[Python-Dev] async/await in Python; v2 (original) (raw)

Wolfgang Langner tds333+pydev at gmail.com
Thu Apr 23 09:30:30 CEST 2015

Hi,

most of the time I am a silent reader but in this discussion I must step in. I use twisted and async stuff a lot over years followed development of asyncio closely.

First it is good to do differentiate async coroutines from generators. So everyone can see it and have this in mind and don't mix up booth. It is also easier to explain for new users. Sometimes generators blows their mind and it takes a while to get used to them. Async stuff is even harder.

  1. I am fine with using something special instead of "yield" or "yield from" for this. C# "await" is ok.

Everything else suggested complicates the language and makes it harder to read.

  1. async def f(): is harder to read and something special also it breaks the symmetry in front (def indent). Also every existing tooling must be changed to support it. Same for def async, def f() async: I thing a decorator is enough here @coroutine def f(): is the best solution to mark something as a coroutine.

  2. async with and async for Bead idea, we clutter the language even more and it is one more thing every newbie could do wrong. for x in y: result = await f() is enough, every 'async' framework lived without it over years. Same for with statement.

The main use case suggested was for database stuff and this is also where most are best with defer something to a thread and keep it none async.

All together it is very late in the development cycle for 3.5 to incorporate such a big change. Best is to give all this some more time and defer it to 3.6 and some alpha releases to experiment with.

Regards,

Wolfgang

On Tue, Apr 21, 2015 at 7:26 PM, Yury Selivanov <yselivanov.ml at gmail.com> wrote:

Hi python-dev,

I'm moving the discussion from python-ideas to here. The updated version of the PEP should be available shortly at https://www.python.org/dev/peps/pep-0492 and is also pasted in this email. Updates: 1. COASYNC flag was renamed to COCOROUTINE; 2. sys.setasyncwrapper() was renamed to sys.setcoroutinewrapper(); 3. New function: sys.getcoroutinewrapper(); 4. types.asyncdef() renamed to types.coroutine(); 5. New section highlighting differences from PEP 3152. 6. New AST node - AsyncFunctionDef; the proposal now is 100% backwards compatible; 7. A new section clarifying that coroutine-generators are not part of the current proposal; 8. Various small edits/typos fixes.

There's is a bug tracker issue to track code review of the reference implementation (Victor Stinner is doing the review): http://bugs.python.org/issue24017 While the PEP isn't accepted, we want to make sure that the reference implementation is ready when such a decision will be made. Let's discuss some open questions: 1. Victor raised a question if we should locate coroutine() function from 'types' module to 'functools'. My opinion is that 'types' module is a better place for 'corotine()', since it adjusts the type of the passed generator. 'functools' is about 'partials', 'lrucache' and 'wraps' kind of things. 2. I propose to disallow using of 'for..in' loops, and builtins like 'list()', 'iter()', 'next()', 'tuple()' etc on coroutines. It's possible by modifying PyObjectGetIter to raise an exception if it receives a coroutine-object. 'yield from' can also be modified to only accept coroutine objects if it is called from a generator with COCOROUTINE flag. This will further separate coroutines from generators, making it harder to screw something up by an accident. I have a branch of reference implementation https://github.com/1st1/cpython/tree/awaitnoiter where this is implemented. I did not observe any performance drop. There is just one possible backwards compatibility issue here: there will be an exception if some user of asyncio actually used to iterate over generators decorated with @coroutine. But I can't imagine why would someone do that, and even if they did -- it's probably a bug or wrong usage of asyncio. That's it! I'd be happy to hear some feedback! Thanks, Yury

