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）当它们的引用计数为0时，Python的对象将被删除。在您的示例中，del fooremoves删除了最后一个引用，因此__del__即刻被调用。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
在gc没有任何与删除您和其他大多数的对象。由于自引用或循环引用而无法进行简单的引用计数时，可以在此进行清理：
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()
给出：
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>
使用创建对象，__new__然后将其传递给__init__as self。在中出现异常后__init__，该对象通常将没有名称（即该f =部件未运行），因此其引用计数为0。这意味着该对象将被正常删除并被__del__调用。
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