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Implemenation of the PEP 492 - Coroutines with async and await syntax #68205

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vstinner opened this issue Apr 20, 2015 · 69 comments
Closed

Implemenation of the PEP 492 - Coroutines with async and await syntax #68205

vstinner opened this issue Apr 20, 2015 · 69 comments
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interpreter-core Interpreter core (Objects, Python, Grammar, and Parser dirs) release-blocker type-feature A feature request or enhancement

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@vstinner
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vstinner commented Apr 20, 2015

BPO 24017
Nosy @gvanrossum, @ncoghlan, @scoder, @vstinner, @asvetlov, @agronholm, @1st1
Dependencies
  • bpo-2292: Missing *-unpacking generalizations
  • bpo-22906: PEP 479: Change StopIteration handling inside generators
  • bpo-24018: add a Generator ABC
  • bpo-24180: PEP 492: Documentation
  • bpo-24184: PEP 492: Add AsyncIterator and AsyncIterable to collections.abc
  • bpo-24315: collections.abc: Coroutine should be derived from Awaitable
  • bpo-24316: Fix types.coroutine to accept objects from Cython
  • bpo-24400: Awaitable ABC incompatible with functools.singledispatch
  • Files
  • await_01.patch
  • await_02.patch
  • await_03.patch
  • await_04.patch
  • await_05.patch
  • await_06.patch
  • with.patch
  • Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = 'https://github.com/1st1'
    closed_at = <Date 2015-05-13.19:46:15.300>
    created_at = <Date 2015-04-20.19:33:55.846>
    labels = ['interpreter-core', 'type-feature', 'release-blocker']
    title = 'Implemenation of the PEP 492 - Coroutines with async and await syntax'
    updated_at = <Date 2015-06-20.19:39:53.655>
    user = 'https://github.com/vstinner'

    bugs.python.org fields:

    activity = <Date 2015-06-20.19:39:53.655>
    actor = 'yselivanov'
    assignee = 'yselivanov'
    closed = True
    closed_date = <Date 2015-05-13.19:46:15.300>
    closer = 'yselivanov'
    components = ['Interpreter Core']
    creation = <Date 2015-04-20.19:33:55.846>
    creator = 'vstinner'
    dependencies = ['2292', '22906', '24018', '24180', '24184', '24315', '24316', '24400']
    files = ['39148', '39155', '39314', '39325', '39337', '39344', '39354']
    hgrepos = []
    issue_num = 24017
    keywords = ['patch']
    message_count = 69.0
    messages = ['241678', '241679', '241687', '241697', '242056', '242064', '242086', '242304', '242326', '242627', '242731', '242779', '242793', '242848', '242860', '242862', '242877', '242912', '242927', '242928', '242931', '242935', '242936', '242937', '242938', '242979', '242982', '242984', '243031', '243033', '243035', '243036', '243037', '243038', '243041', '243042', '243117', '243118', '243119', '243759', '244175', '244252', '244257', '244258', '244261', '244264', '244268', '244269', '244273', '244278', '244280', '244281', '244303', '244307', '244319', '244322', '244374', '244384', '244385', '244567', '244568', '244596', '244759', '244761', '244799', '244818', '244826', '244831', '245075']
    nosy_count = 8.0
    nosy_names = ['gvanrossum', 'ncoghlan', 'scoder', 'vstinner', 'asvetlov', 'alex.gronholm', 'python-dev', 'yselivanov']
    pr_nums = []
    priority = 'release blocker'
    resolution = 'fixed'
    stage = 'resolved'
    status = 'closed'
    superseder = None
    type = 'enhancement'
    url = 'https://bugs.python.org/issue24017'
    versions = ['Python 3.5']

    @scoder scoder added the type-feature A feature request or enhancement label Apr 20, 2015
    @1st1
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    1st1 commented Apr 20, 2015

    Here's the first patch (git diff master..await). Should be easier to review and work from there.

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    1st1 commented Apr 20, 2015

    Attaching a patch generated with mercurial

    @vstinner
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    vstinner commented Apr 20, 2015

    Does the implementation depends on the implementation of the PEP-479? (issue bpo-22906)

    Attaching a patch generated with mercurial

    Next time, if possible, try to skip generated files. Maybe write a script for that, but sorry I don't know how :-(

    @1st1
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    1st1 commented Apr 21, 2015

    Attaching a revised patch (all Victor's comments but asyncio changes)

    @1st1 1st1 self-assigned this Apr 21, 2015
    @scoder
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    scoder commented Apr 26, 2015

    Could we have type slots for the new special methods? Otherwise, implementing the protocol in C would be fairly inefficient, especially for async iteration.

