[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Abdur-Rahmaan Janhangeer]

Thanks for Mr David beazley's materials online, a beginner like me was able
to understand this thread from the beginning. Rarely do i get to understand
what the folks in here are rumbling about.

He was also teaching compiler theory at university. Don't know if he ever
contributed to CPython. Here's a direct effect of him on me:
https://www.pythonkitchen.com/python-generators-in-depth/

Though i consider Mr Paul's comments as near trolling, i was able to
appreciate his perspective this time

If ever there is a newbie like me on this list, i'd recommend to vaccum in
as much as possible from mr Beazley ^^
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[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Brett Cannon]

On Thu, Feb 18, 2021 at 9:04 PM Guido van Rossum  wrote:

> On Thu, Feb 18, 2021 at 8:23 PM Luciano Ramalho 
> wrote:
>
>> Thanks for your reply, Guido.
>>
>> On Fri, Feb 19, 2021 at 12:07 AM Guido van Rossum 
>> wrote:
>> > Reading the doc section you link to, it's pretty clear that
>> `@asyncio.coroutine` will be removed.
>>
>> The *Note* right at the top of [1] says "Support for generator-based
>> coroutines is deprecated and is scheduled for removal in Python 3.10."
>>
>> [1]
>> https://docs.python.org/3/library/asyncio-task.html#generator-based-coroutines
>>
>> But PEP 492 [2] says:
>>
>> "Since, internally, coroutines are a special kind of generators, every
>> await is suspended by a yield somewhere down the chain of await calls"
>>
>> [2] https://www.python.org/dev/peps/pep-0492/#await-expression
>>
>> If that part of PEP 492 is no longer accurate, then I have a couple of
>> questions:
>>
>> 1) What Python construct is to be used at the end of a chain of await
>> calls, if not of a generator-based coroutine decorated with
>> `@types.coroutine` and using a `yield` expression in its body?
>>
>> 2) Given that the sole purpose of `@types.coroutine` is to decorate
>> generator-based coroutines to become awaitable, will that decorator
>> also be removed, along with "support for generator-based coroutines"?
>>
>> Best,
>>
>> Luciano
>>
>
> It looks like types.coroutine will remain (it does not contain code to
> warn about deprecation like asyncio.coroutine does), but I don't think it
> is required to end a chain of coroutines -- it may have been an oversight
> that we did not start deprecating it. (But in any case it won't be
> supported by asyncio.)
>
> At the end of the chain you can call the __await__() method which gives an
> iterator, and then you call next() or send() on that iterator. Each
> next()/send() call then represents an await step, and send() in general is
> used to provide an awaited result. Eventually this will raise StopIteration
> with a value indicating the ultimate result (the return value of the
> top-level async def).
>
> The code used to "drive" a chain of await calls is called a trampoline.
> But writing a trampoline is not easy, and the only example I know of is
> asyncio's Task class, in particular its __step() method, which of course is
> beyond complicated because it has to handle so many special cases.
>

I've found https://github.com/dabeaz/curio to be the simplest,
full-featured Python event loop to read.

-Brett


>
> --
> --Guido van Rossum (python.org/~guido)
> *Pronouns: he/him **(why is my pronoun here?)*
> 
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[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Guido van Rossum]

I think that for this functionality (force the event loop to run), the
trampoline just needs to define a function that returns a magic value and
special-case that. The helper then becomes

def __sleep0():
await 

The magic object may have to be something with an __await__() method.

On Fri, Feb 19, 2021 at 01:34 Luciano Ramalho  wrote:

> On Fri, Feb 19, 2021 at 6:29 AM Luciano Ramalho 
> wrote:
> > async def __sleep():
> > return None
>
> Sorry, I meant to write:
>
> async def __sleep0():
> return None
>
> Since the idea is to replace the generator-based coroutine `__sleep0`
> in tasks.py [1] with a native coroutine.
>
> [1]
> https://github.com/python/cpython/blob/e92d67dfbb4790df37aa6a0961fb6dc7e8d2fbbf/Lib/asyncio/tasks.py#L585
>
> --
> Luciano Ramalho
> |  Author of Fluent Python (O'Reilly, 2015)
> | http://shop.oreilly.com/product/0636920032519.do
> |  Technical Principal at ThoughtWorks
> |  Twitter: @ramalhoorg
>
-- 
--Guido (mobile)
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[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Paul Sokolovsky]

Hello,

On Fri, 19 Feb 2021 06:29:35 -0300
Luciano Ramalho  wrote:

> On Fri, Feb 19, 2021 at 4:08 AM Guido van Rossum 
> wrote:
> > Can you try this?
> >
> > async def __sleep(self):
> > await None  
> 
> That didn't work*, but this does:
> 
> async def __sleep():
> return None
> 
> Was that the idea?

