Python Memory Management and Tips Transcripts
Chapter: Recovering memory in Python
Lecture: Reference counting

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0:00 Alright, we're gonna do some fun programming over here, and it's fun because you've got to be a little bit clever to work and understand
0:08 reference counting in Python, because if you create a variable and you point it at a thing so you can ask questions about it like,
0:14 "is it alive or not?", it's always gonna be alive cause you're giving at least one to that reference count. So we're gonna use some cool little
0:23 utilities we're gonna write. So I'm gonna write one first called "memutil" and I'm going to drop some code in here because
0:30 we don't need to do a whole lot here. So we're gonna use this "ctypes" thing, which actually allows us to get kind of a direct reference to PyObjects,
0:38 that's the same one we talked about, and then give us the field reference count. Now I'm going to add that to the dictionary so it doesn't look broken.
0:45 It's going to come back as a long, like I said, and then what we can do is if we get one of those id's, remember, id of thing,
0:53 we can come down here and use this, say parse it from that address, and then we're going to get the reference count. Okay? We're gonna use that.
1:04 Then we're gonna have our "app_refcount" That's the one we're gonna run. I'll go ahead and set it to run now and I'll just hit the hotkey,
1:12 we don't need it that big, do we? There we go, and I'll do my fmain magic,
1:17 so it's ready to run. Now we're going to need something interesting to work with. First of all, we can work with the garbage collector.
1:27 We're not really talking about the garbage collector, and I only want to work with it to the extent where I say "let's
1:33 not have it do anything". Let's just disable it for now so we don't think about it or worry about it because what's happening has nothing to
1:39 do with the garbage collector. Later we're gonna enable it and focus just on its
1:44 role in this whole world. We're gonna create some variable, call it "v1" and let's just give it the number 7 and then we'll have, I'll call it "oid"
1:53 for object id. It's gonna be the id of v1. Now what we need to do is we need to grab this early so we have the id.
2:00 This number knowing the memory address that the location given to us by id will not keep this thing alive. Right?
2:08 So it could be potentially up for collection. And we could still use this to ask from our memutil here. So we'll say "import memutil" as well.
2:19 We're gonna need that, but in order to actually track this really clearly, we need to do one more cool thing. So I'm gonna create an object,
2:26 a class, and I'll call it "doomed". Why is it doomed? Because its only purpose in life is to get created and
2:32 then get destroyed by the reference counting cleanup system. So let's go in here and find a class "doomed".
2:40 Now, this is going to basically plug into the Python data model,
2:45 the "dunder methods" to capture different lifetime events of this object. So we'll have an "__init__", and this is going to be when it gets created.
2:55 Let's go ahead say that it can have some friends. I don't think we're gonna use this yet,
2:58 but this "*friends" means this will come in as a list and then we'll just print. We'll come over here and say "at {id(self)}".
3:14 So we created this doomed thing wherever it happens to be now. That's what happens when the thing comes to life.
3:20 And then we're gonna have another one run when the thing gets deleted. Then we wanna have some string representation of it
3:31 so it's easy for us to just print it out and see what's going on and we'll just do the id there,
3:36 but we want to have a string Representation so we'll have a "__str__" and we'll return a "__repr__" as well we'll just make them be the same.
3:47 We're just gonna paste some code so you can see what's happening. All right,
3:50 so what we're gonna do is we're gonna say "there's a doomed object at this address and it has however many friends if there are friends,
3:56 Otherwise it's not going to talk about his friends". Doesn't want feel bad. So this is gonna be are doomed object.
4:03 We're going to create one of these over here in our code. Instead of creating 7, I'm gonna create a "doomed" and it's going to start with
4:08 no friends. We'll come back, the friends is more important in the garbage collection side of things. But let's just run this and see what happens.
4:17 Notice it created a doomed object at some address and then it deleted the doomed object
4:22 at that address. And that delete was when this function returned, no more things pointed at it,
4:28 and it went away. But we can be more interesting than that. Let's go see what else we can do.
4:36 Let's start by first printing out how many things refer to it. So let's say "print", and then here we're gonna use our "memutil.refs(oid)".
4:48 Now we run this. You might think it's zero right, cause we're kind of done with it, but it doesn't get cleaned up explicitly.
4:57 Doesn't get cleaned up unless we explicitly do so until line 13. So it should still say "1". There we go. Step 1 ref count is 1.
5:07 Now, let's go to step 2. Step 2 is we're gonna have another variable that is equal to v1.
5:16 So remember the way this works is Python sees this and says "there's a new variable defined that is now pointing over there",
5:23 and so we now have v1 and v2 pointing at our doomed object, and so now it's gonna have to increment that reference count. Let's try that. Sure enough,
5:35 reference count is 2. Alright, let's do some more things. Let's go over here and do this again. Step 3 is we're gonna change where
5:44 variable 2 points. We're going to tell it to point at none, but it could also be at 7. It doesn't matter. If it's just not pointing at the doomed thing
5:52 anymore, that's going to decrement, take away one from the reference count. Can you see it went "1, 2, 1", okay? Pretty cool. And the final step,
6:03 step 4, let's go and tell v1 it also no longer points at doomed. And at that point, nothing, not v1 not 2, nothing else should be pointing
6:15 that v1 and so it should clean itself up. And let's do a "print('End of method')". Right? So we should actually see this
6:23 go to zero. We should see it get cleaned up here, and this should return zero, and you should see all that before it's getting cleaned
6:30 up naturally as part of the method return. Are you ready? Let's see what we got. Here goes. Beautiful. Okay, reference count is 1, then it's 2,
6:40 then it's 1, and this line, you can't really make it happen all at once, but this line 19 is what is causing this right here.
6:49 When we say "the last thing no longer points here" immediately, like on line 19, basically, this thing is getting destroyed,
6:58 the memory is getting reclaimed. And then later we can say, Well, now how many things point at it?
7:03 Nothing. But that's already been the case on line 19 which did the cleanup. And of course, that happened before the end of the method.
7:10 And if we don't do this one, you'll see the cleanup doesn't come until after the end of the method.
7:16 So that's pretty cool. And I want to emphasize this is not non-deterministic. Rather, I should say this is deterministic.
7:24 It will always, always be the case that on line 19 this will get cleaned up. Run it again. You can see it definitely ran between step 3 and 4,
7:33 the destructor, if you will, of our doomed object, right? And it said "hey, I've been destroyed, I'm gone, I've been deleted by memory management,
7:43 either reference counting or the GC". This is really important. This is incredibly lightweight. All you have to do, all Python, rather, has to do
7:53 to implement this is just to keep count of how many things point at it
7:56 and when some variable changes assignment just increment or decrement that number. If that number ever hits
8:02 zero, immediately take it out of the block and tell the block that spot is
8:08 now available again, right? You don't even actually have to clean up the memory. So this is really, really efficient. In the deterministic part,
8:15 a lot of languages use garbage collectors as the primary way of cleaning up their objects, you know .NET, Java,
8:23 those types of things. And because of that, it's non-deterministic. When the garbage collector runs is based on the behavior the program
8:31 has had over time. You can't say on line 19 this thing's gonna get cleaned up.
8:35 You can say "well when the memory kind of gets full enough and the heuristic decides that section of memory is worth looking at again,
8:42 then it'll get cleaned up", and that could be problematic for real-time things, like stock trading, that has to have no latencies of,
8:51 like, 4 milliseconds or whatever it might turn out to be, right? So this deterministic aspect of reference counting is really nice,
8:58 because it's going to behave the same way, memory wise, every single time.

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