New subject: PEP 492: What is the real goal?

28 Apr 2015

      Hi python-dev,

Another round of updates.  Reference implementation
has been updated: https://github.com/1st1/cpython/tree/await
(includes all things from the below summary of updates +
tests).

Summary:

1. "PyTypeObject.tp_await" slot.  Replaces "tp_reserved".
This is to enable implementation of Futures with C API.
Must return an iterator if implemented.

2. New grammar for "await" expressions, see
'Syntax of "await" expression' section

3. inspect.iscoroutine() and inspect.iscoroutineobjects()
functions.

4. Full separation of coroutines and generators.
This is a big one; let's discuss.

a) Coroutine objects raise TypeError (is NotImplementedError
better?) in their __iter__ and __next__.  Therefore it's
not not possible to pass them to iter(), tuple(), next() and
other similar functions that work with iterables.

b) Because of (a), for..in iteration also does not work
on coroutines anymore.

c) 'yield from' only accept coroutine objects from
generators decorated with 'types.coroutine'. That means
that existing asyncio generator-based coroutines will
happily yield from both coroutines and generators.
*But* every generator-based coroutine *must* be
decorated with `asyncio.coroutine()`.  This is
potentially a backwards incompatible change.

d) inspect.isgenerator() and inspect.isgeneratorfunction()
return `False` for coroutine objects & coroutine functions.

e) Should we add a coroutine ABC (for cython etc)?

I, personally, think this is highly necessary. First,
separation of coroutines from generators is extremely
important. One day there won't be generator-based
coroutines, and we want to avoid any kind of confusion.
Second, we only can do this in 3.5.  This kind of
semantics change won't be ever possible.

asyncio recommends using @coroutine decorator, and most
projects that I've seen do use it.  Also there is no
reason for people to use iter() and next() functions
on coroutines when writing asyncio code. I doubt that
this will cause serious backwards compatibility problems
(asyncio also has provisional status).

Thank you,
Yury

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
Post-History: 17-Apr-2015, 21-Apr-2015, 27-Apr-2015

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) and PEP 3152 (Cofunctions).

 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_COROUTINE`` - is introduced
   to enable runtime detection of coroutines (and migrating existing
   code). All coroutines have both ``CO_COROUTINE`` 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.coroutine()
-----------------

A new function ``coroutine(gen)`` is added to the ``types`` module.  It
applies ``CO_COROUTINE`` flag to the passed generator-function's code
object, making it to return 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.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.

   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 ``TypeError`` if ``__await__`` returns anything but an
   iterator.

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

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

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

Syntax of "await" expression
''''''''''''''''''''''''''''

``await`` keyword is defined differently from ``yield`` and ``yield
from``.  The main difference is that *await expressions* do not require
parentheses around them most of the times.

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)``
================================== ==================================

See `Grammar Updates`_ section for details.

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 a ``TypeError`` 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 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

::

     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. New methods ``set_coroutine_wrapper`` and
``get_coroutine_wrapper`` are 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.CoroWrapper(generator)

     sys.set_coroutine_wrapper(async_debug_wrap)

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

     assert isinstance(debug_me(), asyncio.CoroWrapper)

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

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

inspect.iscoroutine() and inspect.iscoroutineobject()
-----------------------------------------------------

Two new functions are added to the ``inspect`` module:

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

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

Differences between coroutines and generators
---------------------------------------------

A great effort has been made to make sure that coroutines and
generators are separate concepts:

1. 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.

2. ``yield from`` does not accept coroutine objects (unless it is used
    in a generator-based coroutine decorated with ``types.coroutine``.)

3. ``yield from`` does not accept coroutine objects from plain Python
    generators (*not* generator-based coroutines.)

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

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

Coroutines are based on generators internally, thus they share the
implementation.  Similarly to generator objects, coroutine objects have
``throw``, ``send`` and ``close`` methods.  ``StopIteration`` and
``GeneratorExit`` play the same role for coroutine objects (although
PEP 479 is enabled by default for coroutines).

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, or a C object with
     ``tp_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.

: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 awaitable                    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
-----------------------

This proposal preserves 100% backwards compatibility.

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

Grammar changes are also 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*

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.coroutine()`` 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
=====================

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.

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 "async for/with" instead of "await for/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
---------------------------

``__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 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 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
--------------

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.

Async lambdas
-------------

Lambda coroutines are not part of this proposal.  In this proposal they
would look like ``async lambda(parameters): expression``.  Unless there
is a strong demand to have them as part of this proposal, it is
recommended to consider them later in a separate PEP.

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, and new
    ``tp_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``.

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()``, and ``inspect.iscoroutine()``.

7. New ``CO_COROUTINE`` 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-funct...

.. [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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PEP 492: async/await in Python; v3

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