How does Python's garbage collector handle cycles?

Most objects die when their reference count hits zero. A cycle, such as a parent referencing a child that references the parent, keeps counts above zero even when nothing external points to the group. The cyclic collector finds those unreachable groups and frees them.

import gc
import weakref

class Node:
    def __init__(self, name):
        self.name = name
        self.peer = None

first = Node("first")
second = Node("second")
first.peer = second
second.peer = first

ref = weakref.ref(first)
del first
print(ref() is not None)
gc.collect()
print(ref() is None)

Generations and tuning

• Objects move through three generations as they survive collections
• Younger generations are collected more often because most objects die young
• gc.get_threshold and gc.set_threshold control when collections trigger
• gc.disable is only reasonable for short-lived batch processes with measured impact

Disabling collection can reduce latency jitter in a batch job that exits soon, but in a long-running service it can grow memory without bound. If you tune it, measure pause times and memory, and remember that some libraries rely on finalisers running promptly.