Re: [Tutor] Question about the object.__del__(self) method
On 22/04/19 3:35 PM, Alan Gauld via Tutor wrote:
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.)
Hello Alan,
Nice explanation. I completly got what is going on. Closing the question
here. :)
--
Thanks,
Arup Rakshit
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Re: [Tutor] Question about the object.__del__(self) method
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
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos
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[Tutor] Question about the object.__del__(self) method
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.
from python_methods import RandomKlass
obj = RandomKlass(10)
del obj
# Deleted...
obj = RandomKlass(10)
obj1 = RandomKlass(10)
del obj
# Deleted...
del obj1
# Deleted...
Now why then the doc
https://docs.python.org/3/reference/datamodel.html#object.__del__ says:
> `del x` doesn’t directly call `x.__del__()` — the former decrements the
> reference count for `x` by one, and the latter is only called when `x`’s
> reference count reaches zero.
Also what the reference count here means? I know that x can hold only one
reference at a time.
Thanks,
Arup Rakshit
a...@zeit.io
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