Python: Delete Objects and Free Up Space

Question

Python documentation said that an object is considered deleted if the reference counts to the object is zero and the difference between del and __del__ is that:

del reduces the reference count to an object by one
__del__ executes when the reference count to an object is zero

That means if we keep delete the reference to an object until the reference count to zero. It would executes __del__ to delete the object to free up memory.

However, what is troubling me is that:

If I create an object and bind the object to the name ObjectX. It would increase the reference count by 1, right?
If I create a list with the name ListY and I append the ListY with ObjectX. It means that it would increase the reference count to 2, right?
Now, if I delete ObjectX and ListY, would the reference count to the object is still remains as 1 and the object is still remaining sitting there in hard disk waiting for some kind soul to come and kill it?

Any comments and thoughts are welcomed....

By the way, if what is happening is really what I understand about deleting object in Python. Is there any good suggestion for how to delete an object that is appended into a list and free up the memory space?

class Slave:

def __init__(self):
    self.name=""

def __del__(self):
    print("SLAVE DEAD")

def sequence():

   SlaveX=Slave()
   slaveList=list()
   slaveList.append(SlaveX)

del SlaveX
   print("I AM NOT DEAD YET")
   del slaveList

It won't be on your hard disk - when you close python it goes away. — doctorlove
– doctorlove, Commented Mar 2, 2018 at 9:04
If you deleted both references, why would the reference count still be 1? — khelwood
– khelwood, Commented Mar 2, 2018 at 9:05
When you delete ObjectX it does down by one., When you delete ListY it goes down by another one. — doctorlove
– doctorlove, Commented Mar 2, 2018 at 9:07
Because the amount of data that I have to handle is quite huge. I afraid memory leak would occur before the program finish running. — Daozui
– Daozui, Commented Mar 2, 2018 at 9:08
You never call __del__, that's called for you when the count goes to zero. You can call del if you like, but just exiting the method where ObjectX and ListY are assigned will be enough to decrement the reference count to zero, so (usually) you don't need to call del. — user9423368
– user9423368, Commented Mar 2, 2018 at 9:17

user9423368 · Accepted Answer · 2018-03-02 09:28:44Z

10

Imagine a simple class:

class Thingy:
    pass

Now I can create an object of this class and call it george:

george = Thingy()

Now I have created an instance of the class and I have one reference to it. Somewhere this count is maintained and set to 1.

I can make a new variable called mary and assign it to the same object:

mary = george

Now I still only have one object, but I have two references to it. If I call methods on george, this affects the object referred to by mary because it's the same instance.

As you mention lists, let's add it to a list as well, but essentially it's exactly the same, it increases the reference count to 3.

my_objects = []
my_objects.append(george)

Now there are three references. Now, still inside this method, let's remove the references again

george = None    # reference count drops to 2
mary = 3         # reference count drops to 1
my_objects.pop() # reference count drops to 0

when the count drops to 0, the python runtime realises this (maybe not straight away, but quickly enough) and calls the __del__ method on the instance. You don't care about this, you just let the references disappear.

Now, if I delete ObjectX and ListY, would the reference count to the object still remain as 1?

No, it would drop to zero.

and the object is still remaining sitting there in hard disk waiting for some kind soul to come and kill it?

No, it was only held in memory, and will be deleted and the memory freed again.

answered Mar 2, 2018 at 9:28

user9423368

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1 Comment

Daozui Over a year ago

I have idea about how the destructor works now. Thanks for the explanation.

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