[Python-Dev] PEP 492: No new syntax is required

[Yury Selivanov]

Hi Mark,

On 2015-04-26 4:21 PM, Mark Shannon wrote:
> Hi,
>
> I was looking at PEP 492 and it seems to me that no new syntax is 
> required.

Mark, all your points are explained in the PEP in a great detail:

>
> Looking at the code, it does four things; all of which, or a 
> functional equivalent, could be done with no new syntax.

Yes, everything that the PEP proposes can be done without new syntax.  
That's how people use asyncio right now, with only what we have in 3.4.

But it's hard.  Iterating through something asynchronously?  Write a 
'while True' loop.  Instead of 1 line you now have 5 or 6.  Want to 
commit your database transaction?  Instead of 'async with' you will 
write 'try..except..finally' block, with a very high probability to 
introduce a bug, because you don't rollback or commit properly or 
propagate exception.

> 1. Make a normal function into a generator or coroutine. This can be 
> done with a decorator.

https://www.python.org/dev/peps/pep-0492/#rationale-and-goals
https://www.python.org/dev/peps/pep-0492/#debugging-features
https://www.python.org/dev/peps/pep-0492/#importance-of-async-keyword

> 2. Support a parallel set of special methods starting with 'a' or 
> 'async'. Why not just use the current set of special methods?

Because you can't reuse them.

https://www.python.org/dev/peps/pep-0492/#why-not-reuse-existing-for-and-with-statements
https://www.python.org/dev/peps/pep-0492/#why-not-reuse-existing-magic-names

> 3. "await". "await" is an operator that takes one argument and 
> produces a single result, without altering flow control and can thus 
> be replaced by an function.

It can't be replaced by a function. Only if you use greenlets or 
Stackless Python.

> 4. Asynchronous with statement. The PEP lists the equivalent as "with 
> (yield from xxx)" which doesn't seem so bad.

There is no equivalent to 'async with'. "with (yield from xxx)" only 
allows you to suspend execution
in __enter__ (and it's not actually in __enter__, but in a coroutine 
that returns a context manager).

https://www.python.org/dev/peps/pep-0492/#asynchronous-context-managers-and-async-with 
see "New Syntax" section to see what 'async with' is equivalent too.

>
> Please don't add unnecessary new syntax.


It is necessary.  Perhaps you haven't spent a lot of time maintaining 
huge code-bases developed with frameworks like asyncio, so I understand 
why it does look unnecessary to you.

Thanks,
Yury