Uncollectable garbage

Objects that have a __del__ method cannot be collected by the garbage collector if they participate in a cycle, prior to Python 3.4.

>>> class Nondestructible(list):
...     def __del__(self):
...         pass

>>> x = Nondestructible()
>>> y = []
>>> z = []
>>> x.append(y)
>>> y.append(z)
>>> z.append(x)

When you remove all other references to these, they end up in gc.garbage.

>>> import objgraph
>>> del x, y, z
>>> import gc
>>> _ = gc.collect()
>>> len(gc.garbage)
1

We highlight these objects by showing the existence of a __del__.

>>> objgraph.show_backrefs(objgraph.by_type('Nondestructible'),
...                        filename='finalizers.png') 
Graph written to ....dot (8 nodes)
Image generated as finalizers.png

Note that classes that define a __del__ method do not have this indicator

>>> objgraph.show_refs(Nondestructible, max_depth=1,
...                    filename='class-with-finalizers.png')
Graph written to ....dot (5 nodes)
Image generated as class-with-finalizers.png