WEBVTT 1 00:00:00.130 --> 00:00:01.960 Hi guys, and welcome back. 2 00:00:01.960 --> 00:00:04.600 Right now all of our models basically everywhere 3 00:00:04.600 --> 00:00:05.870 we wanna use a connection, 4 00:00:05.870 --> 00:00:07.560 we've got a bit of duplication there, 5 00:00:07.560 --> 00:00:09.790 we're actually seeing the pool directly getting 6 00:00:09.790 --> 00:00:12.600 a connection, and then putting it back when we're done. 7 00:00:12.600 --> 00:00:15.120 However, if we ever want to change the way 8 00:00:15.120 --> 00:00:17.290 that we handle the pool, for example, 9 00:00:17.290 --> 00:00:20.210 we're going to have to go everywhere and change that. 10 00:00:20.210 --> 00:00:22.620 A common approach when handling connections 11 00:00:22.620 --> 00:00:25.110 that you want to clean up when you're done is to use 12 00:00:25.110 --> 00:00:26.340 a context manager. 13 00:00:26.340 --> 00:00:28.160 We're going to be developing a context manager 14 00:00:28.160 --> 00:00:30.730 that we can use to get a connection from the pool 15 00:00:30.730 --> 00:00:32.540 and put him back when we're done. 16 00:00:32.540 --> 00:00:34.930 That way, if we later on want to change how the pool 17 00:00:34.930 --> 00:00:37.100 is implemented, we're probably gonna wanna use 18 00:00:37.100 --> 00:00:40.010 a context manager anyway, so we'll be keeping that. 19 00:00:40.010 --> 00:00:41.260 Alright, let's get to it. 20 00:00:42.130 --> 00:00:44.990 You can see that everything here has this duplication 21 00:00:44.990 --> 00:00:48.550 and now our change is going to make it a little bit simpler. 22 00:00:48.550 --> 00:00:51.680 What we're going to do is go over to connection pool.py, 23 00:00:51.680 --> 00:00:55.100 and down here we're going to create our context manager. 24 00:00:55.100 --> 00:00:57.130 If you're not familiar with context managers, 25 00:00:57.130 --> 00:00:59.370 then we've got a resource linked over in 26 00:00:59.370 --> 00:01:00.970 the resources section of this lecture 27 00:01:00.970 --> 00:01:02.490 that you can check out. 28 00:01:02.490 --> 00:01:05.040 You should also be familiar with the yield keyword 29 00:01:05.040 --> 00:01:06.200 in Python. 30 00:01:06.200 --> 00:01:07.440 If you're not familiar with that, 31 00:01:07.440 --> 00:01:09.830 also check out the resources section. 32 00:01:09.830 --> 00:01:11.950 Let's create our context manager. 33 00:01:11.950 --> 00:01:13.960 The first thing we have to do is go ahead 34 00:01:13.960 --> 00:01:16.500 and import something from the standard library. 35 00:01:16.500 --> 00:01:20.570 We do from context lib, import context manager. 36 00:01:20.570 --> 00:01:23.040 And what this does is it turns any function into 37 00:01:23.040 --> 00:01:24.700 a context manager. 38 00:01:24.700 --> 00:01:26.994 So we can do @contextmanager, 39 00:01:26.994 --> 00:01:28.160 this is a decorator by the way, 40 00:01:28.160 --> 00:01:30.200 and so if you're not familiar with decorators, 41 00:01:30.200 --> 00:01:31.610 also we've got something linked in 42 00:01:31.610 --> 00:01:33.400 the resources section of this lecture. 43 00:01:33.400 --> 00:01:35.640 Then this is gonna be called get connection. 44 00:01:35.640 --> 00:01:38.150 And what this does is it gets a connection from the pool, 45 00:01:38.150 --> 00:01:40.120 with pool, get con. 46 00:01:40.120 --> 00:01:42.970 And then it's going to do something, 47 00:01:42.970 --> 00:01:46.100 is going to get to that connection 48 00:01:46.100 --> 00:01:51.100 and the return it but it's going to return it using yield. 49 00:01:52.300 --> 00:01:56.850 Then when it receives the control back from the caller, 50 00:01:56.850 --> 00:01:59.930 we're going to use a finally block here 51 00:01:59.930 --> 00:02:03.470 to put the connection back into the pool. 52 00:02:03.470 --> 00:02:08.110 So quick primer on what a yield does, 53 00:02:08.110 --> 00:02:10.790 we're going to be calling this as 54 00:02:10.790 --> 00:02:13.870 a context manager using the width keyword. 55 00:02:13.870 --> 00:02:15.650 And what that's gonna do is it's going 56 00:02:15.650 --> 00:02:19.140 to run this function up to the yield, 57 00:02:19.140 --> 00:02:21.260 then is going to return connection 58 00:02:21.260 --> 00:02:23.380 and give control to the caller. 59 00:02:23.380 --> 00:02:25.220 When the context manager is done, 60 00:02:25.220 --> 00:02:26.850 we're going to come back to the yield, 61 00:02:26.850 --> 00:02:29.470 and we're gonna run the finally block. 62 00:02:29.470 --> 00:02:31.640 So again, you do have to be familiar with yield 63 00:02:31.640 --> 00:02:33.950 and context managers for this to really make sense. 