No. That doesn't return control to the event loop, like the original
"yield" does.

The purpose of all these shaman dances to obfuscate the relationship
between generators and async functions is unclear. (Well, it's kinda
clear - to allow "async generators", but ughh, don't let me start on
that.) The best solution now indeed appears to allow an "await None"
special construct, to have the same meaning as "yield", but only for
async functions. Any other solution is dirty (and inefficient!)
workaround.


> 
> (*) TypeError: object NoneType can't be used in 'await' expression
> 
> >
> > On Thu, Feb 18, 2021 at 22:31 Luciano Ramalho 
> > wrote:  
> >>
> >> Follow up question: what's the plan to replace this use of
> >> `@types.coroutine` in `asyncio/tasks.py`? [1]
> >>
> >> @types.coroutine
> >> def __sleep0():
> >>  ""
> >>  yield
> >>
> >> [1]
> >> https://github.com/python/cpython/blob/master/Lib/asyncio/tasks.py#L585
> >>
> >> Best,
> >>
> >> Luciano
> >>
> >> On Fri, Feb 19, 2021 at 2:31 AM Luciano Ramalho
> >>  wrote:  
> >> >
> >> > On Fri, Feb 19, 2021 at 1:59 AM Guido van Rossum
> >> >  wrote:  
> >> > >> 1) What Python construct is to be used at the end of a chain
> >> > >> of await calls, if not of a generator-based coroutine
> >> > >> decorated with `@types.coroutine` and using a `yield`
> >> > >> expression in its body?  
> >> >  
> >> > > At the end of the chain you can call the __await__() method
> >> > > which gives an iterator, and then you call next() or send() on
> >> > > that iterator. Each next()/send() call then represents an
> >> > > await step, and send() in general is used to provide an
> >> > > awaited result. Eventually this will raise StopIteration with
> >> > > a value indicating the ultimate result (the return value of
> >> > > the top-level async def).  
> >> >
> >> > All right, that made sense to me. Thank you so much, Guido.
> >> >
> >> > Thanks for the clarification about `@types.coroutine` as well.
> >> >
> >> > Take care,
> >> >
> >> > Luciano
> >> >
> >> >
> >> > --
> >> > Luciano Ramalho
> >> > |  Author of Fluent Python (O'Reilly, 2015)
> >> > | http://shop.oreilly.com/product/0636920032519.do
> >> > |  Technical Principal at ThoughtWorks
> >> > |  Twitter: @ramalhoorg  
> >>
> >>
> >>
> >> --
> >> Luciano Ramalho
> >> |  Author of Fluent Python (O'Reilly, 2015)
> >> | http://shop.oreilly.com/product/0636920032519.do
> >> |  Technical Principal at ThoughtWorks
> >> |  Twitter: @ramalhoorg  
> >
> > --
> > --Guido (mobile)  
> 
> 
> 
> -- 
> Luciano Ramalho
> |  Author of Fluent Python (O'Reilly, 2015)
> | http://shop.oreilly.com/product/0636920032519.do
> |  Technical Principal at ThoughtWorks
> |  Twitter: @ramalhoorg
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-- 
Best regards,
 Paul  mailto:pmis...@gmail.com
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[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Luciano Ramalho]

On Fri, Feb 19, 2021 at 6:29 AM Luciano Ramalho  wrote:
> async def __sleep():
> return None

Sorry, I meant to write:

async def __sleep0():
return None

Since the idea is to replace the generator-based coroutine `__sleep0`
in tasks.py [1] with a native coroutine.

[1] 
https://github.com/python/cpython/blob/e92d67dfbb4790df37aa6a0961fb6dc7e8d2fbbf/Lib/asyncio/tasks.py#L585

-- 
Luciano Ramalho
|  Author of Fluent Python (O'Reilly, 2015)
| http://shop.oreilly.com/product/0636920032519.do
|  Technical Principal at ThoughtWorks
|  Twitter: @ramalhoorg
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[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Luciano Ramalho]

On Fri, Feb 19, 2021 at 4:08 AM Guido van Rossum  wrote:
> Can you try this?
>
> async def __sleep(self):
> await None

That didn't work*, but this does:

async def __sleep():
return None

Was that the idea?

