
``GeneratorExit`` play the same role for coroutines (although
:pep:`479` is enabled by default for coroutines).  See :pep:`342`, :pep:`380`,
and Python Documentation [11]_ for details.

``throw()``, ``send()`` methods for *coroutines* are used to push
values and raise errors into *Future-like* objects.


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

A common beginner mistake is forgetting to use ``yield from`` on
coroutines::

    @asyncio.coroutine
    def useful():
        asyncio.sleep(1) # this will do nothing 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.  *In addition* to a ``RuntimeWarning`` being raised on
coroutines that were never awaited, it is proposed to add two new
functions to the ``sys`` module: ``set_coroutine_wrapper`` and
``get_coroutine_wrapper``.  This is to enable advanced debugging
facilities in asyncio and other frameworks (such as displaying where
exactly coroutine was created, and a more detailed stack trace of where
it was garbage collected).


New Standard Library Functions
------------------------------

* ``types.coroutine(gen)``.  See `types.coroutine()`_ section for
  details.

* ``inspect.iscoroutine(obj)`` returns ``True`` if ``obj`` is a
  *native coroutine* object.

* ``inspect.iscoroutinefunction(obj)`` returns ``True`` if ``obj`` is a
  *native coroutine function*.

* ``inspect.isawaitable(obj)`` returns ``True`` if ``obj`` is an
  *awaitable*.

* ``inspect.getcoroutinestate(coro)`` returns the current state of
  a *native coroutine object* (mirrors
  ``inspect.getfgeneratorstate(gen)``).

* ``inspect.getcoroutinelocals(coro)`` returns the mapping of a
  *native coroutine object's* local variables to their values
  (mirrors ``inspect.getgeneratorlocals(gen)``).

* ``sys.set_coroutine_wrapper(wrapper)`` allows to intercept creation of
  *native coroutine* objects. ``wrapper`` must be either a callable that
  accepts one argument (a *coroutine* object), or ``None``.  ``None``
  resets the wrapper.  If called twice, the new wrapper replaces the
  previous one. The function is thread-specific.  See `Debugging
  Features`_ for more details.

* ``sys.get_coroutine_wrapper()`` returns the current wrapper object.
  Returns ``None`` if no wrapper was set.  The function is
  thread-specific.  See  `Debugging Features`_ for more details.


New Abstract Base Classes
-------------------------

In order to allow better integration with existing frameworks (such as
Tornado, see [13]_) and compilers (such as Cython, see [16]_), two new
Abstract Base Classes (ABC) are added:

* ``collections.abc.Awaitable`` ABC for *Future-like* classes, that
  implement ``__await__`` method.

* ``collections.abc.Coroutine`` ABC for *coroutine* objects, that
  implement ``send(value)``, ``throw(type, exc, tb)``, ``close()`` and
  ``__await__()`` methods.

  Note that generator-based coroutines with ``CO_ITERABLE_COROUTINE``
  flag do not implement ``__await__`` method, and therefore are not
  instances of ``collections.abc.Coroutine`` and
  ``collections.abc.Awaitable`` ABCs::

      @types.coroutine
      def gencoro():
          yield

      assert not isinstance(gencoro(), collections.abc.Coroutine)

      # however:
      assert inspect.isawaitable(gencoro())

To allow easy testing if objects support asynchronous iteration, two
more ABCs are added:

* ``collections.abc.AsyncIterable`` -- tests for ``__aiter__`` method.

* ``collections.abc.AsyncIterator`` -- tests for ``__aiter__`` and
  ``__anext__`` methods.


Glossary
========

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

Native coroutine
    Returned from a native coroutine function. See `Await Expression`_
    for details.

Generator-based coroutine function
    Coroutines based on generator syntax.  Most common example are
    functions decorated with ``@asyncio.coroutine``.

Generator-based coroutine
    Returned from a generator-based coroutine function.

Coroutine
    Either *native coroutine* or *generator-based coroutine*.

Coroutine object
    Either *native coroutine* object or *generator-based coroutine*
    object.

