WEBVTT 1 00:00:01.170 --> 00:00:02.630 Hi guys and welcome back. 2 00:00:02.630 --> 00:00:04.210 In this video we're going to talk about 3 00:00:04.210 --> 00:00:06.312 Mutable default parameters. 4 00:00:06.312 --> 00:00:08.410 This is something that you really need 5 00:00:08.410 --> 00:00:09.770 to know about in Python because if you 6 00:00:09.770 --> 00:00:12.386 make this mistake it's gonna cost you a lot time 7 00:00:12.386 --> 00:00:14.600 trying to find out what the problem is. 8 00:00:14.600 --> 00:00:16.590 So let's get started. 9 00:00:16.590 --> 00:00:19.101 Here we've got a student class and in the init 10 00:00:19.101 --> 00:00:21.070 method we have two parameters. 11 00:00:21.070 --> 00:00:23.659 The name which is a string and the grades 12 00:00:23.659 --> 00:00:26.490 which is a list of integers 13 00:00:26.490 --> 00:00:30.093 But it has a default value of empty list. 14 00:00:31.460 --> 00:00:33.210 So by the way this is how you type hint 15 00:00:33.210 --> 00:00:35.640 when you have both a type hint and a default value. 16 00:00:35.640 --> 00:00:37.290 I'm not sure we even looked at it before 17 00:00:37.290 --> 00:00:39.820 but nonetheless that is how it goes. 18 00:00:39.820 --> 00:00:42.520 Though you've got your grades you can patch in 19 00:00:42.520 --> 00:00:45.860 a list of grades if you want when you create the object 20 00:00:45.860 --> 00:00:49.420 or you can use a default which is an empty list. 21 00:00:49.420 --> 00:00:52.770 Then you save that and when you take an exam 22 00:00:52.770 --> 00:00:55.160 your passing a result such as an integer. 23 00:00:55.160 --> 00:00:57.000 we're going to put that in there actually 24 00:00:57.000 --> 00:01:00.497 and then it appends to self dot grades. 25 00:01:00.497 --> 00:01:02.800 "This is bad" I put a comment in there so you 26 00:01:02.800 --> 00:01:03.730 don't do this. 27 00:01:03.730 --> 00:01:08.730 Never make a parameter equal to a mutual value by default. 28 00:01:08.900 --> 00:01:09.930 Here's why. 29 00:01:09.930 --> 00:01:12.680 You created your student, Bob, and you used the 30 00:01:12.680 --> 00:01:16.113 default grades, then you made him take an exam 31 00:01:16.113 --> 00:01:18.970 which appends 90 to self dot grades 32 00:01:18.970 --> 00:01:20.510 and finally you print, Bob's, grades. 33 00:01:20.510 --> 00:01:23.300 So let's save that and run it and you see you 34 00:01:23.300 --> 00:01:26.670 get 90 out so yeah, no problem. 35 00:01:26.670 --> 00:01:28.960 But then what if you create one more student 36 00:01:30.550 --> 00:01:35.293 such as, Rolf, and you don't make him take any exams. 37 00:01:36.590 --> 00:01:38.040 What are we going to see now? 38 00:01:40.040 --> 00:01:41.290 Well let me run this code 39 00:01:42.430 --> 00:01:45.280 and wallah both student's 40 00:01:45.280 --> 00:01:46.940 have 90. 41 00:01:46.940 --> 00:01:50.370 Even though only Bob took an exam. 42 00:01:50.370 --> 00:01:52.890 This is why mutability is so important 43 00:01:52.890 --> 00:01:55.500 in Python and why this is a bad idea. 44 00:01:55.500 --> 00:01:59.380 The function parameters these default ones 45 00:01:59.380 --> 00:02:02.030 evaluate when the function is defined, 46 00:02:02.030 --> 00:02:05.050 not when the function is called. 47 00:02:05.050 --> 00:02:08.160 When the class is created this function is 48 00:02:08.160 --> 00:02:10.350 evaluated so that Python knows what the 49 00:02:10.350 --> 00:02:11.820 parameters are and what the name is 50 00:02:11.820 --> 00:02:14.550 and so on and this one too. 51 00:02:14.550 --> 00:02:18.303 And there is default value is created. 