WEBVTT 1 00:00:00.110 --> 00:00:01.330 Hi and welcome back. 2 00:00:01.330 --> 00:00:02.940 In this video we're going to look at 3 00:00:02.940 --> 00:00:04.950 how to do multiple things at once, 4 00:00:04.950 --> 00:00:06.336 or sort of at once, 5 00:00:06.336 --> 00:00:11.150 without using either threads or processes. 6 00:00:11.150 --> 00:00:12.720 As you can see, I've created a file here 7 00:00:12.720 --> 00:00:14.050 called generators.py, 8 00:00:14.920 --> 00:00:15.900 so that should give you a hint 9 00:00:15.900 --> 00:00:16.810 as to what we're going to try 10 00:00:16.810 --> 00:00:20.170 to do with this file. 11 00:00:20.170 --> 00:00:22.090 We've looked at generators before; 12 00:00:22.090 --> 00:00:25.240 let me create an example of a generator. 13 00:00:25.240 --> 00:00:27.490 This one is going to be called countdown. 14 00:00:27.490 --> 00:00:28.360 So we're going to take a number 15 00:00:28.360 --> 00:00:30.910 that we're going to start counting down from. 16 00:00:30.910 --> 00:00:34.270 I'm going to say while n is greater than zero, 17 00:00:35.900 --> 00:00:37.690 and we're gonna yield n, 18 00:00:37.690 --> 00:00:40.590 remember that gives the value back to the core 19 00:00:40.590 --> 00:00:43.130 and we're going to subtract one from it. 20 00:00:43.130 --> 00:00:45.100 So, if we have a countdown 21 00:00:45.100 --> 00:00:48.000 like c equals countdown that starts from 10, 22 00:00:49.310 --> 00:00:53.270 we can then do something like, print, next, of c. 23 00:00:53.270 --> 00:00:55.890 This is sort of nothing new here. 24 00:00:55.890 --> 00:00:57.110 What this is doing is 25 00:00:57.110 --> 00:00:59.340 we're creating a new generator 26 00:00:59.340 --> 00:01:01.080 and as soon as we call next, 27 00:01:01.080 --> 00:01:02.530 it starts running the generator 28 00:01:02.530 --> 00:01:05.230 and it goes up to the yield 29 00:01:05.230 --> 00:01:08.440 and it gives back the value that we yielded. 30 00:01:08.440 --> 00:01:11.680 So in this case it's gonna give back 10, 31 00:01:11.680 --> 00:01:15.393 and then when we print next again, 32 00:01:15.393 --> 00:01:17.352 it's going to continue running 33 00:01:17.352 --> 00:01:20.530 all the way back to the next yield. 34 00:01:20.530 --> 00:01:22.400 So instead of going for 10, 35 00:01:22.400 --> 00:01:24.830 it would start with n as 10, 36 00:01:24.830 --> 00:01:26.320 you subtract one from it 37 00:01:26.320 --> 00:01:27.570 and then it will repeat the loop 38 00:01:27.570 --> 00:01:30.170 and it would yield again n again which would be nine. 39 00:01:31.170 --> 00:01:33.210 So as soon as you arrive at the yield, 40 00:01:34.110 --> 00:01:37.560 the correct word here is the function is suspended. 41 00:01:37.560 --> 00:01:39.240 So when you arrive at a yield, 42 00:01:39.240 --> 00:01:41.470 you get the value that was yielded 43 00:01:41.470 --> 00:01:42.990 and the function is suspended, 44 00:01:44.660 --> 00:01:47.030 there and then when you call next again, 45 00:01:47.030 --> 00:01:48.760 the function resumes. 46 00:01:48.760 --> 00:01:50.520 So very similar to a thread, really, 47 00:01:50.520 --> 00:01:51.353 if you think about it, 48 00:01:51.353 --> 00:01:53.170 'cause the thread can at any point 49 00:01:53.170 --> 00:01:56.770 be suspended or removed from a core 50 00:01:56.770 --> 00:01:58.010 and then it can be brought back 51 00:01:58.010 --> 00:01:59.930 and it continues running where it's left off. 52 00:01:59.930 --> 00:02:01.210 So the generator actually behaves 53 00:02:01.210 --> 00:02:02.450 fairly similar to a thread 54 00:02:02.450 --> 00:02:04.620 in that when you arrive at a yield 55 00:02:04.620 --> 00:02:07.010 you're removing it from the core, if you wish, 56 00:02:07.010 --> 00:02:07.900 then when you press next, 57 00:02:07.900 --> 00:02:08.970 you're sort of bringing it back 58 00:02:08.970 --> 00:02:10.220 and it continues running. 