__del__ method being called in python when it is not expected
del, object-lifetime, python
Solution
There are a couple of things going on here. When your `Person4` class is instantiated, it initialises its `population` class variable to 0. From your interactive console, you appear to be running your "test1.py" file multiple times. The second time you run it, the `Person4` class is declared again which makes it technically different from the first one (even though it has the same name). That means it has its own independent `population` count.
Now, `swaroop` and `kaleem` are global variables, shared between both your instances of "test1.py". Python internally uses reference counting for most of its automatic garbage collection, so the original instance of the first `Person4` class is not released until the second assignment to `swaroop`. Assigning to `swaroop` decrements the reference count for the first instance, causing `__del__` to be called because the reference count is now zero. But because you're referring to `Person4` by name inside `__del__()`, when the previous instance disappears it decrements the new `Person4.population` count, instead of the old `Person4` population count.
Hopefully that made sense. I can see why this might be confusing to somebody learning Python. Your use of class variables at the same time as redefining the `Person4` class using `execfile()` is further confusing matters. For what it's worth, I've written a lot of Python code and I don't think I've ever needed to use the `__del__` special method.
Problem
I am new to python and have been working through the examples in Swaroop CH's "A Byte of Python". I am seeing some behavior with the `__del__` method that is puzzling me. Basically, if I run the following script (in Python 2.6.2) ``` class Person4: '''Represents a person''' population = 0 def __init__(self, name): '''Initialize the person's data''' self.name = name print 'Initializing %s'% self.name #When the person is created they increase the population Person4.population += 1 def __del__(self): '''I am dying''' print '%s says bye' % self.name Person4.population -= 1 if Person4.population == 0: print 'I am the last one' else: print 'There are still %d left' % Person4.population swaroop = Person4('Swaroop') kaleem = Person4('Kalem') ``` using the Python console (or the Spyder interactive console) I see the following: execfile(u'C:\1_eric\Python\test1.py') Initializing Swaroop Initializing Kalem execfile(u'C:\1_eric\Python\test1.py') Initializing Swaroop Swaroop says bye I am the last one Initializing Kalem Kalem says bye I am the last one Why is the `__del__` method being called immediately after the `__init__` on the second run? I am guessing that since the same instance names ('swaroop' and 'kaleem') are being used that it is releasing the original instance and garbage collecting it. But, this seems to be playing havoc with the current population count. What is going on here? What is a good way to avoid this sort of confusion? Avoid the use of `__del__`? Check for existing instance names before reusing them? ... Thanks, Eric