[Python-Dev] anonymous blocks (don't combine them with generator finalization)

Bob Ippolito bob at redivi.com
Fri Apr 22 03:47:14 CEST 2005

On Apr 21, 2005, at 8:59 PM, Josiah Carlson wrote:

> Guido van Rossum <gvanrossum at gmail.com> wrote:
>> [Brett]
>>> I think I agree with Samuele that it would be more pertinent to put 
>>> all of this
>>> effort into trying to come up with some way to handle cleanup in a 
>>> generator.
>> I.e. PEP 325.
>> But (as I explained, and you agree) that still doesn't render PEP 310
>> unnecessary, because abusing the for-loop for implied cleanup
>> semantics is ugly and expensive, and would change generator semantics;
>> and it bugs me that the finally clause's reachability depends on the
>> destructor executing.
> Yes and no.  PEP 325 offers a method to generators that handles cleanup
> if necessary and calls it close().  Obviously calling it close is a
> mistake.  Actually, calling it anything is a mistake, and trying to
> combine try/finally handling in generators with __exit__/close (inside
> or outside of generators) is also a mistake.
> Start by saying, "If a non-finalized generator is garbage collected, it
> will be finalized."  Whether this be by an exception or forcing a 
> return,
> so be it.
> If this were to happen, we have generator finalization handled by the
> garbage collector, and don't need to translate /any/ for loop.  As long
> as the garbage collection requirement is documented, we are covered 
> (yay!).

Well, for the CPython implementation, couldn't you get away with using 
garbage collection to do everything?  Maybe I'm missing something..

import weakref

class ResourceHandle(object):
     def __init__(self, acquire, release):
         # if I understand correctly, this is safer than __del__
         self.ref = weakref.ref(self, lambda o:release())

class FakeLock(object):
     def acquire(self):
         print "acquired"
     def release(self):
         print "released"

def with_lock(lock):
     r = ResourceHandle(lock.acquire, lock.release)
     yield None
     del r

 >>> x = with_lock(FakeLock())
 >>> del x
 >>> with_lock(FakeLock()).next()
 >>> for ignore in with_lock(FakeLock()):
...     print ignore

I could imagine someone complaining about generators that are never 
used missing out on the acquire/release.  That could be solved with a 
trivial rewrite:

def with_lock(lock):
     def _with_lock(r):
         yield None
         del r
     return _with_lock(ResourceHandle(lock.acquire, lock.release))

 >>> x = with_lock(FakeLock())
 >>> del x

Of course, this just exaggerates Guido's "it bugs me that the finally 
clause's reachability depends on the destructor executing".. but it 
does work, in CPython.

It seems to me that this pattern would be painless enough to use 
without a syntax change...


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