PEP: 492 Title: Coroutines with async and await syntax Version: RevisionRevisionRevision Last-Modified: DateDateDate Author: Yury Selivanov <yselivanov at 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 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 readdata(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 - COCOROUTINE - is introduced to enable runtime detection of coroutines (and migrating existing code). All coroutines have both COCOROUTINE and COGENERATOR 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 COCOROUTINE 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 readdata(db): data = await db.fetch('SELECT ...') ... await, similarly to yield from, suspends execution of readdata 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 SyntaxError to use await outside of a coroutine. Asynchronous Context Managers and "async with" ---------------------------------------------- An asynchronous context manager is a context manager that is able to suspend execution in its enter and exit methods. To make this possible, a new protocol for asynchronous context managers is proposed. Two new magic methods are added: _aenter_ and _aexit_. Both must return an awaitable. An example of an asynchronous context manager:: class AsyncContextManager: async def aenter(self): await log('entering context') async def aexit(self, exctype, 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 exitres = await aexit(mgr, *sys.excinfo()) if not exitres: 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.fetchdata() if data: return data else: raise StopAsyncIteration async def fetchdata(self): ... New Syntax '''''''''' A new statement for iterating through asynchronous iterators is proposed:: async for TARGET in ITER: BLOCK else: BLOCK2 which is semantically equivalent to:: iter = (ITER) iter = await type(iter).aiter(iter) running = True while running: try: TARGET = await type(iter).anext(iter) except StopAsyncIteration: running = False else: BLOCK else: BLOCK2 It is an error to pass a regular iterable without _aiter_ method to async for. It is a SyntaxError to use async for outside of a coroutine. As for with regular for statement, async for has an optional else clause. Example 1 ''''''''' With asynchronous iteration protocol it is possible to asynchronously buffer data during iteration:: async for data in cursor: ... Where cursor is an asynchronous iterator that prefetches N rows of data from a database after every N iterations. The following code illustrates new asynchronous iteration protocol:: class Cursor: def init(self): self.buffer = collections.deque() def prefetch(self): ... async def aiter(self): return self async def anext(self): if not self.buffer: self.buffer = await self.prefetch() if not self.buffer: raise StopAsyncIteration return self.buffer.popleft() then the Cursor class can be used as follows:: async for row in Cursor(): print(row) which would be equivalent to the following code:: i = await Cursor().aiter() while True: try: row = await i.anext() except StopAsyncIteration: break else: print(row) Example 2 ''''''''' The following is a utility class that transforms a regular iterable to an asynchronous one. While this is not a very useful thing to do, the code illustrates the relationship between regular and asynchronous iterators. :: class AsyncIteratorWrapper: def init(self, obj): self.it = iter(obj) async def aiter(self): return self async def anext(self): try: value = next(self.it) except StopIteration: raise StopAsyncIteration return value async for item in AsyncIteratorWrapper("abc"): print(item) Why StopAsyncIteration? ''''''''''''''''''''''' Coroutines are still based on generators internally. So, before PEP 479, there was no fundamental difference between :: def g1(): yield from fut return 'spam' and :: def g2(): yield from fut raise StopIteration('spam') And since PEP 479 is accepted and enabled by default for coroutines, the following example will have its StopIteration wrapped into a RuntimeError :: async def a1(): await fut raise StopIteration('spam') The only way to tell the outside code that the iteration has ended is to raise something other than StopIteration. Therefore, a new built-in exception class StopAsyncIteration was added. Moreover, with semantics from PEP 479, all StopIteration exceptions raised in coroutines are wrapped in RuntimeError. Debugging Features ------------------ One of the most frequent mistakes that people make when using generators as coroutines is forgetting to use yield from:: @asyncio.coroutine def useful(): asyncio.sleep(1) # this will do noting without 'yield from' For debugging this kind of mistakes there is a special debug mode in asyncio, in which @coroutine decorator wraps all functions with a special object with a destructor logging a warning. Whenever a wrapped generator gets garbage collected, a detailed logging message is generated with information about where exactly the decorator function was defined, stack trace of where it was collected, etc. Wrapper object also provides a convenient _repr_ function with detailed information about the generator. The only problem is how to enable these debug capabilities. Since debug facilities should be a no-op in production mode, @coroutine decorator makes the decision of whether to wrap or not to wrap based on an OS environment variable PYTHONASYNCIODEBUG. This way it is possible to run asyncio programs with asyncio's own functions instrumented. EventLoop.setdebug, 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 setcoroutinewrapper and getcoroutinewrapper 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 debugme(): await asyncio.sleep(1) def asyncdebugwrap(generator): return asyncio.AsyncDebugWrapper(generator) sys.setcoroutinewrapper(asyncdebugwrap) debugme() # <- this line will likely GC the coroutine object and_ _# trigger AsyncDebugWrapper's code._ _assert isinstance(debugme(), AsyncDebugWrapper)_ _sys.setcoroutinewrapper(None) # <- this unsets any_ _# previously set wrapper_ _assert not isinstance(debugme(), AsyncDebugWrapper)_ _If sys.setcoroutinewrapper() is called twice, the new wrapper_ _replaces the previous wrapper. sys.setcoroutinewrapper(None)_ _unsets the wrapper._ _Glossary_ _========_ _:Coroutine:_ _A coroutine function, or just "coroutine", is declared with async_ _def. It uses await and return value; see New Coroutine_ _Declaration Syntax for details._ _:Coroutine object:_ _Returned from a coroutine function. See Await Expression for_ _details._ _:Future-like object:_ _An object with an _await_ method. 