52 00:02:19.540 --> 00:02:22.430 That means that if you use the default 53 00:02:22.430 --> 00:02:27.400 self dot grades is a name for this specific list 54 00:02:27.400 --> 00:02:29.350 And when you create two students. 55 00:02:29.350 --> 00:02:31.875 Self dot grades in both of them are names 56 00:02:31.875 --> 00:02:33.950 to the same list. 57 00:02:33.950 --> 00:02:37.070 This one that was created when the function was created. 58 00:02:37.070 --> 00:02:38.670 Not when the function was called 59 00:02:39.600 --> 00:02:42.474 So as long as your using the default parameters 60 00:02:42.474 --> 00:02:46.370 for all your students they will share grades. 61 00:02:46.370 --> 00:02:48.840 That's probably not what you want. 62 00:02:48.840 --> 00:02:50.454 Rather do note that if you pass in 63 00:02:50.454 --> 00:02:53.726 your own list here then these will be different 64 00:02:53.726 --> 00:02:56.777 because now Bob is using this list 65 00:02:56.777 --> 00:02:59.260 And Rolf is using this one. 66 00:02:59.260 --> 00:03:01.520 If you then define a third one that uses this one 67 00:03:01.520 --> 00:03:02.970 those two will share grades 68 00:03:02.970 --> 00:03:04.410 but this one will be unaffected 69 00:03:04.410 --> 00:03:06.343 because that's using his own list. 70 00:03:07.340 --> 00:03:11.360 So avoid this problem by not using Mutable default 71 00:03:11.360 --> 00:03:12.710 values for parameters. 72 00:03:12.710 --> 00:03:14.580 Instead what you can do is you can make it 73 00:03:14.580 --> 00:03:18.360 equal to none and then you can make this equal 74 00:03:18.360 --> 00:03:19.840 to grades 75 00:03:19.840 --> 00:03:21.810 or empty list 76 00:03:21.810 --> 00:03:23.080 and what happens here is 77 00:03:23.080 --> 00:03:27.340 that if this is none then none or empty list 78 00:03:27.340 --> 00:03:29.710 will evaluate to empty list. 79 00:03:29.710 --> 00:03:32.223 Now instead of list of integers you may want to do 80 00:03:32.223 --> 00:03:35.990 import optional as well and make this an optional 81 00:03:38.250 --> 00:03:39.240 list in integers. 82 00:03:39.240 --> 00:03:43.000 That's just a slightly better way of handling this here 83 00:03:43.000 --> 00:03:44.790 so Python knows that it is optional 84 00:03:44.790 --> 00:03:47.090 and it may not be a list initially it maybe none 85 00:03:47.090 --> 00:03:50.170 initially but your gonna set it to a list later on. 86 00:03:50.170 --> 00:03:52.730 So this is a better way of handling that type of 87 00:03:52.730 --> 00:03:53.563 hint as well. 88 00:03:54.420 --> 00:03:57.150 So now this will work the way we intended to. 89 00:03:57.150 --> 00:04:00.350 By pressing play you see that Bob has 90 90 00:04:00.350 --> 00:04:02.390 and Rolf has nothing. 91 00:04:02.390 --> 00:04:04.806 It is very important my word of advise is to 92 00:04:04.806 --> 00:04:08.590 use default parameter values that are mutable. 93 00:04:08.590 --> 00:04:11.130 Just stick to immutable ones like 94 00:04:11.130 --> 00:04:13.357 integers, strings, floats, bullion, tuples 95 00:04:13.357 --> 00:04:17.120 things like that and if not use this pattern here 96 00:04:17.120 --> 00:04:19.608 to define them inside the function and not 97 00:04:19.608 --> 00:04:21.323 in the parameter list. 98 00:04:22.890 --> 00:04:24.260 Thank you for joining me in this video, 99 00:04:24.260 --> 00:04:27.110 I hope it's been useful and I'll see you in the next one.