如何强制删除python对象？

Question

我很好奇__del__python中的细节,何时以及为什么应该使用它以及它不应该用于什么.我已经学到了很难的方法,它不像人们对析构函数的天真期望,因为它不是__new__/ 的反面__init__.

class Foo(object):

    def __init__(self):
        self.bar = None

    def open(self):
        if self.bar != 'open':
            print 'opening the bar'
            self.bar = 'open'

    def close(self):
        if self.bar != 'closed':
            print 'closing the bar'
            self.bar = 'close'

    def __del__(self):
        self.close()

if __name__ == '__main__':
    foo = Foo()
    foo.open()
    del foo
    import gc
    gc.collect()

我在文档中看到,不保证__del__()在解释器退出时仍然存在的对象调用方法.

1. 怎么能保证Foo解释器退出时存在的任何实例都关闭了吧？
2. 在上面的代码片段中,条形图是关闭del foo还是gc.collect()......或者两者都没有？如果你想更好地控制那些细节(例如,当对象未被引用时应该关闭条形图)实现它的常用方法是什么？
3. 当__del__被调用时能够保证所有的__init__已经叫什么名字？如果__init__举起怎么样？

Answer 1

关闭资源的方法是上下文管理器,即with声明:

class Foo(object):

  def __init__(self):
    self.bar = None

  def __enter__(self):
    if self.bar != 'open':
      print 'opening the bar'
      self.bar = 'open'
    return self # this is bound to the `as` part

  def close(self):
    if self.bar != 'closed':
      print 'closing the bar'
      self.bar = 'close'

  def __exit__(self, *err):
    self.close()

if __name__ == '__main__':
  with Foo() as foo:
    print foo, foo.bar

输出:

opening the bar
<__main__.Foo object at 0x17079d0> open
closing the bar

2)Python的对象在引用计数为0时被删除.在您的示例中,del foo删除最后一个引用,因此__del__立即调用.GC不参与其中.

class Foo(object):

    def __del__(self):
        print "deling", self

if __name__ == '__main__':
    import gc
    gc.disable() # no gc
    f = Foo()
    print "before"
    del f # f gets deleted right away
    print "after"

输出:

before
deling <__main__.Foo object at 0xc49690>
after

这gc与删除您和大多数其他对象无关.当简单引用计数不起作用时,它可以清理,因为自引用或循环引用:

class Foo(object):
    def __init__(self, other=None):
        # make a circular reference
        self.link = other
        if other is not None:
            other.link = self

    def __del__(self):
        print "deling", self

if __name__ == '__main__':
    import gc
    gc.disable()   
    f = Foo(Foo())
    print "before"
    del f # nothing gets deleted here
    print "after"
    gc.collect()
    print gc.garbage # The GC knows the two Foos are garbage, but won't delete
                     # them because they have a __del__ method
    print "after gc"
    # break up the cycle and delete the reference from gc.garbage
    del gc.garbage[0].link, gc.garbage[:]
    print "done"

输出:

before
after
[<__main__.Foo object at 0x22ed8d0>, <__main__.Foo object at 0x22ed950>]
after gc
deling <__main__.Foo object at 0x22ed950>
deling <__main__.Foo object at 0x22ed8d0>
done

3)让我们看看:

class Foo(object):
    def __init__(self):

        raise Exception

    def __del__(self):
        print "deling", self

if __name__ == '__main__':
    f = Foo()

得到:

Traceback (most recent call last):
  File "asd.py", line 10, in <module>
    f = Foo()
  File "asd.py", line 4, in __init__
    raise Exception
Exception
deling <__main__.Foo object at 0xa3a910>

创建对象__new__然后传递给__init__as self.在异常进入后__init__,该对象通常没有名称(即该f =部分未运行),因此它们的引用计数为0.这意味着该对象被正常删除并被__del__调用.

Answer 2

一般来说,为了确保无论发生什么事情,你都会使用

from exceptions import NameError

try:
    f = open(x)
except ErrorType as e:
    pass # handle the error
finally:
    try:
        f.close()
    except NameError: pass

finally无论块中是否存在错误,以及try块中发生的任何错误处理是否存在错误,都将运行except块.如果不处理引发的异常,则在finally块被激活后仍会引发异常.

确保文件关闭的一般方法是使用"上下文管理器".

http://docs.python.org/reference/datamodel.html#context-managers

with open(x) as f:
    # do stuff

这将自动关闭f.

对于你的问题#2,bar当引用计数达到零时立即关闭,del foo如果没有其他引用,则立即关闭.

对象不是由__init__它们创建的,它们是由它们创建的__new__.

http://docs.python.org/reference/datamodel.html#object.新

当你做foo = Foo()两件事实际上正在发生时,首先要创建一个新对象__new__,然后对它进行初始化__init__.因此,del foo在这两个步骤发生之前,你无法打电话.但是,如果存在错误__init__,__del__仍将调用,因为该对象实际上已在其中创建__new__.

编辑:如果引用计数减少到零,则在删除时进行更正.

Answer 3

也许您正在寻找上下文管理器？

>>> class Foo(object):
...   def __init__(self):
...     self.bar = None
...   def __enter__(self):
...     if self.bar != 'open':
...       print 'opening the bar'
...       self.bar = 'open'
...   def __exit__(self, type_, value, traceback):
...     if self.bar != 'closed':
...       print 'closing the bar', type_, value, traceback
...       self.bar = 'close'
... 
>>> 
>>> with Foo() as f:
...     # oh no something crashes the program
...     sys.exit(0)
... 
opening the bar
closing the bar <type 'exceptions.SystemExit'> 0 <traceback object at 0xb7720cfc>