    I'm asking because Cython's generator type is not Python's generator type, but implementing the rest of the proposed protocol doesn't seem to be all that difficult.

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    1st1 commented Apr 26, 2015

    Could we have type slots for the new special methods? Otherwise, implementing the protocol in C would be fairly inefficient, especially for async iteration.

    I don't think it's necessary to have slots for __aiter__, __anext__, __aenter__ and __aexit__. Async iteration will never be as fast as regular iteration, and there is plenty overhead in it. And we don't need slots for async context managers as we don't have slots for regular ones.

    What might be a good idea is to add a slot for __await__ method -- tp_await. This will allow to implement Futures in C efficiently. I'd rename 'tp_reserved' for this purpose.

    Victor, your thoughts?

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    1st1 commented Apr 26, 2015

    In fact I will likely add tp_await in the next PEP iteration. I need it to implement another feature.

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    scoder commented May 1, 2015

    I don't think it's necessary to have slots for __aiter__, __anext__, __aenter__ and __aexit__. Async iteration will never be as fast as regular iteration, and there is plenty overhead in it.

    You seem to be assuming that the outer loop is the asyncio I/O loop. That might not be the case. It might be a thread-pool executor, it might be an in-memory task switcher, or it might just be something passing on items from a list. At least "__anext__" is worth being fast, IMHO.

    Also note that one advantage of slots is that the user can cache their value to keep calling the same C function multiple times. That is not the case for a Python method, which can easily be replaced. Some iterators do that with their __next__ method, and it's perfectly valid. Having to look up the Python method on each iteration and calling through it sounds like unnecessary overhead.

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    gvanrossum commented May 1, 2015

    I think we can continue this discussion *after* the PEP's been accepted.

    @1st1
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    1st1 commented May 6, 2015

    I'll upload the most recent patch soon.

    @1st1
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    1st1 commented May 7, 2015

    Third patch attached. Victor, it would be great if you can review it!

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    1st1 commented May 8, 2015

    Another iteration:

    • support of new syntax in lib2to3
    • collections.abc.Awaitable

    @scoder
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    scoder commented May 9, 2015

    We also need a Coroutine ABC. Both the "GeneratorType" and "CO_COROUTINE" checks are too restrictive. Also see bpo-24018, which this one should in fact depend on.

    @ncoghlan
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    ncoghlan commented May 10, 2015

    Review sent - very nice work on this Yury.

    Highlights:

    • I concur with Stefan that we should have a full PyCoroutineMethods struct at the C level, with a "tp_as_coroutine" pointer to that replacing the current tp_reserved slot

    • I also concur with Stefan about adding a Coroutine ABC

    • PyType_FromSpec (and typeslots.h) will need updating once we agree on a slot structure (with my recommendation being "define C level slots for all of the new PEP-492 methods")

    • I found CO_COROUTINE/CO_NATIVE_COROUTINE confusing as a reader of the implementation, as they only told me how the objects were defined, rather than telling me why I should care. Based on what I gleaned of their intended purpose from reading the implementation, I suggest switching this to instead use CO_COROUTINE (set for all coroutines, regardless of how they were defined) and CO_ITERABLE_COROUTINE (set only for those coroutines that also support iteration), and adjusting the naming of other APIs accordingly.

    • I found the names of the WITH_CLEANUP_ENTER and WITH_CLEANUP_EXIT bytecodes misleading, as they don't refer to the corresponding context management phases - they're both related to the "exit" phase. WITH_CLEANUP_START and WITH_CLEANUP_FINISH should be clearer for readers (both of the implementation and of the disassembled bytecode).

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    1st1 commented May 10, 2015

    Review sent - very nice work on this Yury.

    Thanks a lot, Nick!

    Highlights:

    • I concur with Stefan that we should have a full PyCoroutineMethods struct at the C level, with a "tp_as_coroutine" pointer to that replacing the current tp_reserved slot

    Do you think that tp_as_async is a better name? (I explained my point of view in code review comments)

    Also, do we need slots for __aenter__ and __aexit__? We don't have slots for regular context manager protocol, fwiw.

    • I also concur with Stefan about adding a Coroutine ABC

    I will. We definitely need it.