(*) TypeError: object NoneType can't be used in 'await' expression

>
> On Thu, Feb 18, 2021 at 22:31 Luciano Ramalho  wrote:
>>
>> Follow up question: what's the plan to replace this use of
>> `@types.coroutine` in `asyncio/tasks.py`? [1]
>>
>> @types.coroutine
>> def __sleep0():
>>  ""
>>  yield
>>
>> [1] https://github.com/python/cpython/blob/master/Lib/asyncio/tasks.py#L585
>>
>> Best,
>>
>> Luciano
>>
>> On Fri, Feb 19, 2021 at 2:31 AM Luciano Ramalho  wrote:
>> >
>> > On Fri, Feb 19, 2021 at 1:59 AM Guido van Rossum  wrote:
>> > >> 1) What Python construct is to be used at the end of a chain of await
>> > >> calls, if not of a generator-based coroutine decorated with
>> > >> `@types.coroutine` and using a `yield` expression in its body?
>> >
>> > > At the end of the chain you can call the __await__() method which gives 
>> > > an iterator, and then you call next() or send() on that iterator. Each 
>> > > next()/send() call then represents an await step, and send() in general 
>> > > is used to provide an awaited result. Eventually this will raise 
>> > > StopIteration with a value indicating the ultimate result (the return 
>> > > value of the top-level async def).
>> >
>> > All right, that made sense to me. Thank you so much, Guido.
>> >
>> > Thanks for the clarification about `@types.coroutine` as well.
>> >
>> > Take care,
>> >
>> > Luciano
>> >
>> >
>> > --
>> > Luciano Ramalho
>> > |  Author of Fluent Python (O'Reilly, 2015)
>> > | http://shop.oreilly.com/product/0636920032519.do
>> > |  Technical Principal at ThoughtWorks
>> > |  Twitter: @ramalhoorg
>>
>>
>>
>> --
>> Luciano Ramalho
>> |  Author of Fluent Python (O'Reilly, 2015)
>> | http://shop.oreilly.com/product/0636920032519.do
>> |  Technical Principal at ThoughtWorks
>> |  Twitter: @ramalhoorg
>
> --
> --Guido (mobile)



-- 
Luciano Ramalho
|  Author of Fluent Python (O'Reilly, 2015)
| http://shop.oreilly.com/product/0636920032519.do
|  Technical Principal at ThoughtWorks
|  Twitter: @ramalhoorg
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[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Luciano Ramalho]

On Fri, Feb 19, 2021 at 4:08 AM Guido van Rossum  wrote:
> Can you try this?
>
> async def __sleep(self):
> await None

Perhaps you meant this?

async def __sleep0():
  await None

Either way, `await None` raises "TypeError: object NoneType can't be
used in 'await' expression".

Maybe I misunderstood your suggestion?



-- 
Luciano Ramalho
|  Author of Fluent Python (O'Reilly, 2015)
| http://shop.oreilly.com/product/0636920032519.do
|  Technical Principal at ThoughtWorks
|  Twitter: @ramalhoorg
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[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Guido van Rossum]

Can you try this?

async def __sleep(self):
await None

On Thu, Feb 18, 2021 at 22:31 Luciano Ramalho  wrote:

> Follow up question: what's the plan to replace this use of
> `@types.coroutine` in `asyncio/tasks.py`? [1]
>
> @types.coroutine
> def __sleep0():
>  ""
>  yield
>
> [1]
> https://github.com/python/cpython/blob/master/Lib/asyncio/tasks.py#L585
>
> Best,
>
> Luciano
>
> On Fri, Feb 19, 2021 at 2:31 AM Luciano Ramalho 
> wrote:
> >
> > On Fri, Feb 19, 2021 at 1:59 AM Guido van Rossum 
> wrote:
> > >> 1) What Python construct is to be used at the end of a chain of await
> > >> calls, if not of a generator-based coroutine decorated with
> > >> `@types.coroutine` and using a `yield` expression in its body?
> >
> > > At the end of the chain you can call the __await__() method which
> gives an iterator, and then you call next() or send() on that iterator.
> Each next()/send() call then represents an await step, and send() in
> general is used to provide an awaited result. Eventually this will raise
> StopIteration with a value indicating the ultimate result (the return value
> of the top-level async def).
> >
> > All right, that made sense to me. Thank you so much, Guido.
> >
> > Thanks for the clarification about `@types.coroutine` as well.
> >
> > Take care,
> >
> > Luciano
> >
> >
> > --
> > Luciano Ramalho
> > |  Author of Fluent Python (O'Reilly, 2015)
> > | http://shop.oreilly.com/product/0636920032519.do
> > |  Technical Principal at ThoughtWorks
> > |  Twitter: @ramalhoorg
>
>
>
> --
> Luciano Ramalho
> |  Author of Fluent Python (O'Reilly, 2015)
> | http://shop.oreilly.com/product/0636920032519.do
> |  Technical Principal at ThoughtWorks
> |  Twitter: @ramalhoorg
>
-- 
--Guido (mobile)
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[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Luciano Ramalho]

