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

Well, for new syntax, the bar is pretty high... And we should probably move to python-ideas. On Sun, Apr 6, 2014 at 4:44 PM, Andrew Svetlov wrote:

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 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@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 _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/

-- --Guido van Rossum (python.org/~guido)

-- Thanks, Andrew Svetlov

-- --Guido van Rossum (python.org/~guido)