创建实例失败时调用的析构函数？

Question

在试图理解一些Python概念时，我遇到了以下问题：

class A:
    def __init__(self, x):
        self.x = x

    def __del__(self):
        print("del")

a1 = A()

输出：

$ python test.py
del
Traceback (most recent call last):
  File "testdest.py", line 9, in <module>
    a1 = A()
TypeError: __init__() takes exactly 2 arguments (1 given)

错误很明显（实例化时缺少参数），但我想知道为什么在有实例之前调用析构函数？

除非在尝试实例化时，Python 甚至在调用构造函数之前就创建了一种实例，并且需要在最后清理？

由于 self 被传递给构造函数，我可以假设这个 self 是实例吗？原来如此，那么调用构造函数时实例已经存在了，是吗？

这是垃圾收集器的一种行为，可能取决于当前的实现吗？

Answer 1

来自 Python 文档：

Objects are never explicitly destroyed; however, when they become unreachable they may be garbage-collected. An implementation is allowed to postpone garbage collection or omit it altogether — it is a matter of implementation quality how garbage collection is implemented, as long as no objects are collected that are still reachable.
object.__init__(self[, ...])
Called after the instance has been created (by new()), but before it is returned to the caller. [...]
object.__del__(self)
Called when the instance is about to be destroyed. [...]

因此调用__init__时对象实例已经存在，因为它是由__new__创建的。但是对于 Python 一般而言，不能保证 __del__ 会被调用。

以下仅适用于CPython，Python的参考实现。

Note (for object.__del__(self))

del x doesn’t directly call x.__del__() — the former decrements the reference count for x by one, and the latter is only called when x‘s reference count reaches zero. [...]

这里 __del__ 每当实例的引用计数下降到 0 时调用。这与垃圾收集无关。一个小实验：

>>> class A:
...   def __del__(self): print "del"
... 
>>> a = A()
>>> a = None
del
>>> import gc
>>> gc.disable() 
>>> a = A()
>>> a = None
del

如您所见，即使明确禁用 GC，也会调用析构函数。

请注意，这也意味着如果您的对象层次结构中有循环，您最终会得到 __del__ 永远不会被调用的对象，因为 Python GC 无法处理引用周期。

>>> a1 = A()
>>> a2 = A()
>>> a1.x = a2
>>> a2.x = a1
>>> a1 = None
>>> a2 = None
>>> import gc
>>> gc.collect()
4
>>> gc.garbage
[<__main__.A instance at 0x7f2c66a1d7e8>, <__main__.A instance at 0x7f2c66a1d830>]

创建实例失败时调用的析构函数？

destructor called when failing to create the instance?

python-3.x

garbage-collection

destructor

instantiation