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.