Future-like object
    An object with an ``__await__`` method, or a C object with
    ``tp_as_async->am_await`` function, returning an *iterator*.  Can be
    consumed by an ``await`` expression in a coroutine. A coroutine
    waiting for a Future-like object is suspended until the Future-like
    object's ``__await__`` completes, and returns the result.  See
    `Await Expression`_ for details.

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

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.


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;

* while tokenizing ``async def`` block, it replaces ``'async'``
  ``NAME`` token with ``ASYNC``, and ``'await'`` ``NAME`` token with
  ``AWAIT``;

* while tokenizing ``def`` block, it yields ``'async'`` and ``'await'``
  ``NAME`` tokens as is.

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
-----------------------

This proposal preserves 100% backwards compatibility.


asyncio
'''''''

``asyncio`` module was adapted and tested to work with coroutines and
new statements.  Backwards compatibility is 100% preserved, i.e. all
existing code will work as-is.

The required changes are mainly:

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

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

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


asyncio migration strategy
''''''''''''''''''''''''''

Because *plain generators* cannot ``yield from`` *native coroutine
objects* (see `Differences from generators`_ section for more details),
it is advised to make sure that all generator-based coroutines are
decorated with ``@asyncio.coroutine`` *before* starting to use the new
syntax.


async/await in CPython code base
''''''''''''''''''''''''''''''''

There is no use of ``await`` names in CPython.

``async`` is mostly used by asyncio.  We are addressing this by
renaming ``async()`` function to ``ensure_future()`` (see `asyncio`_
section for details).

Another use of ``async`` keyword is in ``Lib/xml/dom/xmlbuilder.py``,
to define an ``async = False`` attribute for ``DocumentLS`` class.
There is no documentation or tests for it, it is not used anywhere else
in CPython.  It is replaced with a getter, that raises a
``DeprecationWarning``, advising to use ``async_`` attribute instead.
'async' attribute is not documented and is not used in CPython code
base.


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

Grammar changes are fairly minimal::

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

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

    async_stmt: ASYNC (funcdef | with_stmt | for_stmt)

    power: atom_expr ['**' factor]
    atom_expr: [AWAIT] atom trailer*


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.


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

PEP 3152
--------

:pep:`3152` by Gregory Ewing proposes a different mechanism for coroutines
(called "cofunctions").  Some key points:

1. A new keyword ``codef`` to declare a *cofunction*.  *Cofunction* is
   always a generator, even if there is no ``cocall`` expressions
   inside it.  Maps to ``async def`` in this proposal.

2. A new keyword ``cocall`` to call a *cofunction*.  Can only be used
   inside a *cofunction*.  Maps to ``await`` in this proposal (with
   some differences, see below).

3. It is not possible to call a *cofunction* without a ``cocall``
   keyword.

4. ``cocall`` grammatically requires parentheses after it::

    atom: cocall | <existing alternatives for atom>
    cocall: 'cocall' atom cotrailer* '(' [arglist] ')'
    cotrailer: '[' subscriptlist ']' | '.' NAME

5. ``cocall f(*args, **kwds)`` is semantically equivalent to
   ``yield from f.__cocall__(*args, **kwds)``.

Differences from this proposal:

1. There is no equivalent of ``__cocall__`` in this PEP, which is
   called and its result is passed to ``yield from`` in the ``cocall``
   expression. ``await`` keyword expects an *awaitable* object,
   validates the type, and executes ``yield from`` on it.  Although,
   ``__await__`` method is similar to ``__cocall__``, but is only used
   to define *Future-like* objects.

2. ``await`` is defined in almost the same way as ``yield from`` in the
   grammar (it is later enforced that ``await`` can only be inside
   ``async def``).  It is possible to simply write ``await future``,
   whereas ``cocall`` always requires parentheses.

3. To make asyncio work with :pep:`3152` it would be required to modify
   ``@asyncio.coroutine`` decorator to wrap all functions in an object
   with a ``__cocall__`` method, or to implement ``__cocall__`` on
   generators.  To call *cofunctions* from existing generator-based
   coroutines it would be required to use ``costart(cofunc, *args,
   **kwargs)`` built-in.

