[Tutor] Python __del__ method

boB Stepp robertvstepp at gmail.com
Tue Jul 11 20:34:16 EDT 2017

On Tue, Jul 11, 2017 at 7:24 PM, Alan Gauld via Tutor <tutor at python.org> wrote:
> On 11/07/17 15:47, Jia Yue Kee wrote:
>> I am new to Python and I came across the Python __del__ method
> The __del__() method is a bit of an oddity and often not
> used in industrial strength Python code.
>> if __name__ == "__main__":
>>     x = Robot("Tik-Tok")
>>     y = Robot("Jenkins")
>>     z = x
>>     del x
>>     del z
>>     del y
>> My question being why is that "Robot has been destroyed"
>> is only printed once in Case 2
> Because __del__() only got called once.
> Which is really quite good, although you don't know which
> object's del() was called!
> The problem is that python does not promise to
> call __del__() even when it is defined! It only promises
> not to call it until all references are deleted.
> So we can say when __del__() will not be called but
> not when it will be called, if ever. The problem is
> that __del__() is called by the garbage collector
> when it actually collects the deleted object and that
> could be a long time after the reference count goes
> to zero, or possibly even never!
> This means __del__() is of limited usefulness since
> you can't rely on it being called. So you can't safely
> use it as a destructor to release resources.
>> ...after the del x and del z statements, the refcount
>> to the Robot object should reach zero and subsequently
>> triggers the __del__ method right?
> Nearly.
> It *might* trigger the __del__() method, eventually.
> Or it might not, there is no guarantee. Mostly it
> will get called, but you can't count on it.

I am assuming that when the OP ran his code from a file, that upon the
script's completion, both object instances were garbage collected.
Surely upon program completion, everything _is_ garbage collected?

> I confess I don't fully understand the reasons why
> it's been done that way, it just is...

Now you have me wondering why, too!  ~(:>))


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