    • PyType_FromSpec (and typeslots.h) will need updating once we agree on a slot structure (with my recommendation being "define C level slots for all of the new PEP-492 methods")
    • I found CO_COROUTINE/CO_NATIVE_COROUTINE confusing as a reader of the implementation, as they only told me how the objects were defined, rather than telling me why I should care. Based on what I gleaned of their intended purpose from reading the implementation, I suggest switching this to instead use CO_COROUTINE (set for all coroutines, regardless of how they were defined) and CO_ITERABLE_COROUTINE (set only for those coroutines that also support iteration), and adjusting the naming of other APIs accordingly.

    I agree that CO_COROUTINE is something that we should use for 'async def' functions (instead of CO_NATIVE_COROUTINE). However, CO_ITERABLE_COROUTINE sounds a bit odd to me, as generator-based coroutines (at least in asyncio) aren't supposed to be iterated over. How about CO_GENBASED_COROUTINE flag?

    • I found the names of the WITH_CLEANUP_ENTER and WITH_CLEANUP_EXIT bytecodes misleading, as they don't refer to the corresponding context management phases - they're both related to the "exit" phase. WITH_CLEANUP_START and WITH_CLEANUP_FINISH should be clearer for readers (both of the implementation and of the disassembled bytecode).

    Big +1. Your names are much better.

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    asvetlov commented May 10, 2015

    On Sun, May 10, 2015 at 7:21 PM, Yury Selivanov <report@bugs.python.org> wrote:

    Yury Selivanov added the comment:

    > Review sent - very nice work on this Yury.

    Thanks a lot, Nick!

    Highlights:

    > * I concur with Stefan that we should have a full PyCoroutineMethods struct at the C level, with a "tp_as_coroutine" pointer to that replacing the current tp_reserved slot

    Do you think that tp_as_async is a better name? (I explained my point of view in code review comments)

    Also, do we need slots for __aenter__ and __aexit__? We don't have slots for regular context manager protocol, fwiw.

    > * I also concur with Stefan about adding a Coroutine ABC

    I will. We definitely need it.

    > * PyType_FromSpec (and typeslots.h) will need updating once we agree on a slot structure (with my recommendation being "define C level slots for all of the new PEP-492 methods")

    > * I found CO_COROUTINE/CO_NATIVE_COROUTINE confusing as a reader of the implementation, as they only told me how the objects were defined, rather than telling me why I should care. Based on what I gleaned of their intended purpose from reading the implementation, I suggest switching this to instead use CO_COROUTINE (set for all coroutines, regardless of how they were defined) and CO_ITERABLE_COROUTINE (set only for those coroutines that also support iteration), and adjusting the naming of other APIs accordingly.

    I agree that CO_COROUTINE is something that we should use for 'async def' functions (instead of CO_NATIVE_COROUTINE). However, CO_ITERABLE_COROUTINE sounds a bit odd to me, as generator-based coroutines (at least in asyncio) aren't supposed to be iterated over. How about CO_GENBASED_COROUTINE flag?

    Maybe CO_ASYNC_COROUTINE and CO_OLDSTYLE_COROUTINE?
    This is wild proposal, feel free to ignore it.

    > * I found the names of the WITH_CLEANUP_ENTER and WITH_CLEANUP_EXIT bytecodes misleading, as they don't refer to the corresponding context management phases - they're both related to the "exit" phase. WITH_CLEANUP_START and WITH_CLEANUP_FINISH should be clearer for readers (both of the implementation and of the disassembled bytecode).

    Big +1. Your names are much better.

    ----------


    Python tracker <report@bugs.python.org>
    <http://bugs.python.org/issue24017\>


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    1st1 commented May 11, 2015

    New patch is attached.

    Nick, I think that all of your feedback should be addressed in this patch.

    Major changes:

    1. There are two code flags now: CO_COROUTINE and CO_GENBASED_COROUTINE (I'm OK to use another name, see my older comments). CO_COROUTINE is assigned to all 'async def' code objects. CO_GENBASED_COROUTINE is assigned to generator-based coroutines decorated with types.coroutine().

    2. tp_await renamed to tp_as_async. (I'm OK to use another name, please see my older comment first) PyAsyncMethods struct holds three slots: am_await, am_aiter, am_anext. Implementing am_exit would be tricky, because of how SETUP_WITH opcode is engineered. I'd really prefer to not to add it.

    3. collections.abc.Coroutine.

    4. etc (all other feedback from you).

