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)

Now we can see that our Canary object is alive in memory

>>> import objgraph
>>> objgraph.count('Canary')

and we can see what holds it in memory

>>> objgraph.show_backrefs(objgraph.by_type('Canary'),
...                        max_depth=7,
...                        filename='canary.png') 
Graph written to (15 nodes)
Image generated as canary.png
[graph of objects from which the canary is reachable]

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') 
Graph written to (11 nodes)
Image generated as canary-chain.png
[chain of objects from a module to the canary]

To a first approximation, modules are garbage-collection roots, which makes the latter technique most useful.