All 15 Memory Management & Garbage Collection questions
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How does Python primarily manage memory for objects? What does it mean that Python has 'automatic' memory management, and what's the trade-off compared to manual memory management in languages like C? How does Python's generational garbage collector work, and why generations? What is the sys.getsizeof() function, and what are its limitations for measuring memory usage? How do context managers and the `del` statement relate to timely memory cleanup versus relying on garbage collection? How does the gc module let you inspect and debug memory leaks caused by uncollectable objects? How do mutable default arguments create a subtle memory and correctness pitfall related to object lifetime? How does the tracemalloc module help track where memory is being allocated in a running program? How does the weakref module help avoid reference cycles and memory leaks? How does object interning (like small integer caching and string interning) affect memory and identity comparisons?
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Python › Memory Management & Garbage Collection

What is a reference cycle, and why can't simple reference counting free it?

Intermediate

Answer

A reference cycle occurs when two or more objects reference each other (directly or indirectly), keeping their reference counts above zero even when nothing outside the cycle refers to them -- so pure reference counting never reaches zero for these objects, causing a memory leak without a separate cycle-detecting collector.
class Node:
    def __init__(self):
        self.parent = None
        self.child = None

a, b = Node(), Node()
a.child = b
b.parent = a  # cycle: a -> b -> a
del a, b  # refcounts don't reach 0 due to the cycle
Real-world example A tree data structure where each node stores both a parent and child reference forms a natural cycle that relies on the cyclic garbage collector, not refcounting, to eventually be reclaimed.

Common follow-ups: How does the generational garbage collector find cycles?;How can weakref break such cycles intentionally?

Multiple Inheritance & MRO;Memory Management & Garbage Collection
All 15 Memory Management & Garbage Collection questions