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    1st1 commented May 11, 2015

    Nick, Guido,
    Updated patch attached.

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    ncoghlan commented May 12, 2015

    Latest version looks good to me (aside from a quibble about whether StopAsyncIteration should inherit from BaseException instead of Exception - see my review for details).

    Based on Guido's explanation in the review, I also suggested adding the following example method to the PEP as part of the rationale for StopAsyncIteration:

        def __anext__(self):
            try:
                data = await self._get_data()
            except EOFError:
                raise StopAsyncIteration
            return data

    The trick is that when __anext__ is itself a coroutine, we really do have 3 exit paths:

    • suspension to wait for events (await)
    • returning the next value (return)
    • terminating iteration (raise StopAsyncIteration)

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    ncoghlan commented May 12, 2015

    Guido convinced me that having StopAsyncIteration inherit from Exception was the right approach, as it means errors are more likely to be of the "we caught it when we shouldn't have" variety, rather than the harder to debug "an exception escaped when it shouldn't have" variety. This isn't like SystemExit, KeyboardInterrupt or GeneratorExit where they're specifically designed to reliably terminate a thread of execution.

    That means I can offer an unreserved +1 on the current patch (#6) for beta 1 :)

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    python-dev mannequin commented May 12, 2015

    New changeset 957478e95b26 by Yury Selivanov in branch '3.4':
    asyncio: Support PEP-492. Issue bpo-24017.
    https://hg.python.org/cpython/rev/957478e95b26

    New changeset 44c1db190525 by Yury Selivanov in branch 'default':
    asyncio: Merge 3.4 -- Support PEP-492. Issue bpo-24017.
    https://hg.python.org/cpython/rev/44c1db190525

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    python-dev mannequin commented May 12, 2015

    New changeset eeeb666a5365 by Yury Selivanov in branch 'default':
    PEP-0492 -- Coroutines with async and await syntax. Issue bpo-24017.
    https://hg.python.org/cpython/rev/eeeb666a5365

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    1st1 commented May 12, 2015

    Guido, Nick, Victor,
    Thanks for your reviews and guidance! The patch has been committed to the default branch.

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    gvanrossum commented May 12, 2015

    Thank you Yury! You are a coding machine.

    On Mon, May 11, 2015 at 8:06 PM, Yury Selivanov <report@bugs.python.org>
    wrote:

    Yury Selivanov added the comment:

    Guido, Nick, Victor,
    Thanks for your reviews and guidance! The patch has been committed to the
    default branch.

    ----------


    Python tracker <report@bugs.python.org>
    <http://bugs.python.org/issue24017\>


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    python-dev mannequin commented May 12, 2015

    New changeset 3a3cc2b9a1b2 by Yury Selivanov in branch 'default':
    bpo-24017: Update NEWS file.
    https://hg.python.org/cpython/rev/3a3cc2b9a1b2

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    python-dev mannequin commented May 12, 2015

    New changeset 0dc3b61f1dfa by Yury Selivanov in branch 'default':
    Issue bpo-24017: Plug ref leak.
    https://hg.python.org/cpython/rev/0dc3b61f1dfa

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    python-dev mannequin commented May 12, 2015

    New changeset ee7d2c9c70ab by Yury Selivanov in branch '3.4':
    asyncio: Make sure sys.set_coroutine_wrapper is called *only* when loop is running.
    https://hg.python.org/cpython/rev/ee7d2c9c70ab

    New changeset 874edaa34b54 by Yury Selivanov in branch 'default':
    asyncio: Make sure sys.set_coroutine_wrapper is called *only* when loop is running.
    https://hg.python.org/cpython/rev/874edaa34b54

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    python-dev mannequin commented May 27, 2015

    New changeset 843fe7e831a8 by Yury Selivanov in branch '3.5':
    bpo-24297: Update symbol.py. See also bpo-24017.
    https://hg.python.org/cpython/rev/843fe7e831a8

    New changeset 87509d71653b by Yury Selivanov in branch 'default':
    bpo-24297: Update symbol.py. See also bpo-24017.
    https://hg.python.org/cpython/rev/87509d71653b

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    scoder commented May 28, 2015

    I added a couple of review comments to patch 6, but since no-one has responded so far, I guess they simply haven't been noticed. So I'll just repeat them here.

    getawaitablefunc / aiternextfunc / getaiterfunc

    Is there a reason why these need to have their specific C type name instead of just reusing unaryfunc, or at least the existing iternextfunc / getiterfunc? They are unprefixed global names in the C namespace and I think we should be careful when adding more of those.

