Pop vs Del

Hi! Thank you so much for this.

Though I still have a question. So how come we can still access the value deleted using del when we assign it to a variable before deleting it(I tried accessing them before deleting it with pop() and del, which made me really confused), if del means to delete it permanently, as we say it, garbage collected? Does del also provide id reference?

I do understand that it is recommended to use pop() when we want to access it still, after deleting it. Just curious why we can still do it with del. Thank you! :)

Assigning a variable is more like giving a label to an object reference. This increases the “reference count” of an object. Think of del as deleting the label reference to an object rather than deleting the object itself. If all label references to an object are removed (reference count become zero), then garbage collection will eventually remove the object from memory because it has no labels referencing that object.

>>> import ctypes
>>> a = 'some string'
>>> id(a)
4511143024
>>> b = a
>>> id(b)
4511143024
>>> del(a)
>>> ctypes.cast(4511143024, ctypes.py_object).value
‘some string’
>>> del(b)
>>> ctypes.cast(4511143024, ctypes.py_object).value
‘some string’

In the case above, this repl does not have strong garbage so the object can still be referenced by id value even though the last label had been removed. A bit later, the same attempt crashed the repl because garbage collection had removed the object!

Ohhh, okay. Now, I see the difference between the two. Though I did not understand the ctypes thing in the code coz I'm just a beginner in Python, but it doesn't matter. It still made sense to me. Thank you so much!

very cool example

Great answer Chris, but how can we assign a new label to an object after it's been returned using pop?

nicolaspeterson, I might be miss understanding your question, but it’s as simple as: popped_obj = some_list[2]. What’s really happening is a bit deeper. Comments added below to show a different way to think about how things work.

# create 5 objects, put their ids in a list
>>> some_list = ["a",  "b", "c", "d", "e"]
# id of object at position 2
>>> id(some_list[2])
4556992672
# get the id at position 2, return that object
>>> some_list[2]
'c'
# remove id at position 2
# assign new label to this id
>>> popped_obj = some_list[2]
# id of object pointed at by label popped_obj
>>> id(popped_obj)
4556992672
# get the id for popped_obj; return that object
>>> popped_obj
'c'

In the end, it’s all about assigning labels to object ids. some_list[2] is just a reference for an id. A list is just an array of ids!