Mailman 3 May 2015 - Python-Dev

Ancient use of generators
by Guido van Rossum 06 May '15

06 May '15

For those interested in tracking the history of generators and coroutines in Python, I just found out that PEP 342 <https://www.python.org/dev/peps/pep-0342/> (which introduced send/throw/close and made "generators as coroutines" a mainstream Python concept) harks back to PEP 288 <https://www.python.org/dev/peps/pep-0288/>, which was rejected. PEP 288 also proposed some changes to generators. The interesting bit though is in the references: there are two links to old articles by David Mertz that describe using generators in state machines and other interesting and unconventional applications of generators. All these well predated PEP 342, so yield was a statement and could not receive a value from the function calling next() -- communication was through a shared class instance. http://gnosis.cx/publish/programming/charming_python_b5.txt http://gnosis.cx/publish/programming/charming_python_b7.txt Enjoy! -- --Guido van Rossum (python.org/~guido)

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Accepting PEP 492 (async/await)
by Guido van Rossum 06 May '15

06 May '15

Everybody, In order to save myself a major headache I'm hereby accepting PEP 492. I've been following Yury's efforts carefully and I am fully confident that we're doing the right thing here. There is only so much effort we can put into clarifying terminology and explaining coroutines. Somebody should write a tutorial. (I started to write one, but I ran out of time after just describing basic yield.) I've given Yury clear instructions to focus on how to proceed -- he's to work with another core dev on getting the implementation ready in time for beta 1 (scheduled for May 24, but I think the target date should be May 19). The acceptance is provisional in the PEP 411 sense (stretching its meaning to apply to language changes). That is, we reserve the right to change the specification (or even withdraw it, in a worst-case scenario) until 3.6, although I expect we won't need to do this except for some peripheral issues (e.g. the backward compatibility flags). I now plan to go back to PEP 484 (type hints). Fortunately in that case there's not much *implementation* that will land (just the typing.py module), but there's still a lot of language in the PEP that needs updating (check the PEP 484 tracker <https://github.com/ambv/typehinting/issues>). -- --Guido van Rossum (python.org/~guido)

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PEP 492: async/await in Python; v3
by Yury Selivanov 06 May '15

06 May '15

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(a)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. ``(a)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 ``(a)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 ``(a)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 ``(a)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 ``(a)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-func… .. [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. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End:

16 72

Sub-claasing pathlib.Path seems impossible
by Christophe Bal 05 May '15

05 May '15

Hello. In this post <http://stackoverflow.com/questions/29850801/simple-subclassing-pathlib-path…>, I have noticed a problem with the following code. from pathlib import Path > class PPath(Path): > def __init__(self, *args, **kwargs): > super().__init__(*args, **kwargs) > > test = PPath("dir", "test.txt") > > This gives the following error message. > Traceback (most recent call last): > File "/Users/projetmbc/test.py", line 14, in <module> > test = PPath("dir", "test.txt") > File "/anaconda/lib/python3.4/pathlib.py", line 907, in __new__ > self = cls._from_parts(args, init=False) > File "/anaconda/lib/python3.4/pathlib.py", line 589, in _from_parts > drv, root, parts = self._parse_args(args) > File "/anaconda/lib/python3.4/pathlib.py", line 582, in _parse_args > return cls._flavour.parse_parts(parts)AttributeError: type object 'PPath' has no attribute '_flavour' > > This breaks the sub-classing from Python point of view. In the post <http://stackoverflow.com/questions/29850801/simple-subclassing-pathlib-path…>, I give a hack to sub-class Path but it's a bit Unpythonic. *Christophe BAL* *Enseignant de mathématiques en Lycée **et développeur Python amateur* *---* *French math teacher in a "Lycée" **and **Python **amateur developer*

4 3

PEP 492 and types.coroutine
by Ethan Furman 02 May '15

02 May '15

According to https://www.python.org/dev/peps/pep-0492/#id31: The [types.coroutine] function applies CO_COROUTINE flag to generator-function's code object, making it return a coroutine object. Further down in https://www.python.org/dev/peps/pep-0492/#id32: [await] uses the yield from implementation with an extra step of validating its argument. await only accepts an awaitable, which can be one of: ... - A generator-based coroutine object returned from a generator decorated with types.coroutine(). If I'm understanding this correctly, type.coroutine's only purpose is to add a flag to a generator object so that await will accept it. This raises the question of why can't await simply accept a generator object? There is no code change to the gen obj itself, there is no behavior change in the gen obj, it's the exact same byte code, only a flag is different. types.coroutine feels a lot like unnecessary boiler-plate. -- ~Ethan~

