Tim Peters <t...@python.org> added the comment:
A simple (finalizer-only) example of what an SCC-based DAG topsort ordering
would accomplish:
import gc
class C:
def __init__(self, val):
self.val = val
def __del__(self):
print("finalizing", self.val)
c, b, a = map(C, "cba")
a.next = b
b.next = c
#a.loop = a
del c, b, a
gc.collect()
That finalizes in the order a, b, c. Refcount semantics force that. But,
uncomment the "a.loop = a" line, and the order changes to c, b, a. They all
look exactly the same to gc, so it runs finalizers in the order they happen to
appear in the list gc is crawling over. A DAG topsort ordering would force a,
b, c order again.
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue40312>
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