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The other day I was wondering why pickling a particular object errored out with an error deep in one of the subobjects.

>>> class MyUnpicklableObject(object):
...     def __getstate__(self):
...         raise NotImplementedError
...
>>> my_object = dict(foo=dict(unrelated='things'),
...                  bar=[dict(nesting='fun'),
...                       dict(boobytrap=MyUnpicklableObject())])
>>> import objgraph
>>> objgraph.show_chain(
...     objgraph.find_ref_chain(
...         my_object,
...         lambda x: isinstance(x, MyUnpicklableObject)),
...     backrefs=False,
...     filename='forward-chain.png')
Graph written to ...dot (4 nodes)
Image generated as forward-chain.png