64 00:02:33.950 --> 00:02:35.650 But we've got some resources linked 65 00:02:35.650 --> 00:02:38.190 in the resources section of this lecture. 66 00:02:38.190 --> 00:02:40.470 Now that we've got this, we can go over to pull.py, 67 00:02:40.470 --> 00:02:42.770 and instead of importing the pool here, 68 00:02:42.770 --> 00:02:45.000 we're gonna import get connection. 69 00:02:45.000 --> 00:02:49.920 And now we can do with get connection as connection. 70 00:02:49.920 --> 00:02:51.380 And we no longer need that. 71 00:02:51.380 --> 00:02:54.960 We need a colon and we can put that in there. 72 00:02:54.960 --> 00:02:57.060 We don't need to put the connection back. 73 00:02:57.060 --> 00:02:59.800 But we do need to have this in there as well. 74 00:02:59.800 --> 00:03:02.810 So what happens now is and just go through it again, 75 00:03:02.810 --> 00:03:07.610 we run get connection, this goes over here and runs up 76 00:03:07.610 --> 00:03:10.550 to the yield, it returns connection. 77 00:03:10.550 --> 00:03:13.910 But it essentially pauses this function. 78 00:03:13.910 --> 00:03:15.790 Connection then becomes the value of 79 00:03:15.790 --> 00:03:17.560 this connection variable. 80 00:03:17.560 --> 00:03:19.780 We use it in this block. 81 00:03:19.780 --> 00:03:21.810 And when we reach the end of the block, 82 00:03:21.810 --> 00:03:25.270 the yield resumes, and we run the finally, 83 00:03:25.270 --> 00:03:27.683 which puts the connection back into the pool. 84 00:03:28.970 --> 00:03:31.660 So that's essentially how a yield keyword works. 85 00:03:31.660 --> 00:03:33.510 But again, more information in the resources section 86 00:03:33.510 --> 00:03:35.020 of this lecture. 87 00:03:35.020 --> 00:03:37.890 So we can copy this and use it everywhere 88 00:03:37.890 --> 00:03:39.510 that we're getting a connection, 89 00:03:39.510 --> 00:03:42.450 and that is gonna simplify our approach a little bit. 90 00:03:42.450 --> 00:03:45.190 We no longer need to put the connections back 91 00:03:45.190 --> 00:03:46.690 because that's happening for us. 92 00:03:46.690 --> 00:03:49.763 And we put the rest of the stuff in the context manager. 93 00:03:50.790 --> 00:03:52.460 Notice that we can put return statements 94 00:03:52.460 --> 00:03:53.760 inside the context manager. 95 00:03:53.760 --> 00:03:54.640 That's okay. 96 00:03:54.640 --> 00:03:57.040 Because we've got this inside a finally block 97 00:03:57.040 --> 00:03:58.730 is always gonna run. 98 00:03:58.730 --> 00:04:02.340 Alright, let's go and do this over an option.py as well. 99 00:04:02.340 --> 00:04:05.460 I'm gonna copy this there, and we're gonna put it in there, 100 00:04:05.460 --> 00:04:07.010 then I'm gonna come back to pull.py 101 00:04:07.010 --> 00:04:10.140 and copy this context manager and use it everywhere, 102 00:04:10.140 --> 00:04:11.700 as well. 103 00:04:11.700 --> 00:04:14.560 Notice that this does decrease our code length 104 00:04:14.560 --> 00:04:16.700 by essentially one line per function. 105 00:04:16.700 --> 00:04:20.990 But also, it makes it so that all the interactions with 106 00:04:20.990 --> 00:04:24.120 the pool happen in the context manager. 107 00:04:24.120 --> 00:04:27.270 If we ever want to change how we're using the pool, 108 00:04:27.270 --> 00:04:28.610 or how we're connecting, 109 00:04:28.610 --> 00:04:31.150 or whether we're using psychopg2 at all, 110 00:04:31.150 --> 00:04:33.630 all we have to do is change the context manager. 111 00:04:33.630 --> 00:04:34.840 And like I said earlier on, 112 00:04:34.840 --> 00:04:37.480 using a context manager to get things 113 00:04:37.480 --> 00:04:39.680 and clean up after yourself afterwards, 114 00:04:39.680 --> 00:04:42.030 is so common in Python that we'll probably wanna do 115 00:04:42.030 --> 00:04:42.943 this anyway. 116 00:04:44.490 --> 00:04:46.470 The last place to use the context manager 117 00:04:46.470 --> 00:04:48.130 is over in app.py. 118 00:04:48.130 --> 00:04:50.520 So here instead of getting and putting the connection back, 119 00:04:50.520 --> 00:04:52.520 we're gonna do the same thing. 120 00:04:52.520 --> 00:04:55.180 Putting that in there and now we can go back to the top 121 00:04:55.180 --> 00:04:57.750 and use this instead. 122 00:04:57.750 --> 00:04:59.350 Alright, thank you guys for watching this video. 123 00:04:59.350 --> 00:05:00.183 Thanks for joining me. 124 00:05:00.183 --> 00:05:01.190 I hope you've learnt something 125 00:05:01.190 --> 00:05:02.840 and I'll see you in the next one.