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Stack frames and generators
===========================
Let's define a custom class
>>> class Canary(object):
... pass
Suppose we've a generator that uses it
>>> def count_to_three():
... tweety = Canary()
... yield 1
... yield 2
... yield 3
and we make it active
>>> it = count_to_three()
>>> next(it)
1
Now we can see that our Canary object is alive in memory
>>> import objgraph
>>> objgraph.count('Canary')
1
and we can see what holds it in memory
>>> objgraph.show_backrefs(objgraph.by_type('Canary'),
... max_depth=7,
... filename='canary.png') # doctest: +NODES_VARY
Graph written to ....dot (15 nodes)
Image generated as canary.png
.. figure:: canary.png
:alt: [graph of objects from which the canary is reachable]
:scale: 50%
Or we can examine just one of the reference chains leading straight to a module.
>>> objgraph.show_chain(
... objgraph.find_backref_chain(objgraph.by_type('Canary')[0],
... objgraph.is_proper_module),
... filename='canary-chain.png') # doctest: +NODES_VARY
Graph written to ....dot (11 nodes)
Image generated as canary-chain.png
.. figure:: canary-chain.png
:alt: [chain of objects from a module to the canary]
:scale: 50%
To a first approximation, modules are garbage-collection roots, which makes
the latter technique most useful.
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