3 5

Summary of Python tracker Issues
by Python tracker 01 May '15

01 May '15

ACTIVITY SUMMARY (2015-04-24 - 2015-05-01) Python tracker at http://bugs.python.org/ To view or respond to any of the issues listed below, click on the issue. Do NOT respond to this message. Issues counts and deltas: open 4841 (+27) closed 31025 (+25) total 35866 (+52) Open issues with patches: 2254 Issues opened (39) ================== #24054: Invalid syntax in inspect_fodder2.py (on Python 2.x) http://bugs.python.org/issue24054 opened by ddriddle #24055: unittest package-level set up & tear down module http://bugs.python.org/issue24055 opened by demian.brecht #24056: Expose closure & generator status in function repr() http://bugs.python.org/issue24056 opened by ncoghlan #24060: Clearify necessities for logging with timestamps http://bugs.python.org/issue24060 opened by krichter #24063: Support Mageia and Arch Linux in the platform module http://bugs.python.org/issue24063 opened by akien #24064: Make the property doctstring writeable http://bugs.python.org/issue24064 opened by rhettinger #24065: Outdated *_RESTRICTED flags in structmember.h http://bugs.python.org/issue24065 opened by berker.peksag #24066: send_message should take all the addresses in the To: header i http://bugs.python.org/issue24066 opened by kirelagin #24067: Weakproxy is an instance of collections.Iterator http://bugs.python.org/issue24067 opened by ereuveni #24068: statistics module - incorrect results with boolean input http://bugs.python.org/issue24068 opened by wolma #24069: Option to delete obsolete bytecode files http://bugs.python.org/issue24069 opened by Sworddragon #24076: sum() several times slower on Python 3 http://bugs.python.org/issue24076 opened by lukasz.langa #24078: inspect.getsourcelines ignores context and returns wrong line http://bugs.python.org/issue24078 opened by siyuan #24079: xml.etree.ElementTree.Element.text does not conform to the doc http://bugs.python.org/issue24079 opened by jlaurens #24080: asyncio.Event.wait() Task was destroyed but it is pending http://bugs.python.org/issue24080 opened by matt #24081: Obsolete caveat in reload() docs http://bugs.python.org/issue24081 opened by encukou #24082: Obsolete note in argument parsing (c-api/arg.rst) http://bugs.python.org/issue24082 opened by encukou #24084: pstats: sub-millisecond display http://bugs.python.org/issue24084 opened by Romuald #24085: large memory overhead when pyc is recompiled http://bugs.python.org/issue24085 opened by bukzor #24086: Configparser interpolation is unexpected http://bugs.python.org/issue24086 opened by tbekolay #24087: Documentation doesn't explain the term "coroutine" (PEP 342) http://bugs.python.org/issue24087 opened by paul.moore #24088: yield expression confusion http://bugs.python.org/issue24088 opened by Jim.Jewett #24089: argparse crashes with AssertionError http://bugs.python.org/issue24089 opened by spaceone #24090: Add a "copy variable to clipboard" option to the edit menu http://bugs.python.org/issue24090 opened by irdb #24091: Use after free in Element.extend (1) http://bugs.python.org/issue24091 opened by pkt #24092: Use after free in Element.extend (2) http://bugs.python.org/issue24092 opened by pkt #24093: Use after free in Element.remove http://bugs.python.org/issue24093 opened by pkt #24094: Use after free during json encoding (PyType_IsSubtype) http://bugs.python.org/issue24094 opened by pkt #24095: Use after free during json encoding a dict (2) http://bugs.python.org/issue24095 opened by pkt #24096: Use after free in get_filter http://bugs.python.org/issue24096 opened by pkt #24097: Use after free in