Follow up question: what's the plan to replace this use of
`@types.coroutine` in `asyncio/tasks.py`? [1]

@types.coroutine
def __sleep0():
 ""
 yield

[1] https://github.com/python/cpython/blob/master/Lib/asyncio/tasks.py#L585

Best,

Luciano

On Fri, Feb 19, 2021 at 2:31 AM Luciano Ramalho  wrote:
>
> On Fri, Feb 19, 2021 at 1:59 AM Guido van Rossum  wrote:
> >> 1) What Python construct is to be used at the end of a chain of await
> >> calls, if not of a generator-based coroutine decorated with
> >> `@types.coroutine` and using a `yield` expression in its body?
>
> > At the end of the chain you can call the __await__() method which gives an 
> > iterator, and then you call next() or send() on that iterator. Each 
> > next()/send() call then represents an await step, and send() in general is 
> > used to provide an awaited result. Eventually this will raise StopIteration 
> > with a value indicating the ultimate result (the return value of the 
> > top-level async def).
>
> All right, that made sense to me. Thank you so much, Guido.
>
> Thanks for the clarification about `@types.coroutine` as well.
>
> Take care,
>
> Luciano
>
>
> --
> Luciano Ramalho
> |  Author of Fluent Python (O'Reilly, 2015)
> | http://shop.oreilly.com/product/0636920032519.do
> |  Technical Principal at ThoughtWorks
> |  Twitter: @ramalhoorg



-- 
Luciano Ramalho
|  Author of Fluent Python (O'Reilly, 2015)
| http://shop.oreilly.com/product/0636920032519.do
|  Technical Principal at ThoughtWorks
|  Twitter: @ramalhoorg
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[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Luciano Ramalho]

On Fri, Feb 19, 2021 at 1:59 AM Guido van Rossum  wrote:
>> 1) What Python construct is to be used at the end of a chain of await
>> calls, if not of a generator-based coroutine decorated with
>> `@types.coroutine` and using a `yield` expression in its body?

> At the end of the chain you can call the __await__() method which gives an 
> iterator, and then you call next() or send() on that iterator. Each 
> next()/send() call then represents an await step, and send() in general is 
> used to provide an awaited result. Eventually this will raise StopIteration 
> with a value indicating the ultimate result (the return value of the 
> top-level async def).

All right, that made sense to me. Thank you so much, Guido.

Thanks for the clarification about `@types.coroutine` as well.

Take care,

Luciano


-- 
Luciano Ramalho
|  Author of Fluent Python (O'Reilly, 2015)
| http://shop.oreilly.com/product/0636920032519.do
|  Technical Principal at ThoughtWorks
|  Twitter: @ramalhoorg
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[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Guido van Rossum]

On Thu, Feb 18, 2021 at 8:23 PM Luciano Ramalho  wrote:

> Thanks for your reply, Guido.
>
> On Fri, Feb 19, 2021 at 12:07 AM Guido van Rossum 
> wrote:
> > Reading the doc section you link to, it's pretty clear that
> `@asyncio.coroutine` will be removed.
>
> The *Note* right at the top of [1] says "Support for generator-based
> coroutines is deprecated and is scheduled for removal in Python 3.10."
>
> [1]
> https://docs.python.org/3/library/asyncio-task.html#generator-based-coroutines
>
> But PEP 492 [2] says:
>
> "Since, internally, coroutines are a special kind of generators, every
> await is suspended by a yield somewhere down the chain of await calls"
>
> [2] https://www.python.org/dev/peps/pep-0492/#await-expression
>
> If that part of PEP 492 is no longer accurate, then I have a couple of
> questions:
>
> 1) What Python construct is to be used at the end of a chain of await
> calls, if not of a generator-based coroutine decorated with
> `@types.coroutine` and using a `yield` expression in its body?
>
> 2) Given that the sole purpose of `@types.coroutine` is to decorate
> generator-based coroutines to become awaitable, will that decorator
> also be removed, along with "support for generator-based coroutines"?
>
> Best,
>
> Luciano
>

It looks like types.coroutine will remain (it does not contain code to warn
about deprecation like asyncio.coroutine does), but I don't think it is
required to end a chain of coroutines -- it may have been an oversight that
we did not start deprecating it. (But in any case it won't be supported by
asyncio.)