59 00:02:11.350 --> 00:02:12.760 So let's run this file. 60 00:02:12.760 --> 00:02:15.090 I'm going to right-click generators.py and run it, 61 00:02:15.090 --> 00:02:17.730 and as you can see we get 10 and nine, 62 00:02:17.730 --> 00:02:19.320 which is what you sort of expect, 63 00:02:19.320 --> 00:02:21.750 the first next gives you the 10, 64 00:02:21.750 --> 00:02:23.310 then when you go next again, 65 00:02:23.310 --> 00:02:24.910 it continues running up to the next yield 66 00:02:24.910 --> 00:02:26.010 and it gives you nine. 67 00:02:26.910 --> 00:02:28.510 So this is all good. 68 00:02:28.510 --> 00:02:29.780 What if we had, 69 00:02:30.647 --> 00:02:32.715 c1 which is a countdown from 10 70 00:02:32.715 --> 00:02:35.800 and c2 which is a countdown say from 20? 71 00:02:37.530 --> 00:02:38.980 Yeah? 72 00:02:38.980 --> 00:02:41.580 Now you have two tasks, 73 00:02:41.580 --> 00:02:43.720 a countdown that starts counting down from 10, 74 00:02:43.720 --> 00:02:45.940 and one that starts counting down from 20. 75 00:02:45.940 --> 00:02:48.140 What you could do is this. 76 00:02:54.320 --> 00:02:56.920 So here you're printing the next value of c1, 77 00:02:56.920 --> 00:02:59.240 goes up to the yield and gives you 10. 78 00:02:59.240 --> 00:03:00.780 Then you're printing the next value of c2, 79 00:03:00.780 --> 00:03:03.440 which goes down to the yield and gives you 20. 80 00:03:03.440 --> 00:03:04.950 Then you could do this again, 81 00:03:04.950 --> 00:03:07.200 which resumes c1, 82 00:03:07.200 --> 00:03:09.610 subtracts minus one from n, 83 00:03:09.610 --> 00:03:12.930 goes and repeats the loop, and now gives you nine. 84 00:03:12.930 --> 00:03:14.350 That suspends and then you move 85 00:03:14.350 --> 00:03:15.370 over to the next line, 86 00:03:15.370 --> 00:03:17.680 which gives you the next of c2. 87 00:03:17.680 --> 00:03:19.013 So that, instead of being 20, 88 00:03:19.013 --> 00:03:21.410 now you subtract one and you can repeat 89 00:03:21.410 --> 00:03:23.500 and you now yield 19. 90 00:03:23.500 --> 00:03:24.333 Let's run this. 91 00:03:26.100 --> 00:03:27.980 You can see that this is what's happening. 92 00:03:27.980 --> 00:03:30.010 You are running one task, 93 00:03:30.010 --> 00:03:31.300 then you are suspending it, 94 00:03:31.300 --> 00:03:32.660 and you're running another, 95 00:03:32.660 --> 00:03:33.590 and then you're suspending that 96 00:03:33.590 --> 00:03:35.110 and you're going back to the first one. 97 00:03:35.110 --> 00:03:35.943 You're suspending that 98 00:03:35.943 --> 00:03:37.930 and going back to the other one. 99 00:03:37.930 --> 00:03:39.250 This is essentially 100 00:03:39.250 --> 00:03:43.490 some very slow multi-threading 101 00:03:43.490 --> 00:03:45.160 but, without using threads. 102 00:03:45.160 --> 00:03:46.440 Notice there's no import of threads 103 00:03:46.440 --> 00:03:48.090 or anything here. 104 00:03:48.090 --> 00:03:50.530 What you've got is two tasks 105 00:03:50.530 --> 00:03:54.000 yielding control of the main thread to one another. 106 00:03:54.000 --> 00:03:57.530 Think of it as passing the baton in these races. 107 00:03:59.100 --> 00:04:00.850 So if we can do this really quickly, 108 00:04:00.850 --> 00:04:04.730 if we can do c1 and then c2 really, really quickly, 109 00:04:04.730 --> 00:04:06.550 it may look as though 110 00:04:06.550 --> 00:04:09.080 they are running at the same time. 111 00:04:09.080 --> 00:04:11.030 This is essentially the purpose with threads, remember? 112 00:04:11.030 --> 00:04:13.550 You cannot run two threads at once in Python, 113 00:04:13.550 --> 00:04:17.740 but you can slice their time on the CPU 114 00:04:17.740 --> 00:04:19.740 quickly enough that it looks like they're running 115 00:04:19.740 --> 00:04:20.700 at the same time. 116 00:04:22.200 --> 00:04:24.510 So this looks a bit like a hack, 117 00:04:24.510 --> 00:04:27.170 like something that shouldn't really work, 118 00:04:27.170 --> 00:04:30.890 but actually, asynchronous development in Python 119 00:04:30.890 --> 00:04:34.560 is built around this generator. 120 00:04:34.560 --> 00:04:36.220 Not this generator specifically, 121 00:04:36.220 --> 00:04:38.290 but generators as a whole. 122 00:04:38.290 --> 00:04:39.900 So let's look in the next video 123 00:04:39.900 --> 00:04:43.680 at how we can build our first tasks scheduler 124 00:04:43.680 --> 00:04:46.620 using generators instead of threads. 125 00:04:46.620 --> 00:04:47.800 I'll see you there.