[Python-Dev] Combining the best of PEP 288 and PEP 325: generator exceptions and cleanup

[Phillip J. Eby]

At 12:36 PM 5/19/2005 -0400, James Y Knight wrote:
>On May 19, 2005, at 8:43 AM, Phillip J. Eby wrote:
>>At 06:09 PM 5/19/2005 +1200, Greg Ewing wrote:
>>>Guido van Rossum wrote:
>>>>- When a generator is GC'ed, its close() method is called (which
>>>>is a
>>>>no-op if it is already closed).
>>>
>>>Does this mean that all generators will be ineligible
>>>for cyclic garbage collection (since they implicitly
>>>have something equivalent to a __del__ method)?
>>
>>No, since it's implemented in C.  (The C equivalent to __del__ does
>>not
>>interfere with cyclic GC.)
>
>But you're missing the point -- there's a *reason* that __del__
>interferes with cyclic GC. It doesn't just do it for the heck of it!
>You can't simply have the C delete call into python code...the
>objects the generator has references to may be invalid objects
>already because they've been cleared to break a cycle. If you want to
>execute python code on collection of a generator, it must be done via
>__del__, or else it'll be horribly, horribly broken.

Eeyowch.  Good point.  OTOH, the only way a generator-iterator can become 
part of a cycle is via  an action taken outside the generator.  (E.g. 
passing it into itself via 'continue', creating a link from one of its 
arguments to it, etc.)  So, it's probably not a terrible limitation in 
practice.