[Python-ideas] Relax __exit__ method to be a generator

[Andrew Svetlov]

Well, this is good point..

But from my experience try/finally is a common point of errors.
Beginners use "with" properly as:

with open(filename) as f:
  f.read()

but switch to try/finally produces very common error.

Instead of

f = open(filename)
try:
  f.read()
finally:
  f.close()

people usually write

try:
 f = open(filename)
 f.read()
finally:
  f.close()

I saw it constantly many times. When I wrote an article about the
problem and true solution in my blog I got several emails from my
readers: please, tell me again why I need to move open() out of try
block if it can generate exception on file opening?

So maybe you would like some other syntax? For example

with from obj:
   BLOCK

That form can use magic methods with names different than
__enter__/__exit__ (and get rid of "with (yield from lock)" BTW). And
it's obviously? points on two suspending points: at BLOCK enter and
exit.

On Mon, Apr 7, 2014 at 2:12 AM, Guido van Rossum <guido at python.org> wrote:
> I prefer to use a try/finally statement. One of the points of using
> yield-from is that you can always tell where your code may be suspended by
> searching for "yield from". With your proposed change that would no longer
> be true -- any with statement would also have to be inspected, and there
> would no longer be a way to know from the source alone whether it might
> yield or not (because it would dynamic -- there's no way to be sure at
> compile time which context manager is being used).
>
>
> On Sun, Apr 6, 2014 at 12:02 PM, Andrew Svetlov <andrew.svetlov at gmail.com>
> wrote:
>>
>> Literally it may be generator itself or function that returns generator
>> object.
>>
>> Now I'm working on postgres library for asyncio
>> (http://aiopg.readthedocs.org/).
>>
>> And I would to use *with statement* for transaction handling like:
>>
>> with (yield from cursor.transaction()):
>>    yield from cursor.execute(sql)
>>
>> The problem is: at exit of *with statement* I need to call `yield from
>> cursor.execute('COMMIT')` or `yield from cursor.execute('ROLLBACK')`.
>>
>> I can do it only in __exit__ in *context manager*, but python
>> understand only if __exit__:
>> - returns true value, that suppresses exception from code block
>> - returns None or any false value to propagate exception if any
>> - raises exception itself
>>
>> I propose to add new rule:
>> IF the code object is generator (co_flags & CO_GENERATOR) and __exit__
>> returns generator object (isinstance(ret, types.GeneratorType))
>> THEN do `yield from ret`.
>>
>> That's work fine if __exit__ itself is a *generator function*
>> (contains `yield` or `yield from` statements): call to *generator
>> function* returns *generator object*.
>>
>> The proposal:
>> 1. Doesn't break any existing code except if user accidentally
>> returns generator object instead of True from __exit__ call (he should
>> not to do this and I sure this is very rare case).
>> 2. Don't requires new syntax.
>>
>> asyncio itself uses:
>>
>> with (yield from lock):
>>    BLOCK
>>
>> for locking etc but unlocking for asyncio objects doesn't requires any
>> `yield from`, so __exit__ code is just plain function but not
>> generator.
>>
>> Also I can live with asyncio trick for __enter__:
>> https://code.google.com/p/tulip/source/browse/asyncio/locks.py#156
>> The way is a but annoying but unrolling a value returned by __enter__
>> if the value is generator object will break existing code, sure.
>>
>> Thoughts?
>>
>>
>> --
>> Thanks,
>> Andrew Svetlov
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>> Python-ideas at python.org
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>
>
>
>
> --
> --Guido van Rossum (python.org/~guido)



-- 
Thanks,
Andrew Svetlov