Delete all objects in a list

Question

I create many object then I store in a list. But I want to delete them after some time because I create news one and don't want my memory goes high (in my case, it jumps to 20 gigs of ram if I don't delete it).

Here is a little code to illustrate what I trying to do:

class test:     def __init__(self):         self.a = "Hello World"     def kill(self):         del self  a = test() b = test() c = [a,b]  print("1)Before:",a,b)  for i in c:     del i  for i in c:     i.kill()     print("2)After:",a,b) 

A and B are my objects. C is a list of these two objects. I'm trying to delete it definitely with a for-loop in C: one time with DEL and other time with a function. It's not seem to work because the print continue to show the objects.

I need this because I create 100 000 objects many times. The first time I create 100k object, the second time another 100k but I don't need to keep the previous 100k. If I don't delete them, the memory usage goes really high, very quickly.

Answer 1

tl;dr;

mylist.clear()  # Added in Python 3.3 del mylist[:] 

are probably the best ways to do this. The rest of this answer tries to explain why some of your other efforts didn't work.

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cpython at least works on reference counting to determine when objects will be deleted. Here you have multiple references to the same objects. a refers to the same object that c[0] references. When you loop over c (for i in c:), at some point i also refers to that same object. the del keyword removes a single reference, so:

for i in c:    del i 

creates a reference to an object in c and then deletes that reference -- but the object still has other references (one stored in c for example) so it will persist.

In the same way:

def kill(self):     del self 

only deletes a reference to the object in that method. One way to remove all the references from a list is to use slice assignment:

mylist = list(range(10000)) mylist[:] = [] print(mylist) 

Apparently you can also delete the slice to remove objects in place:

del mylist[:]  #This will implicitly call the `__delslice__` or `__delitem__` method. 

This will remove all the references from mylist and also remove the references from anything that refers to mylist. Compared that to simply deleting the list -- e.g.

mylist = list(range(10000)) b = mylist del mylist #here we didn't get all the references to the objects we created ... print(b) #[0, 1, 2, 3, 4, ...] 

Finally, more recent python revisions have added a clear method which does the same thing that del mylist[:] does.

mylist = [1, 2, 3] mylist.clear() print(mylist) 

Answer 2

Here's how you delete every item from a list.

del c[:] 

Here's how you delete the first two items from a list.

del c[:2] 

Here's how you delete a single item from a list (a in your case), assuming c is a list.

del c[0]