构造函数和析构函数如何工作？ [英] How do constructors and destructors work?

问题描述

我正在尝试理解此代码：

I'm trying to understand this code:

class Person:
    '''Represents a person '''
    population = 0

    def __init__(self,name):
          //some statements and population += 1
    def __del__(self):
          //some statements and population -= 1 
    def sayHi(self):
        '''grettings from person'''
        print 'Hi My name is %s' % self.name

    def howMany(self):
        '''Prints the current population'''
        if Person.population == 1:
            print 'i am the only one here'
        else:
            print 'There are still %d guyz left ' % Person.population
rohan = Person('Rohan')
rohan.sayHi()
rohan.howMany()


sanju = Person('Sanjivi')
sanju.howMany()

del rohan # am i doing this correctly? 

析构函数是如何被调用的-自动还是我必须在 main中添加一些东西

How does the destructor get invoked -- automatically or do I have to add something in the "main" program/class like above?

输出：

初始化人员数据
>
******************************************

初始化Rohan

************************************* *****

人口现在是：1

我叫Rohan

i是这里唯一的人

正在初始化的人数据

**********************************************

初始化Sanjivi

********************************** ********

现在的人口是：2

如果有人去世：

*********** **************************************

Sanjivi Bye Bye世界

在那里还剩1个人吗？

i是这里唯一的一个人

如果有人去世：

************** ********************************

Rohan Bye Bye world < br>
i是地球上最后一个人

现在的人口是：0

Initializing person data
******************************************
Initializing Rohan
******************************************
Population now is: 1
Hi My name is Rohan
i am the only one here
Initializing person data
******************************************
Initializing Sanjivi
******************************************
Population now is: 2
In case Person dies:
******************************************
Sanjivi Bye Bye world
there are still 1 people left
i am the only one here
In case Person dies:
******************************************
Rohan Bye Bye world
i am the last person on earth
Population now is: 0

如果需要，我也可以粘贴整个课程。我正在学习：
http://www.ibiblio.org/swaroopch / byteofpython / read /

If required I can paste the whole lesson as well. I'm learning from: http://www.ibiblio.org/swaroopch/byteofpython/read/

推荐答案

这里是一个有点自以为是的答案。

Here is a slightly opinionated answer.

不要使用 __ del __ 。这不是C ++或为析构函数构建的语言。 __ del __ 方法确实应该在Python 3.x中消失，尽管我确信有人会发现一个有意义的用例。如果您需要使用 __ del __ ，请注意 http://docs.python.org/reference/datamodel.html ：

Don't use __del__. This is not C++ or a language built for destructors. The __del__ method really should be gone in Python 3.x, though I'm sure someone will find a use case that makes sense. If you need to use __del __, be aware of the basic limitations per http://docs.python.org/reference/datamodel.html:

• __ del __ 在垃圾收集器恰巧正在收集对象时调用，而不是在丢失对对象的最后一个引用时，而不是在执行 del对象。


• __ del __ 负责在其中调用任何 __ del __ 一个超类，尽管尚不清楚
  是按方法解析顺序（MRO）还是只是调用每个超类。


• 具有 __ del __ 意味着垃圾收集器放弃了检测和清除任何循环链接的操作，例如丢失对链接列表的最后一个引用。您可以从 gc.garbage 获取被忽略对象的列表。您有时可以使用弱引用来完全避免循环。时不时对此进行辩论：请参见 http：//mail.python .org / pipermail / python-ideas / 2009-October / 006194.html 。


• __ del __ 函数可能会作弊，保存对对象的引用并停止垃圾回收。


• __ del __ 中显式引发的异常将被忽略。


• __ del __ 对 __ new __ 的补充远远超过 __ init __ 。这变得令人困惑。参见 http://www.algorithm.co.il/blogs/index.php/programming/python/python-gotchas-1- del -is-not-the-opposite- - init / 进行解释和了解。


• __ del __ 不是 Python中深受喜爱的孩子。您会注意到sys.exit（）文档没有指定是否在退出之前收集垃圾，并且存在很多奇怪的问题。在全局变量上调用 __ del __ 会引起奇怪的排序问题，例如 http： //bugs.python.org/issue5099 。即使 __ init __ 失败，也应该调用 __ del __ 吗？参见 http://mail.python.org/pipermail/ python-dev / 2000-March / thread.html＃2423 来获取长线程。

• __del__ is called when the garbage collector happens to be collecting the objects, not when you lose the last reference to an object and not when you execution del object.
• __del__ is responsible for calling any __del__ in a superclass, though it is not clear if this is in method resolution order (MRO) or just calling each superclass.
• Having a __del__ means that the garbage collector gives up on detecting and cleaning any cyclic links, such as losing the last reference to a linked list. You can get a list of the objects ignored from gc.garbage. You can sometimes use weak references to avoid the cycle altogether. This gets debated now and then: see http://mail.python.org/pipermail/python-ideas/2009-October/006194.html.
• The __del__ function can cheat, saving a reference to an object, and stopping the garbage collection.
• Exceptions explicitly raised in __del__ are ignored.
• __del__ complements __new__ far more than __init__. This gets confusing. See http://www.algorithm.co.il/blogs/index.php/programming/python/python-gotchas-1-del-is-not-the-opposite-of-init/ for an explanation and gotchas.
• __del__ is not a "well-loved" child in Python. You will notice that sys.exit() documentation does not specify if garbage is collected before exiting, and there are lots of odd issues. Calling the __del__ on globals causes odd ordering issues, e.g., http://bugs.python.org/issue5099. Should __del__ called even if the __init__ fails? See http://mail.python.org/pipermail/python-dev/2000-March/thread.html#2423 for a long thread.

但是，另一方面：

• __ del __ 意味着您不会忘记调用关闭语句。参见 http://eli.thegreenplace.net/2009 / 06/12 / safe-using-destructors-in-python / 获取专业的 __ del __ 观点。这通常是关于释放ctype或其他特殊资源的。

• __del__ means you do not forget to call a close statement. See http://eli.thegreenplace.net/2009/06/12/safely-using-destructors-in-python/ for a pro __del__ viewpoint. This is usually about freeing ctypes or some other special resource.

我的个人理由是不喜欢 __ del__ 函数。

And my pesonal reason for not liking the __del__ function.

• 每次有人提出 __ del __ 时，分解成三十个混乱的消息。


• 它打破了Python Zen中的这些项目：
  
  
  
  
  • 复杂度比
  
  
  • 特殊情况还不足以打破规则。
  
  
  • 错误绝不能默默传递。
  
  
  • 面对歧义，拒绝诱惑。
  
  
  • 应该有一种-最好只有一种-显而易见的方法。
  
  
  • 如果难以解释该实现，则是个坏主意。
  
  
  • Everytime someone brings up __del__ it devolves into thirty messages of confusion.
  • It breaks these items in the Zen of Python:
    • Complex is better than complicated.
    • Special cases aren't special enough to break the rules.
    • Errors should never pass silently.
    • In the face of ambiguity, refuse the temptation to guess.
    • There should be one-- and preferably only one --obvious way to do it.
    • If the implementation is hard to explain, it's a bad idea.

    因此，找到不使用 __ del __ 的原因。

    So, find a reason not to use __del__.

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