    Awaitable.register(Coroutine)

    I think this is incorrect. A Coroutine is not Awaitable unless it also implements "__await__". How else should it be awaited?

    I propose to use this wrapping code as a fallback for types.coroutine() in the case that a Generator (ABC) is passed instead of a generator (yield):

      class types_coroutine(object):
        def __init__(self, gen):
            self._gen = gen
    
        class as_coroutine(object):
            def __init__(self, gen):
                self._gen = gen
                self.send = gen.send
                self.throw = gen.throw
                self.close = gen.close
    
            def __await__(self):
                return self._gen
    
        def __call__(self, *args, **kwargs):
            return self.as_coroutine(self._gen(*args, **kwargs))

    Note that the resulting Awaitable Coroutine type is not an Iterable. This differs from a (yield) coroutine, but it matches the Coroutine and Awaitable protocols, and the intention to separate both in order to avoid mistakes on user side.

    Additionally, regarding the tests:
    4)

        def test_func_2(self):
            async def foo():
                raise StopIteration
        with self.assertRaisesRegex(
                RuntimeError, "generator raised StopIteration"):
    
                run_async(foo())

    Why is this actually necessary? I'm aware that it's also mentioned in the PEP, but is there an actual reason why a coroutine should behave the same as a generator here? Is it just an implementation detail for legacy reasons because generators and coroutines happen to share the same type implementation? (I don't think they need to, BTW.)

    1. def test_func_8(self):
      @types.coroutine
      def bar():
      return (yield from foo())
            async def foo():
                return 'spam'
        self.assertEqual(run_async(bar()), ([], 'spam') )
    

    I find it surprising that this works at all. Yield-from iterates, and a coroutine is not supposed to be iterable, only awaitable (at least, that's what all error messages tell me when I try it). So why should "yield from" work on them? What if foo() was not an Iterable but a Coroutine? Should "yield from" then call "__await__" on it internally? I would find that *really* surprising, but given the above, I think it would be necessary to achieve consistency.

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    1st1 commented May 28, 2015

    1. Awaitable.register(Coroutine)
      I think this is incorrect. A Coroutine is not Awaitable unless it also implements "__await__". How else should it be awaited?

    It *is* correct, see PEP-492. Awaitable is either a coroutine *or* an object with an __await__ method. Generally, being an awaitable means that the object can be used in "await" expression.

    I propose to use this wrapping code as a fallback for types.coroutine() in the case that a Generator (ABC) is passed instead of a generator (yield):

    Just implement tp_await/await for coroutine-like objects coming from C-API or Cython. In general, iteration protocol is still the foundation for Future-like objects, so there is nothing wrong with this.

    Generator ABC isn't supposed to be used with "await" expression.

    def test_func_2(self):
        async def foo():
            raise StopIteration
    
        with self.assertRaisesRegex(
                RuntimeError, "generator raised StopIteration"):
    
            run_async(foo())
    

    Why is this actually necessary? I'm aware that it's also mentioned in the PEP, but is there an actual reason why a coroutine should behave the same as a generator here? Is it just an implementation detail for legacy reasons because generators and coroutines happen to share the same type implementation? (I don't think they need to, BTW.)

    Coroutines are implemented on top of generators. Until we clearly separate them (in 3.6?) I don't think we should allow coroutines to bubble up StopIteration.

    1. def test_func_8(self):
      @types.coroutine
      def bar():
      return (yield from foo())

      async def foo():
           return 'spam'
      
       self.assertEqual(run_async(bar()), ([], 'spam') )
      

    I find it surprising that this works at all. Yield-from iterates, and a coroutine is not supposed to be iterable, only awaitable (at least, that's what all error messages tell me when I try it). So why should "yield from" work on them? What if foo() was not an Iterable but a Coroutine? Should "yield from" then call "__await__" on it internally? I would find that *really* surprising, but given the above, I think it would be necessary to achieve consistency.

    This is a special backwards-compatibility thing. In general, generators cannot yield-from coroutines (unless they are decorated with @types.coroutine).

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    scoder commented May 28, 2015

    Another question: is it ok if Cython implements and uses the "tp_as_async" slot in all Py3.x versions (3.2+)? It shouldn't hurt, but it would speed up async/await in Cython at least under Py3.x. Only Py2.6/7 would then have to resort to calling "__await__()" etc. at the Python level.

