[Tutor] Question about the object.__del__(self) method

[Alan Gauld]

On 22/04/2019 10:18, Arup Rakshit wrote:
> Consider the below in simple class:
> 
> class RandomKlass:
>     def __init__(self, x):
>         self.x = x
>     
>     def __del__(self):
>         print("Deleted…")
> 
> Now when I delete the object created from RandomKlass using `del` operator I see the output “Deleted…”. That means `del` operator calls the __del__ method if available.

No it doesn't, it just means that is what seemed to
happen in this specific scenario.

Now try:


>>> from python_methods import RandomKlass
>>> o1 = RandomKlass(10)
>>> o2 = o1
>>> oblist = [o1,o2]
>>> del(o1)
>>> del(o2)
>>> del(oblist)
Deleted...

So your __del__() is only called once, after all the
references to the instance have been deleted.

> Also what the reference count here means? 
> I know that x can hold only one reference at a time.

Remember that variables in Python are just names that
refer to objects. So, while the name 'x' can only refer
to one object at a time, many other names can also refer
to that same object, as in the example above.
o1, o2 and oblist[0] and oblist[1] all refer to
the same original instance of your class.

Each time a new variable references the instance an
internal "reference count" is incremented. When a referring
name is deleted the reference count is decremented.
Once the count reaches zero the instance is deleted
and its __del__() method, if it exists, is called.
So, only when all the names referring to the instance
have been deleted is the __del__() method called. (And it
is important to realise that there are cases where
__del__() is never called. Do not rely on __del__()
for any critical actions - such as saving instance
data or shutting down the nuclear reactor.)

-- 
Alan G
Author of the Learn to Program web site
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