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::_ _awaitexpr: AWAIT test_ _awaitstmt: awaitexpr_ _decorated: decorators (classdef | funcdef | asyncfuncdef)_ _asyncfuncdef: ASYNC funcdef_ _asyncstmt: ASYNC (funcdef | withstmt | forstmt)_ _compoundstmt: (ifstmt | whilestmt | forstmt | trystmt |_ _withstmt | funcdef | classdef | decorated |_ _asyncstmt)_ _atom: ('(' [yieldexpr|awaitexpr|testlistcomp] ')' |_ _'[' [testlistcomp] ']' |_ _'{' [dictorsetmaker] '}' |_ _NAME | NUMBER | STRING+ | '...' | 'None' | 'True' | 'False’)_ _exprstmt: testliststarexpr_ _(augassign (yieldexpr|awaitexpr|testlist) |_ _('=' (yieldexpr|awaitexpr|testliststarexpr))*)_ _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_ _adefault = await fut_ _def nested(a=adefault):_ _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 ensuretask() 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 | 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. To call cofunctions from existing generator-based coroutines it would be required to use costart built-in. In this proposal @asyncio.coroutine simply sets COCOROUTINE on the wrapped function's code object and everything works automatically. 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. 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 withandasync for, where async`` is a modifier, stating that the statement is asynchronous. It is also more consistent with the existing grammar. Why not a future import --------------------------- _future_ imports are inconvenient and easy to forget to add. Also, they are enabled for the whole source file. Consider that there is a big project with a popular module named "async.py". With future imports it is required to either import it using _import_() or importlib.importmodule() 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 _asynciter_. However, to align new magic methods with the existing ones, such as _radd_ and _iadd_ it was decided to use a shorter version. Why not reuse existing magic names ---------------------------------- An alternative idea about new asynchronous iterators and context managers was to reuse existing magic methods, by adding an async keyword to their declarations:: class CM: async def enter(self): # instead of aenter ... This approach has the following downsides: * it would not be possible to create an object that works in both with and async with statements; * it would look confusing and would require some implicit magic behind the scenes in the interpreter; * one of the main points of this proposal is to make coroutines as simple and foolproof as possible. Comprehensions -------------- For the sake of restricting the broadness of this PEP there is no new syntax for asynchronous comprehensions. This should be considered in a separate PEP, if there is a strong demand for this feature. 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/RELEASEX8664 x8664 i386 Total CPU cores: 8 ### etreeiterparse ### 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: djangov2, 2to3, etreegenerate, etreeparse, etreeprocess, fastpickle, fastunpickle, jsondumpv2, jsonload, nbody, regexv8, tornadohttp. Tokenizer modifications ----------------------- There is no observable slowdown of parsing python files with the modified tokenizer: parsing of one 12Mb file (Lib/test/testbinop.py repeated 1000 times) takes the same amount of time. async/await ----------- The following micro-benchmark was used to determine performance difference between "async" functions and generators:: import sys import time def binary(n): if n <= 0: return 1 l = yield from binary(n - 1) r = yield from binary(n - 1) return l + 1 + r async def abinary(n): if n <= 0: return 1 l = await abinary(n - 1) r = await abinary(n - 1) return l + 1 + r def timeit(gen, depth, repeat): t0 = time.time() for in range(repeat): list(gen(depth)) t1 = time.time() print('{}({}) * {}: total {:.3f}s'.format( gen.name, depth, repeat, t1-t0)) The result is that there is no observable performance difference. Minimum timing of 3 runs :: abinary(19) * 30: total 12.985s binary(19) * 30: total 12.953s Note that depth of 19 means 1,048,575 calls. Reference Implementation ======================== The reference implementation can be found here: [3]. List of high-level changes and new protocols -------------------------------------------- 1. New syntax for defining coroutines: async def and new await keyword. 2. New _await_ method for Future-like objects. 3. New syntax for asynchronous context managers: async with. And associated protocol with _aenter_ and _aexit_ methods. 4. New syntax for asynchronous iteration: async for. And associated protocol with _aiter_, _aexit_ and new built- in exception StopAsyncIteration. 5. New AST nodes: AsyncFunctionDef, AsyncFor, AsyncWith, Await. 6. New functions: sys.setcoroutinewrapper(callback), sys.getcoroutinewrapper(), and types.coroutine(gen). 7. New COCOROUTINE 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 echoserver(): print('Serving on localhost:8000') await asyncio.startserver(handleconnection, 'localhost', 8000) async def handleconnection(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.geteventloop() loop.rununtilcomplete(echoserver()) try: loop.runforever() 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:asyncfunctions .. [3] https://github.com/1st1/cpython/tree/await .. [4] https://hg.python.org/benchmarks .. [5] https://msdn.microsoft.com/en-us/library/hh191443.aspx .. [6] http://docs.hhvm.com/manual/en/hack.async.php .. [7] https://www.dartlang.org/articles/await-async/ .. [8] http://docs.scala-lang.org/sips/pending/async.html .. [9] https://github.com/google/traceur-compiler/wiki/LanguageFeatures#async-functions-experimental .. [10] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3722.pdf (PDF) Acknowledgments =============== I thank Guido van Rossum, Victor Stinner, Elvis Pranskevichus, Andrew Svetlov, and Łukasz Langa for their initial feedback. Copyright ========= This document has been placed in the public domain.

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