    One drawback is that Py<3.5 currently (needlessly) checks that "tp_reserved" and "tp_richcompare" are both implemented, but that can be worked around by also implementing the latter...

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    scoder commented May 28, 2015

    It *is* correct, see PEP-492. Awaitable is either a coroutine *or* an object with an __await__ method.

    "coroutine", yes. But "Coroutine"? Shouldn't the Coroutine ABC then require
    "__await__" to be implemented? Maybe even by inheriting from Awaitable?

    Just implement tp_await/await for coroutine-like objects coming from C-API or Cython.

    Sure, that's how it's done. (Specifically, Coroutine is not an
    Iterable/Iterator, but its __await__() returns a thin Iterator that simply
    calls into the Generator code. A bit annoying and slowish, but that's what
    it takes.)

    I was just wondering how Cython should compile Python code that makes use
    of this decorator. The Coroutine and Generator types are separated in
    Cython now, and I think that's actually the right thing to do. This
    types.coroutine() decorator and special casing in CPython's code base gets
    a bit in the way here.

    In general, iteration protocol is still the foundation for Future-like objects

    That's not really reflected in the ABCs, is it?

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    1st1 commented May 28, 2015

    > It *is* correct, see PEP-492. Awaitable is either a coroutine *or* an object with an __await__
    method.

    "coroutine", yes. But "Coroutine"? Shouldn't the Coroutine ABC then require
    "__await__" to be implemented? Maybe even by inheriting from Awaitable?

    This is an interesting idea.

    Practically, when you register something as a Coroutine, you expect it to
    be compatible with ‘await’ expressions. And that’s only possible if
    __await__ is implemented.

    I’m curious what Guido and Nick think about this. I think that we can
    derive Coroutine from Awaitable.

    > Just implement tp_await/await for coroutine-like objects coming from C-API
    or Cython.

    Sure, that's how it's done. (Specifically, Coroutine is not an
    Iterable/Iterator, but its __await__() returns a thin Iterator that simply
    calls into the Generator code. A bit annoying and slowish, but that's what
    it takes.)

    Can't your Coroutine object return itself from its __await__, and implement
    __next__? Like genobject in CPython simply returns self from its __iter__.

    I was just wondering how Cython should compile Python code that makes use
    of this decorator. The Coroutine and Generator types are separated in
    Cython now, and I think that's actually the right thing to do. This
    types.coroutine() decorator and special casing in CPython's code base gets
    a bit in the way here.

    I think we can update types.coroutine to continue using CO_ITERABLE_COROUTINE
    for pure python generator functions. And for something foreign we can use your
    proposed design. Would that be OK?

    > In general, iteration protocol is still the foundation for Future-like objects

    That's not really reflected in the ABCs, is it?

    Awaitable has its __await__ defined as a generator...

    @ncoghlan
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    ncoghlan commented May 28, 2015

    Thanks for highlighting these Stefan - you guessed correctly that I'd missed them on the review.

    For your first question, I agree getawaitablefunc / aiternextfunc / getaiterfunc should all be dropped and replaced with the existing "unaryfunc".

    For your second question, I agree it makes more sense for Coroutine to inherit from Awaitable than it does to have it registered with it.

    For the other three, I don't have a strong opinion, except that we should make sure that whatever we do on the CPython side works by default for Cython.

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    1st1 commented May 28, 2015

    For your first question, I agree getawaitablefunc / aiternextfunc / getaiterfunc should all be dropped and replaced with the existing "unaryfunc".

    I have no problem with that. But why do we have iternextfunc & getiterfunc (no "a" prefix)?

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    ncoghlan commented May 28, 2015

    Given that tp_call is just "ternaryfunc", my guess would be "because when the iterator protocol was added, someone went with function-pointer-type-per-slot rather than function-pointer-type-per-call-signature".

    We *are* a little inconsistent here (e.g. "reprfunc" could also just use the "unaryfunc" signature), but Stefan's right that that isn't a good reason to *add* to the inconsistency.

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    scoder commented May 28, 2015

    Can't your Coroutine object return itself from its __await__, and implement
    __next__? Like genobject in CPython simply returns self from its __iter__.

