描述Python中的Python垃圾收集。

Python的垃圾回收機制通過引用計數和周期性垃圾收集來自動管理內存。其核心方法是引用計數，當對象的引用數為零時立即釋放內存；但無法處理循環引用，因此引入了垃圾收集模塊（gc）來檢測並清理循環。垃圾回收通常在程序運行中引用計數減少、分配與釋放差值超過閾值或手動調用gc.collect()時觸發。用戶可通過gc.disable()關閉自動回收、gc.collect()手動執行、gc.set_threshold()調整閾值以實現控制。並非所有對像都參與循環回收，如不包含引用的對象由引用計數處理，內置類型如int和string不參與循環回收，而定義__del__方法的類可能影響回收行為。

Python handles memory management automatically, and a big part of that is garbage collection. The main idea is that Python keeps track of which objects are still in use and cleans up the ones that aren't — freeing up memory without you having to do it manually.

How Python's Garbage Collector Works

At its core, Python uses reference counting as the primary method. Every object has a count of how many references point to it. When that count drops to zero, the memory is immediately freed.

But reference counting alone can't catch everything — especially circular references , where two or more objects refer to each other but are otherwise unreachable.

For those cases, Python also includes a garbage collector module (gc) that runs periodically to detect and clean up these cycles.

When Does Garbage Collection Happen?

Garbage collection usually happens behind the scenes. Here's when it kicks in:

• During normal program execution, when reference counts drop.
• When the number of allocations minus deallocations exceeds a threshold — this triggers the cyclic garbage collector.
• You can also trigger it manually using gc.collect() if needed.

This automatic behavior works well for most applications, but in performance-sensitive or long-running programs, understanding when GC runs can help avoid unexpected pauses.

Controlling Garbage Collection

If you're working with large data structures or need more control over memory cleanup, Python lets you tweak the garbage collector via the gc module.

Some common things you might do:

• Turn off automatic collection: gc.disable()
• Run a manual collection: gc.collect()
• Adjust thresholds: gc.set_threshold()

This level of control is useful in things like game loops, real-time systems, or services where timing consistency matters.

What Gets Collected (and What Doesn't)

Not all objects are treated the same during garbage collection. For example:

• Objects that don't contain references to other objects may be handled purely by reference counting.
• Objects involved in circular references (like lists containing themselves) are tracked by the garbage collector.
• Some built-in types (like ints or strings) don't participate in cyclic GC because they can't form cycles.

Also, if your class defines __del__ , it can affect how objects are collected — sometimes delaying or complicating the process.

Basically, Python's garbage collection system does most of the heavy lifting for you, but knowing how it works helps you write better-performing and memory-efficient code.

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