PyObject_GetState http://bugs.python.org/issue24097 opened by pkt #24098: Multiple use after frees in obj2ast_* methods http://bugs.python.org/issue24098 opened by pkt #24099: Use after free in siftdown (1) http://bugs.python.org/issue24099 opened by pkt #24100: Use after free in siftdown (2) http://bugs.python.org/issue24100 opened by pkt #24101: Use after free in siftup http://bugs.python.org/issue24101 opened by pkt #24102: Multiple type confusions in unicode error handlers http://bugs.python.org/issue24102 opened by pkt #24103: Use after free in xmlparser_setevents (1) http://bugs.python.org/issue24103 opened by pkt #24104: Use after free in xmlparser_setevents (2) http://bugs.python.org/issue24104 opened by pkt #24105: Use after free during json encoding a dict (3) http://bugs.python.org/issue24105 opened by pkt Most recent 15 issues with no replies (15) ========================================== #24105: Use after free during json encoding a dict (3) http://bugs.python.org/issue24105 #24104: Use after free in xmlparser_setevents (2) http://bugs.python.org/issue24104 #24103: Use after free in xmlparser_setevents (1) http://bugs.python.org/issue24103 #24102: Multiple type confusions in unicode error handlers http://bugs.python.org/issue24102 #24101: Use after free in siftup http://bugs.python.org/issue24101 #24100: Use after free in siftdown (2) http://bugs.python.org/issue24100 #24099: Use after free in siftdown (1) http://bugs.python.org/issue24099 #24098: Multiple use after frees in obj2ast_* methods http://bugs.python.org/issue24098 #24097: Use after free in PyObject_GetState http://bugs.python.org/issue24097 #24095: Use after free during json encoding a dict (2) http://bugs.python.org/issue24095 #24094: Use after free during json encoding (PyType_IsSubtype) http://bugs.python.org/issue24094 #24093: Use after free in Element.remove http://bugs.python.org/issue24093 #24092: Use after free in Element.extend (2) http://bugs.python.org/issue24092 #24091: Use after free in Element.extend (1) http://bugs.python.org/issue24091 #24090: Add a "copy variable to clipboard" option to the edit menu http://bugs.python.org/issue24090 Most recent 15 issues waiting for review (15) ============================================= #24087: Documentation doesn't explain the term "coroutine" (PEP 342) http://bugs.python.org/issue24087 #24084: pstats: sub-millisecond display http://bugs.python.org/issue24084 #24082: Obsolete note in argument parsing (c-api/arg.rst) http://bugs.python.org/issue24082 #24081: Obsolete caveat in reload() docs http://bugs.python.org/issue24081 #24076: sum() several times slower on Python 3 http://bugs.python.org/issue24076 #24068: statistics module - incorrect results with boolean input http://bugs.python.org/issue24068 #24066: send_message should take all the addresses in the To: header i http://bugs.python.org/issue24066 #24064: Make the property doctstring writeable http://bugs.python.org/issue24064 #24056: Expose closure & generator status in function repr() http://bugs.python.org/issue24056 #24054: Invalid syntax in inspect_fodder2.py (on Python 2.x) http://bugs.python.org/issue24054 #24053: Define EXIT_SUCCESS and EXIT_FAILURE constants in sys http://bugs.python.org/issue24053 #24042: Convert os._getfullpathname() and os._isdir() to Argument Clin http://bugs.python.org/issue24042 #24037: Argument Clinic: add the boolint converter http://bugs.python.org/issue24037 #24036: GB2312 codec is using a wrong covert table http://bugs.python.org/issue24036 #24034: Make fails Objects/typeslots.inc http://bugs.python.org/issue24034 Top 10 most discussed issues (10) ================================= #24053: Define EXIT_SUCCESS and EXIT_FAILURE constants in sys http://bugs.python.org/issue24053 30 msgs #24067: Weakproxy is an instance of collections.Iterator http://bugs.python.org/issue24067 12 msgs #24076: sum() several times slower on Python 3 http://bugs.python.org/issue24076 11 msgs #24085: large memory overhead when pyc is recompiled http://bugs.python.org/issue24085 11 msgs #23749: asyncio missing wrap_socket http://bugs.python.org/issue23749 10 msgs #24018: add a Generator ABC http://bugs.python.org/issue24018 10 msgs #24052: sys.exit(code) returns "success" to the OS for some nonzero va http://bugs.python.org/issue24052 9 msgs #23955: Add python.ini file for embedded/applocal installs http://bugs.python.org/issue23955 8 msgs #17908: Unittest runner needs an option to call gc.collect() after eac http://bugs.python.org/issue17908 7 msgs #24043: Implement mac_romanian and mac_croatian encodings http://bugs.python.org/issue24043 6 msgs Issues closed (23) ================== #9246: os.getcwd() hardcodes max path len http://bugs.python.org/issue9246 closed by haypo #21354: PyCFunction_New no longer exposed by python DLL breaking bdist http://bugs.python.org/issue21354 closed by asvetlov #23058: argparse silently ignores arguments http://bugs.python.org/issue23058 closed by barry #23342: run() - unified high-level interface for subprocess http://bugs.python.org/issue23342 closed by gregory.p.smith #23356: In argparse docs simplify example about argline http://bugs.python.org/issue23356 closed by berker.peksag #23668: Support os.ftruncate on Windows http://bugs.python.org/issue23668 closed by steve.dower #23852: Wrong computation of max_fd on OpenBSD http://bugs.python.org/issue23852 closed by gregory.p.smith #23910: property_descr_get reuse argument tuple http://bugs.python.org/issue23910 closed by rhettinger #23996: _PyGen_FetchStopIterationValue() crashes on unnormalised excep http://bugs.python.org/issue23996 closed by pitrou #24035: When Caps Locked, <Shift> + alpha-character still displayed as http://bugs.python.org/issue24035 closed by principia1687 #24057: trivial typo in datetime.rst: needing a preceding dot http://bugs.python.org/issue24057 closed by python-dev #24058: Compiler warning for readline extension http://bugs.python.org/issue24058 closed by python-dev #24059: Minor speed and readability improvement to the random module http://bugs.python.org/issue24059 closed by rhettinger #24061: Python 2.x breaks with address sanitizer http://bugs.python.org/issue24061 closed by python-dev #24062: links to os.stat() in documentation lead to stat module instea http://bugs.python.org/issue24062 closed by berker.peksag #24070: Exceptions and arguments disappear when using argparse inside http://bugs.python.org/issue24070 closed by benjamin.peterson #24071: Python 2.7.8, 2.7.9 re.MULTILINE failure http://bugs.python.org/issue24071 closed by serhiy.storchaka #24072: xml.etree.ElementTree.Element does not catch text http://bugs.python.org/issue24072 closed by ned.deily #24073: sys.stdin.mode can not give the right mode and os.fdopen does http://bugs.python.org/issue24073 closed by ned.deily #24074: string, center, ljust, rjust, width paramter should accept Non http://bugs.python.org/issue24074 closed by rhettinger #24075: list.sort() should do quick exit if len(list) <= 1 http://bugs.python.org/issue24075 closed by benjamin.peterson #24077: man page says -I implies -S. code says -s. http://bugs.python.org/issue24077 closed by ned.deily #24083: MSVCCompiler.get_msvc_path() doesn't work on Win x64 http://bugs.python.org/issue24083 closed by lemburg