    That was my first try, sure, and it mostly worked. It has a drawback,
    though: it's an incomplete implementation of the Iterator protocol. It's
    still (mostly) an Iterator, but not an Iterable, so it depends on how you
    use it whether you notice or not, and whether it works at all with other
    code or not. There's a test for a failing "next(coro)" in your test suite,
    for example, which would then not fail in Cython. OTOH, code cannot assume
    that calling iter() or for-looping over on an Iterable is a sane thing to
    do, because it doesn't work for Python's generator type based coroutine
    either, so we might get away with it...

    All of these little details make this trick appear like a potential source
    of subtle inconsistencies or incompatibilities. But given that there will
    almost certainly remain inconsistencies for compiled Python code, I'm not
    sure yet which approach is better. It's not impossible that I'll end up
    going back to the original design. I guess I'll eventually have to include
    some benchmarks in the decision.

    On a related note, my testing made me stumble over this code in
    asyncio.tasks.Task():

            if coro.__class__ is types.GeneratorType:
                self._coro = coro
            else:
                self._coro = iter(coro)  # Use the iterator just in case.

    This seems wrong regardless of how you look at it. And it definitely fails
    with both designs.

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    scoder commented May 28, 2015

    BTW, given that "iter(iterator)" works and returns the iterator, should we also allow "await x.__await__()" to work? I guess that would be tricky to achieve given that __await__() is only required to return any kind of arbitrary Iterator, and Iterators cannot be awaited due to deliberate restrictions. But it might be nice to have for wrapping purposes.

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    scoder commented May 28, 2015

    > Yield-from iterates, and a coroutine is not supposed to be iterable, only awaitable (at least, that's what all error messages tell me when I try it). So why should "yield from" work on them? What if foo() was not an Iterable but a Coroutine? Should "yield from" then call "__await__" on it internally? I would find that *really* surprising, but given the above, I think it would be necessary to achieve consistency.

    This is a special backwards-compatibility thing.

    That only answers the half-serious first part of my question. ;)

    This code only works if foo() returns an Iterable, including a (yield)
    coroutine:

        @types.coroutine
        def bar():
            return (yield from foo())

    It does not work for arbitrary Coroutines as they are not iterable, but it
    might trick people into writing code that fails for non-coroutine
    Coroutines. I'd rather like to have this either work for any Coroutine or
    not at all.

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    python-dev mannequin commented May 28, 2015

    New changeset 09327f653ec5 by Yury Selivanov in branch '3.4':
    asyncio: Drop some useless code from tasks.py.
    https://hg.python.org/cpython/rev/09327f653ec5

    New changeset adf72cffceb7 by Yury Selivanov in branch '3.5':
    asyncio: Drop some useless code from tasks.py.
    https://hg.python.org/cpython/rev/adf72cffceb7

    New changeset 9c0a00247021 by Yury Selivanov in branch 'default':
    asyncio: Drop some useless code from tasks.py.
    https://hg.python.org/cpython/rev/9c0a00247021

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    python-dev mannequin commented May 28, 2015

    New changeset dfa0288c91fd by Yury Selivanov in branch '3.5':
    bpo-24017: Drop getawaitablefunc and friends in favor of unaryfunc.
    https://hg.python.org/cpython/rev/dfa0288c91fd

    New changeset 99dcca3466d3 by Yury Selivanov in branch 'default':
    bpo-24017: Drop getawaitablefunc and friends in favor of unaryfunc.
    https://hg.python.org/cpython/rev/99dcca3466d3

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    1st1 commented May 28, 2015

    Stefan,

    I've already committed fixes for:

    1. getawaitablefunc / aiternextfunc / getaiterfunc -> unaryfunc
    2. strange code in tasks.py doing "coro = iter(coro)" is now removed

    I've also opened a couple of new issues (with patches for a review):

    1. abc.Coroutine derived from abc.Awaitable: bpo-24315
    2. types.coroutine() to support Cython objects: bpo-24316

    I'll reply to some of your messages below:

    Another question: is it ok if Cython implements and uses the "tp_as_async" slot in all Py3.x versions (3.2+)? It shouldn't hurt, [..]

    I think it's totally OK, given that you can workaround the drawback you mentioned.

    > Can't your Coroutine object return itself from its __await__, and implement
    > __next__? Like genobject in CPython simply returns self from its __iter__.

    That was my first try, sure, and it mostly worked. It has a drawback,
    though: it's an incomplete implementation of the Iterator protocol. It's
    still (mostly) an Iterator, but not an Iterable, so it depends on how you
    use it whether you notice or not, and whether it works at all with other
    code or not. There's a test for a failing "next(coro)" in your test suite,
    for example, which would then not fail in Cython. [..]