4. Since it is impossible to call a *cofunction* without a ``cocall``
   keyword, it automatically prevents the common mistake of forgetting
   to use ``yield from`` on generator-based coroutines.  This proposal
   addresses this problem with a different approach, see `Debugging
   Features`_.

5. A shortcoming of requiring a ``cocall`` keyword to call a coroutine
   is that if is decided to implement coroutine-generators --
   coroutines with ``yield`` or ``async yield`` expressions -- we
   wouldn't need a ``cocall`` keyword to call them.  So we'll end up
   having ``__cocall__`` and no ``__call__`` for regular coroutines,
   and having ``__call__`` and no ``__cocall__`` for
   coroutine-generators.

6. Requiring parentheses grammatically also introduces a whole lot
   of new problems.

   The following code::

       await fut
       await function_returning_future()
       await asyncio.gather(coro1(arg1, arg2), coro2(arg1, arg2))

   would look like::

       cocall fut()  # or cocall costart(fut)
       cocall (function_returning_future())()
       cocall asyncio.gather(costart(coro1, arg1, arg2),
                             costart(coro2, arg1, arg2))

7. There are no equivalents of ``async for`` and ``async with`` in PEP
   3152.


Coroutine-generators
--------------------

With ``async for`` keyword it is desirable to have a concept of a
*coroutine-generator* -- a coroutine with ``yield`` and ``yield from``
expressions.  To avoid any ambiguity with regular generators, we would
likely require to have an ``async`` keyword before ``yield``, and
``async yield from`` would raise a ``StopAsyncIteration`` exception.

While it is possible to implement coroutine-generators, we believe that
they are out of scope of this proposal.  It is an advanced concept that
should be carefully considered and balanced, with a non-trivial changes
in the implementation of current generator objects.  This is a matter
for a separate PEP.


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__" does not return an awaitable
--------------------------------------------

:pep:`492` was accepted in CPython 3.5.0 with ``__aiter__`` defined as
a method, that was expected to return an awaitable resolving to an
asynchronous iterator.

In 3.5.2 (as :pep:`492` was accepted on a provisional basis) the
``__aiter__`` protocol was updated to return asynchronous iterators
directly.

The motivation behind this change is to make it possible to
implement asynchronous generators in Python.  See [19]_ and [20]_ for
more details.


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 "await for" and "await with"
------------------------------------

``async`` is an adjective, and hence it is a better choice for a
*statement qualifier* keyword.  ``await for/with`` would imply that
something is awaiting for a completion of a ``for`` or ``with``
statement.


Why "async def" and not "def async"
-----------------------------------

``async`` keyword is a *statement qualifier*.  A good analogy to it are
"static", "public", "unsafe" keywords from other languages.  "async
for" is an asynchronous "for" statement, "async with" is an
asynchronous "with" statement, "async def" is an asynchronous function.

Having "async" after the main statement keyword might introduce some
confusion, like "for async item in iterator" can be read as "for each
asynchronous item in iterator".

Having ``async`` keyword before ``def``, ``with`` and ``for`` also
makes the language grammar simpler.  And "async def" better separates
coroutines from regular functions visually.


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

`Transition Plan`_ section explains how tokenizer is modified to treat
``async`` and ``await`` as keywords *only* in ``async def`` blocks.
Hence ``async def`` fills the role that a module level compiler
declaration like ``from __future__ import async_await`` would otherwise
fill.


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 ``__anext__``
becomes ``__async_next__``. 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 break backwards compatibility, as nothing prohibits from
  returning a Future-like objects from ``__enter__`` and/or
  ``__exit__`` in Python <= 3.4;

* one of the main points of this proposal is to make native coroutines
  as simple and foolproof as possible, hence the clear separation of
  the protocols.


