Hello, On Tue, 28 Apr 2015 21:00:17 +0100 Mark Shannon wrote: []

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CO_COROUTINE = 0x0080 CO_GENERATOR = 0x0020

def coroutine(f): 'Converts a function to a generator function' old_code = f.__code__ new_code = CodeType( old_code.co_argcount, old_code.co_kwonlyargcount,

This is joke right? Well it was partly for entertainment value, although it works on PyPy.

The point is that something that can be done with a decorator, whether in pure Python or as builtin, does not require new syntax.

And that's exactly not what Python is and not how it evolves. Unlike Scheme, it doesn't offer some minimal orthogonal basis out of which everything can be derived by functional application. Instead, it's more pragmatic and offers plethora of (well defined, unlike many other languages) concepts and implementations to choose from. And if so happens that practice shows that some concepts needs "slight" redefinition, such concept is defined as first-class, despite the fact that it matches 90% semantics of another concept. Fortunately, on implementation level, those 90% of semantics are shared, so it is not outright "bloat". The current wishful thinking of this PEP is that more people will know and use "await", while "yield from" will keep being understood and used by quite not every Python programmer. (Just to state the obvious, all the above is actually my own trying to grasp it, and is reverse causation - trying to explain Python progress in terms of how this particular PEP482 and its older friends progress, it may be quite different for other aspects of language. I for one rather surprised that BDFL is so positive about this PEP).

Cheers, Mark.

-- Best regards, Paul mailto:pmiscml@gmail.com

On 28/04/15 21:06, Guido van Rossum wrote:

On Tue, Apr 28, 2015 at 11:44 AM, Mark Shannon mailto:mark@hotpy.org> wrote:

Hi,

I still think that there are several issues that need addressing with PEP 492. This time, one issue at a time :)

"async"

The "Rationale and Goals" of PEP 492 states that PEP 380 has 3 shortcomings. The second of which is: """It is not possible to natively define a coroutine which has no yield or yield from statements.""" This is incorrect, although what is meant by 'natively' is unclear.

A coroutine without a yield statement can be defined simply and concisely, thus:

@coroutine def f(): return 1

This is only a few character longer than the proposed new syntax, perfectly explicit and requires no modification the language whatsoever. A pure-python definition of the "coroutine" decorator is given below.

So could the "Rationale and Goals" be correctly accordingly, please. Also, either the "async def" syntax should be dropped, or a new justification is required.

So here's *my* motivation for this. I don't want the code generator to have to understand decorators. To the code generator, a decorator is just an expression, and it shouldn't be required to understand decorators in sufficient detail to know that *this* particular decorator means to generate different code. The code generator knows nothing about it. The generated bytecode is identical, only the flags are changed. The decorator can just return a copy of the function with modified co_flags.

And it's not just generating different code -- it's also the desire to issue static errors (SyntaxError) when await (or async for/with) is used outside a coroutine, or when yield [from] is use inside one.

Would raising a TypeError at runtime be sufficient to catch the sort of errors that you are worried about?

The motivation is clear enough to me (and AFAIR I'm the BDFL for this PEP :-).

Can't argue with that. Cheers, Mark.