[Python-ideas] Automatic context managers

[anatoly techtonik]

On Fri, Apr 26, 2013 at 6:39 PM, Ned Batchelder <ned at nedbatchelder.com>wrote:

>
> On 4/26/2013 11:21 AM, anatoly techtonik wrote:
>
>    """
>>> If a QObject falls out of scope in Python, it will get deleted. You have
>>> to take care of keeping a reference to the object:
>>>
>>> * Store it as an attribute of an object you keep around, e.g.
>>> self.window = QMainWindow()
>>>  * Pass a parent QObject to the object’s constructor, so it gets owned
>>> by the parent
>>>
>>> """
>>>
>>>     This thread on the PySide mailing list suggests that you are
>>> mistaken, PySide does not have superpowers over and
>>>     above Python's garbage collector, and is subject to the exact same
>>> non-deterministic destructors as any other Python
>>>     object. Whether you call that destructor __del__ or __exit__ makes
>>> no difference.
>>>
>>>
>>> http://www.mail-archive.com/__pyside@lists.openbossa.org/__msg01029.html
>>>     <
>>> http://www.mail-archive.com/pyside@lists.openbossa.org/msg01029.html>
>>>
>>
>> You'll notice it doesn't say "gets /immediately/ deleted" -- because it
>> doesn't.  It gets deleted when it gets garbage collected.
>>
>
>  Are you sure about that? The example on the PySide wiki is pretty
> reproducible. With current garbage collector lazyness it should be at least
> in some cases non-reliable.
>
>
> Again, I suspect we are falling prey to fuzzy language.  CPython will
> reclaim objects as soon as their reference count reaches zero.  This is not
> the garbage collector.  The garbage collector is a separate facility which
> kicks in every once in a while to find objects that have non-zero reference
> counts, even though they are unreachable, because of circular references.
> Some people say "garbage collector" or "garbage collection" to mean the
> usual reclamation of objects when their refcount reaches zero, but this is
> imprecise and confusing when mixed with people who use the term differently.
>
> I know nothing about the internals of QObjects, but like most others, I
> *strongly* suspect that they are doing nothing special above and beyond
> what Python does to determine the lifetime of an object.  Their cleanup
> happens when the object is reclaimed (note I am careful not to say, "when
> the object is garbage collected").
>
> The example on the PySide wiki is reproducible because it is not subject
> to garbage collector laziness.  The "animation" name is a local in
> animate_stuff.  At the end of that function, the name "animation" falls out
> of scope, decrementing the reference count on the QPropertyAnimation object
> it referenced.  That object now has a reference count of zero, so it is
> reclaimed.  The cleanup code is then invoked, destroying the native objects
> as well.
>

Thanks. That makes it more clear. So if I create a circular reference with
two QObjects and lose a link to both of them - they won't be reclaimed
until GC starts, because their mutual reference count is not zero, right?
And the same will be true for the object returned by open() involved in
circular reference.

I guess that's the reason why the example in file.close() could not be
updated to note that "with" is not needed anymore.
http://docs.python.org/2/library/stdtypes.html#file.close
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