At the end of the chain you can call the __await__() method which gives an
iterator, and then you call next() or send() on that iterator. Each
next()/send() call then represents an await step, and send() in general is
used to provide an awaited result. Eventually this will raise StopIteration
with a value indicating the ultimate result (the return value of the
top-level async def).

The code used to "drive" a chain of await calls is called a trampoline. But
writing a trampoline is not easy, and the only example I know of is
asyncio's Task class, in particular its __step() method, which of course is
beyond complicated because it has to handle so many special cases.

-- 
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*

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[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Luciano Ramalho]

Thanks for your reply, Guido.

On Fri, Feb 19, 2021 at 12:07 AM Guido van Rossum  wrote:
> Reading the doc section you link to, it's pretty clear that 
> `@asyncio.coroutine` will be removed.

The *Note* right at the top of [1] says "Support for generator-based
coroutines is deprecated and is scheduled for removal in Python 3.10."

[1] 
https://docs.python.org/3/library/asyncio-task.html#generator-based-coroutines

But PEP 492 [2] says:

"Since, internally, coroutines are a special kind of generators, every
await is suspended by a yield somewhere down the chain of await calls"

[2] https://www.python.org/dev/peps/pep-0492/#await-expression

If that part of PEP 492 is no longer accurate, then I have a couple of
questions:

1) What Python construct is to be used at the end of a chain of await
calls, if not of a generator-based coroutine decorated with
`@types.coroutine` and using a `yield` expression in its body?

2) Given that the sole purpose of `@types.coroutine` is to decorate
generator-based coroutines to become awaitable, will that decorator
also be removed, along with "support for generator-based coroutines"?

Best,

Luciano

-- 
Luciano Ramalho
|  Author of Fluent Python (O'Reilly, 2015)
| http://shop.oreilly.com/product/0636920032519.do
|  Technical Principal at ThoughtWorks
|  Twitter: @ramalhoorg
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[Python-Dev] Re: Clarification on the removal of generator-based coroutines in 3.10

[Guido van Rossum]

On Thu, Feb 18, 2021 at 3:57 PM Luciano Ramalho  wrote:

> I am not a Python core developer, but my question relates
> to changes that are expected in Python 3.10, so I felt this was the
> best forum to ask. Please let me know if I should discuss this
> elsewhere or file a documentation bug.
>
> First of all, thank you Yuri Selivanov and everybody who contributed
> to the overhaul of the asyncio docs which are now much better
> organized.
>
> In the process of updating "Fluent Python" to cover Python 3.9, I
> learned that the "Generator-based coroutines" section in the asyncio
> docs warns that "Support for generator-based coroutines is deprecated
> and is scheduled for removal in Python 3.10." [1]
>
> [1]
> https://docs.python.org/3/library/asyncio-task.html#generator-based-coroutines
>
> I don't understand that warning, given that PEP 492 lists as one of
> the awaitable types:
>
> * A generator-based coroutine object returned from a function
> decorated with types.coroutine().
>
> Perhaps what is going to be removed is support for old style
> generator-based coroutines decorated with @asyncio.coroutine, or not
> decorated (which historically also worked)?
>
> I'd appreciate some clarification on exactly what is going to be removed.
>

Reading the doc section you link to, it's pretty clear that
`@asyncio.coroutine` will be removed.

The fact that it's mentioned in PEP 492 is irrelevant -- there is no rule
that says we can't evolve Python to invalidate APIs specified in past PEPs,
as long as the deprecation process is followed (which it is in this case).


> Maybe we also need updates to the Glossary [2] to bridge the gap
> between our previous use of the word "coroutine" (as in PEP 342) and
> the way we are using it today, when we are not being explicit about
> "native coroutines". In particular, the Glossary has no entry at all
> for "generator-based coroutine"—those marked with @types.coroutine.
>
> [2] https://docs.python.org/3/glossary.html
>

Why would it require a glossary entry? It's right there in the first link
you give as the section header. And it's an outmoded concept (which was
clear from the moment 'async def' was invented).

-- 
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*

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