Exception error when accessing the class variable at the termination of the program
duncan.booth at invalid.invalid
Mon Feb 2 04:37:21 EST 2009
jwalana at vsnl.net wrote:
> Can someone please explain why the exception happens in the case
> where there is no explicit del statement?
When Python is ecleaning up as it exits, it clears all the global
variables in each module by setting them to None. This happens in an
arbitrary and unpredictable order: in this particular case it set Person
to None before setting x2 to None.
If you must have a __del__ method on a class then you should take care
not to access any globals from inside the __del__ method (or any
function it calls). Alternatively you could just catch and ignore any
exceptions thrown from your __del__ method.
In the example you gave, provided you never subclass Person you could
use self.__class__.Count to access your class variable. However in many
cases you'll probably find that a much better solution is to use weak
references. e.g. a WeakValueDictionary mapping id(self) to self:
from weakref import WeakValueDictionary
_instances = WeakValueDictionary()
def __init__(self, name):
self.name = name
self._instances[id(self)] = self
print name, 'is now created'
print self.name, 'is now deleted'
print 'The last object of Person class is now deleted'
print 'There are still', self.Count, 'objects of class
x2 = Person('Krishna')
# del x2
Duncan Booth http://kupuguy.blogspot.com
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