    I think if "next(cython_coro)" does not fail is acceptable. It's not ideal, but
    the purpose of Cython is to make Python code as fast as possible, so I'd try
    to avoid having any kind of "thin wrappers" around Cyhton coroutines.

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    scoder commented May 28, 2015

    Thanks Yury, I'll give it a try ASAP.

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    1st1 commented May 29, 2015

    Stefan,

    Because of https://mail.python.org/pipermail/python-committers/2015-May/003410.html I've decided to commit 24315 and 24316 today. Please try to check that everything works before new beta 2.

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    scoder commented May 29, 2015

    Tried it, works for me. Thanks!

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    1st1 commented May 29, 2015

    Tried it, works for me. Thanks!

    This is really good news! Thanks!

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    python-dev mannequin commented Jun 1, 2015

    New changeset 0708aabefb55 by Yury Selivanov in branch '3.4':
    bpo-24017: Fix asyncio.CoroWrapper to support 'async def' coroutines
    https://hg.python.org/cpython/rev/0708aabefb55

    New changeset 1dc232783012 by Yury Selivanov in branch '3.5':
    bpo-24017: Fix asyncio.CoroWrapper to support 'async def' coroutines
    https://hg.python.org/cpython/rev/1dc232783012

    New changeset 2e7c45560c38 by Yury Selivanov in branch 'default':
    bpo-24017: Fix asyncio.CoroWrapper to support 'async def' coroutines
    https://hg.python.org/cpython/rev/2e7c45560c38

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    python-dev mannequin commented Jun 1, 2015

    New changeset a0a699b828e7 by Yury Selivanov in branch '3.5':
    bpo-24017: Add a test for CoroWrapper and 'async def' coroutines
    https://hg.python.org/cpython/rev/a0a699b828e7

    New changeset 89521ac669f0 by Yury Selivanov in branch 'default':
    bpo-24017: Add a test for CoroWrapper and 'async def' coroutines
    https://hg.python.org/cpython/rev/89521ac669f0

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    python-dev mannequin commented Jun 1, 2015

    New changeset 1e9e0664ee9b by Yury Selivanov in branch '3.5':
    bpo-24017: Make PyEval_(Set|Get)CoroutineWrapper private
    https://hg.python.org/cpython/rev/1e9e0664ee9b

    New changeset 6fcb64097b1c by Yury Selivanov in branch 'default':
    bpo-24017: Make PyEval_(Set|Get)CoroutineWrapper private
    https://hg.python.org/cpython/rev/6fcb64097b1c

    @agronholm
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    agronholm mannequin commented Jun 3, 2015

    Was __await__() deliberately left out of concurrent.futures.Future or was that an oversight? Or am I misunderstanding something?

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    1st1 commented Jun 3, 2015

    Was __await__() deliberately left out of concurrent.futures.Future or was that an oversight? Or am I misunderstanding something?

    I don't think concurrent.Future is supposed to be used with asyncio (in 'yield from' or 'await' expressions).

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    scoder commented Jun 4, 2015

    Hmm, but IMHO a) the new syntax isn't just for asyncio and b) awaiting a Future seems like a *very* reasonable thing to do. I think opening a new ticket for this is a good idea.

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    1st1 commented Jun 4, 2015

    Hmm, but IMHO a) the new syntax isn't just for asyncio and b) awaiting a Future seems like a *very* reasonable thing to do. I think opening a new ticket for this is a good idea.

    Stefan, I honestly have bo idea what concurrent.Future.__await__ would do. There is no loop for concurrent module. If you have a patch with tests in mind, please open a separate issue (targeting 3.6).

    @gvanrossum
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    gvanrossum commented Jun 4, 2015

    Maybe it's possible to give an interpretation to awaiting on a threaded
    Future? __await__ could return a new asyncio Future, and add a completion
    callback to the original Future that makes the asyncio Future ready and
    transfers the result/exception. This would have to use
    loop.call_soon_threadsafe() to transfer control from the exector thread to
    the thread where the loop is running.

    The only thing I don't know is whether it's possible for __await__ to
    return a Future.

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    1st1 commented Jun 4, 2015

    Guido, Stefen, please see bpo-24383.

    @scoder
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    scoder commented Jun 9, 2015

    See bpo-24400 regarding a split of yield-based generators and async-def coroutines at a type level.

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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