1 0

PEP 492 vs. PEP 3152, new round
by Guido van Rossum 01 May '15

01 May '15

I've tried to catch up with the previous threads. A summary of issues brought up: 1. precise syntax of `async def` (or do we need it at all) 2. do we need `async for` and `async with` (and how to spell them) 3. syntactic priority of `await` 4. `cocall` vs. `await` 5. do we really need `__aiter__` and friends 6. StopAsyncException 7. compatibility with asyncio and existing users of it (I've added a few myself.) I'll try to take them one by one. *1. precise syntax of `async def`* Of all the places to put `async` I still like *before* the `def` the best. I often do "imprecise search" for e.g. /def foo/ and would be unhappy if this didn't find async defs. Putting it towards the end (`def foo async()` or `def foo() async`) makes it easier to miss. A decorator makes it hard to make the syntactic distinctions required to reject `await` outside an async function. So I still prefer *`async def`*. *2. do we need `async for` and `async with`* Yes we do. Most of you are too young to remember, but once upon a time you couldn't loop over the lines of a file with a `for` loop like you do now. The amount of code that was devoted to efficiently iterate over files was tremendous. We're back in that stone age with the asyncio `StreamReader` class -- it supports `read()`, `readline()` and so on, but you can't use it with `for`, so you have to write a `while True` loop. `asyncio for` makes it possible to add a simple `__anext__` to the `StreamReader` class, as follows: ``` async def __anext__(self): line = await self.readline() if not line: raise StopAsyncIteration return line ``` A similar argument can be made for `async with`; the transaction commit is pretty convincing, but it also helps to be able to wait e.g. for a transport to drain upon closing a write stream. As for how to spell these, I think having `async` at the front makes it most clear that this is a special form. (Though maybe we should consider `await for` and `await with`? That would have the advantage of making it easy to scan for all suspension points by searching for /await/. But being a verb it doesn't read very well.) *3. syntactic priority of `await`* Yury, could you tweak the syntax for `await` so that we can write the most common usages without parentheses? In particular I'd like to be able to write ``` return await foo() with await foo() as bar: ... foo(await bar(), await bletch()) ``` (I don't care about `await foo() + await bar()` but it would be okay.) ``` I think this is reasonable with some tweaks of the grammar (similar to what Greg did for cocall, but without requiring call syntax at the end). *4. `cocall` vs. `await`* Python evolves. We couldn't have PEP 380 (`yield from`) without prior experience with using generators as coroutines (PEP 342), which in turn required basic generators (PEP 255), and those were a natural evolution of Python's earlier `for` loop. We couldn't PEP 3156 (asyncio) without PEP 380 and all that came before. The asyncio library is getting plenty of adoption and it has the concept of separating the *getting* of a future[1] from *waiting* for it. IIUC this is also how `await` works in C# (it just requires something with an async type). This has enabled a variety of operations that take futures and produce more futures. [1] I write `future` with a lowercase 'f' to include concepts like coroutine generator objects. *I just can't get used to this aspect of PEP 3152, so I'm rejecting it.* Sorry Greg, but that's the end. We must see `await` as a refinement of `yield from`, not as an alternative. (Yury: PEP 492 is not accepted yet, but you're getting closer.) One more thing: this separation is "Pythonic" in the sense that it's similar to the way *getting* a callable object is a separate act from *calling* it. While this is a cause for newbie bugs (forgetting to call an argument-less function) it has also enabled the concept of "callable" as more general and more powerful in Python: any time you need to pass a callable, you can pass e.g. a bound method or a class or something you got from `functools.partial`, and that's a useful thing (other languages require you to introduce something like a lambda in such cases, which can be painful if the thing you wrap has a complex signature -- or they don't support function parameters at all, like Java). I know that Greg defends it by explaining that `cocal f(args)` is not a `cocall` operator applied to `f(args)`, it is the *single* operator `cocall ...(args)` applied to `f`. But this is too subtle, and it just doesn't jive with the long tradition of using `yield from f` where f is some previously obtained future. *5. do we really need `__aiter__` and friends* There's a lot of added complexity, but I think it's worth it. I don't think we need to make the names longer, the 'a' prefix is fine for these methods. I think it's all in the protocols: regular `with` uses `__enter__` and `__exit__`; `async with` uses `__aenter__` and `__aexit__` (which must return futures). Ditto for `__aiter__` and `__anext__`. I guess this means that the async equivalent to obtaining an iterator through `it = iter(xs)` followed by `for x over it` will have to look like `ait = await aiter(xs)` followed by `for x over ait`, where an iterator is required to have an `__aiter__` method that's an async function and returns self immediately. But what if you left out the `await` from the first call? I.e. can this work? ``` ait = aiter(xs) async for x in ait: print(x) ``` The question here is whether the object returned by aiter(xs) has an `__aiter__` method. Since it was intended to be the target of `await`, it has an `__await__` method. But that itself is mostly an alias for `__iter__`, not `__aiter__`. I guess it can be made to work, the object just has to implement a bunch of different protocols. *6. StopAsyncException* I'm not sure about this. The motivation given in the PEP seems to focus on the need for `__anext__` to be async. But is this really the right pattern? What if we required `ait.__anext__()` to return a future, which can either raise good old `StopIteration` or return the next value from the iteration when awaited? I'm wondering if there are a few alternatives to be explored around the async iterator protocol still. *7. compatibility with asyncio and existing users of it* This is just something I want to stress. On the one hand it should be really simple to take code written for pre-3.5 asyncio and rewrite it to PEP 492 -- simple change `(a)asyncio.coroutine` to `async def` and change `yield from` to `await`. (Everything else is optional; existing patterns for loops and context managers should continue to work unchanged, even if in some cases you may be able to simplify the code by using `async for` and `async with`.) But it's also important that *even if you don't switch* (i.e. if you want to keep your code compatible with pre-3.5 asyncio) you can still use the PEP 492 version of asyncio -- i.e. the asyncio library that comes with 3.5 must seamlessly support mixing code that uses `await` and code that uses `yield from`. And this should go both ways -- if you have some code that uses PEP 492 and some code that uses pre-3.5 asyncio, they should be able to pass their coroutines to each other and wait for each other's coroutines. That's all I have for now. Enjoy! -- --Guido van Rossum (python.org/~guido)

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Python-Dev May 2015