[Python-Dev] reference leaks, __del__, and annotations

[Nick Coghlan]

Tim Peters wrote:
> Note that it's very easy to do this with __del__.  The trick is for
> your type not to have a __del__ method itself, but to point to a
> simple "cleanup object" with a __del__ method.  Give that "contained"
> object references to the resources you want to close, and you're done.
>  Because your "real" objects don't have __del__ methods, cycles
> involving them can't inhibit gc.  The cleanup object's only purpose in
> life is to close resources.  Like:
> 
> class _RealTypeResourceCleaner:
>     def __init__(self, *resources):
>         self.resources = resources
> 
>     def __del__(self):
>         if self.resources is not None:
>             for r in self.resources:
>                 r.close()
>             self.resources = None
> 
>     # and typically no other methods are needed, or desirable, in
>     # this helper class
> 
> class RealType:
>     def __init__(*args):
>         ...
>         # and then, e.g.,
>         self.cleaner = _ResourceCleaner(resource1, resource2)
> 
>     ... tons of stuff, but no __del__ ....
> 
> That's the simple, general way to mix cycles and __del__ without problems.

So, stealing this trick for generators would involve a "helper" object with a 
close() method, a __del__ method that invoked it, and access to the 
generator's frame stack (rather than to the generator itself).

   class _GeneratorCleaner:
       __slots__ = ["_gen_frame"]
       def __init__(self, gen_frame):
           self._gen_frame = gen_frame
       def close(self):
           # Do whatever gen.close() currently does to the
           # raise GeneratorExit in the frame stack
           # and catch it again
       def __del__(self):
           self.close()

The generator's close() method would then change to be:

   def close(self):
       self._cleaner.close()

Would something like that eliminate the current cycle problem?

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
---------------------------------------------------------------
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