Why not reuse existing "for" and "with" statements
--------------------------------------------------

The vision behind existing generator-based coroutines and this proposal
is to make it easy for users to see where the code might be suspended.
Making existing "for" and "with" statements to recognize asynchronous
iterators and context managers will inevitably create implicit suspend
points, making it harder to reason about the code.


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

Syntax for asynchronous comprehensions could be provided, but this
construct is outside of the scope of this PEP.


Async lambda functions
----------------------

Syntax for asynchronous lambda functions could be provided, but this
construct is outside of the scope of this PEP.


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

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

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

.. code:: text

    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(func, depth, repeat):
        t0 = time.time()
        for _ in range(repeat):
            o = func(depth)
            try:
                while True:
                    o.send(None)
            except StopIteration:
                pass
        t1 = time.time()
        print('{}({}) * {}: total {:.3f}s'.format(
            func.__name__, depth, repeat, t1-t0))

The result is that there is no observable performance difference::

    binary(19) * 30: total 53.321s
    abinary(19) * 30: total 55.073s

    binary(19) * 30: total 53.361s
    abinary(19) * 30: total 51.360s

    binary(19) * 30: total 49.438s
    abinary(19) * 30: total 51.047s

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, and new
   ``tp_as_async.am_await`` slot in ``PyTypeObject``.

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``.  New ``tp_as_async.am_aiter``
   and ``tp_as_async.am_anext`` slots in ``PyTypeObject``.

5. New AST nodes: ``AsyncFunctionDef``, ``AsyncFor``, ``AsyncWith``,
   ``Await``.

6. New functions: ``sys.set_coroutine_wrapper(callback)``,
   ``sys.get_coroutine_wrapper()``, ``types.coroutine(gen)``,
   ``inspect.iscoroutinefunction(func)``, ``inspect.iscoroutine(obj)``,
   ``inspect.isawaitable(obj)``, ``inspect.getcoroutinestate(coro)``,
   and ``inspect.getcoroutinelocals(coro)``.

7. New ``CO_COROUTINE`` and ``CO_ITERABLE_COROUTINE`` bit flags for code
   objects.

8. New ABCs: ``collections.abc.Awaitable``,
   ``collections.abc.Coroutine``, ``collections.abc.AsyncIterable``, and
   ``collections.abc.AsyncIterator``.

9. C API changes: new ``PyCoro_Type`` (exposed to Python as
   ``types.CoroutineType``) and ``PyCoroObject``.
   ``PyCoro_CheckExact(*o)`` to test if ``o`` is a *native coroutine*.


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.

.. code:: python

    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()


Acceptance
==========

:pep:`492` was accepted by Guido, Tuesday, May 5, 2015 [14]_.


Implementation
==============

The implementation is tracked in issue 24017 [15]_. It was
committed on May 11, 2015.


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)

.. [11] https://docs.python.org/3/reference/expressions.html#generator-iterator-methods

.. [12] https://docs.python.org/3/reference/expressions.html#primaries

.. [13] https://mail.python.org/pipermail/python-dev/2015-May/139851.html

.. [14] https://mail.python.org/pipermail/python-dev/2015-May/139844.html

.. [15] http://bugs.python.org/issue24017

.. [16] https://github.com/python/asyncio/issues/233

.. [17] https://hg.python.org/cpython/rev/7a0a1a4ac639

.. [18] http://bugs.python.org/issue24400

.. [19] http://bugs.python.org/issue27243

.. [20] https://docs.python.org/3/reference/datamodel.html#async-iterators

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

I thank Guido van Rossum, Victor Stinner, Elvis Pranskevichus, Andrew
Svetlov, Łukasz Langa, Greg Ewing, Stephen J. Turnbull, Jim J. Jewett,
Brett Cannon, Nick Coghlan, Steven D'Aprano, Paul Moore, Nathaniel
Smith, Ethan Furman, Stefan Behnel, Paul Sokolovsky, Victor Petrovykh,
and many others for their feedback, ideas, edits, criticism, code
reviews, and discussions around this PEP.


Copyright
=========

This document has been placed in the public domain.