On 28/04/15 21:06, Guido van Rossum wrote:
On Tue, Apr 28, 2015 at 11:44 AM, Mark Shannon <m...@hotpy.org
<mailto:m...@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.
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