1 00:00:00,520 --> 00:00:01,970 Hi guys, and welcome back! 2 00:00:01,970 --> 00:00:03,230 In this video, we're going to learn 3 00:00:03,230 --> 00:00:05,680 about destructuring variables, 4 00:00:05,680 --> 00:00:08,060 which is something we looked at in the previous video, 5 00:00:08,060 --> 00:00:09,910 in the for-loop, but now we're going 6 00:00:09,910 --> 00:00:11,750 to explain how it works. 7 00:00:11,750 --> 00:00:13,790 Normally in Python when you assign a variable, 8 00:00:13,790 --> 00:00:16,963 you can do something like x equal five, say. 9 00:00:17,890 --> 00:00:22,890 But, you can also do x equal five comma 11. 10 00:00:22,990 --> 00:00:25,240 And, this, here is a tuple. 11 00:00:25,240 --> 00:00:27,720 We've learned about these in the past. 12 00:00:27,720 --> 00:00:31,980 But, what a lot of people don't know, is that the brackets 13 00:00:31,980 --> 00:00:34,920 are not necessary, the brackets are only necessary 14 00:00:34,920 --> 00:00:39,180 when Python might be confused as to whether you wanna create 15 00:00:39,180 --> 00:00:43,310 a tuple, or these are values in another collection. 16 00:00:43,310 --> 00:00:45,500 So if you have a tuple like this one, 17 00:00:45,500 --> 00:00:48,060 you can actually remove the brackets. 18 00:00:48,060 --> 00:00:52,580 But, if you wanted to create a list with a tuple inside it, 19 00:00:52,580 --> 00:00:54,320 then, you would need the brackets. 20 00:00:54,320 --> 00:00:56,180 Because, if you don't put the brackets in, 21 00:00:56,180 --> 00:00:58,400 you'll actually end with a list with two values, 22 00:00:58,400 --> 00:01:01,610 not a list with a tuple inside it. 23 00:01:01,610 --> 00:01:04,660 So, the brackets are only needed when you want to explicitly 24 00:01:04,660 --> 00:01:09,420 say, I want a tuple here, I don't want Python to treat these 25 00:01:09,420 --> 00:01:11,380 as two separate values. 26 00:01:11,380 --> 00:01:14,640 But most of the time, the brackets are not necessary, 27 00:01:14,640 --> 00:01:16,740 so you can leave it as this. 28 00:01:16,740 --> 00:01:19,390 Now, regarding destructuring variables, 29 00:01:19,390 --> 00:01:22,770 this tuple here can be destructured or decomposed, 30 00:01:22,770 --> 00:01:26,790 or split-out into two variables if you want. 31 00:01:26,790 --> 00:01:31,070 X, comma, y, and Python is smart enough to assign 32 00:01:31,070 --> 00:01:33,340 x to five and y to 11. 33 00:01:33,340 --> 00:01:35,880 It's just a shorthand, instead of defining 34 00:01:35,880 --> 00:01:37,720 two variables separately. 35 00:01:37,720 --> 00:01:40,400 The extra benefit that we get from being able 36 00:01:40,400 --> 00:01:44,270 to do something like this is that if you have another 37 00:01:44,270 --> 00:01:49,020 tuple, which is five and 11, you can then actually say 38 00:01:49,020 --> 00:01:52,570 something like this, x comma y equal t. 39 00:01:52,570 --> 00:01:55,000 And then, we can print x and y. 40 00:01:55,000 --> 00:01:58,540 So if we run this, you'll see that you get five and 11 41 00:01:58,540 --> 00:02:00,550 as the two separate variables. 42 00:02:00,550 --> 00:02:04,100 So, Python was smart enough to say, okay, t is a tuple, 43 00:02:04,100 --> 00:02:06,630 and you're asking me to assign this tuple to two variables, 44 00:02:06,630 --> 00:02:08,600 what I'm gonna do is I'm gonna split it out 45 00:02:08,600 --> 00:02:10,560 into it's components. 46 00:02:10,560 --> 00:02:13,713 So this is what destructuring really means. 47 00:02:16,270 --> 00:02:18,930 In the last video, we had some code like this one, 48 00:02:18,930 --> 00:02:23,360 where we have a dictionary of strings to integers, 49 00:02:23,360 --> 00:02:28,250 and then we iterated over student attendance dot items. 50 00:02:28,250 --> 00:02:33,250 Now, I'm going to print the list of that, which is going 51 00:02:33,560 --> 00:02:36,940 to take this stuff here, turn it into a list, 52 00:02:36,940 --> 00:02:38,230 and print it out. 53 00:02:38,230 --> 00:02:41,420 So, let me do that, and not print that, 54 00:02:41,420 --> 00:02:43,510 and show you what happens. 55 00:02:43,510 --> 00:02:48,050 Notice that what you get back is a list of tuples. 56 00:02:48,050 --> 00:02:51,570 So, if you iterate over a list of tuples, 57 00:02:51,570 --> 00:02:55,853 that means that, by doing something like this, 58 00:02:57,050 --> 00:03:01,280 what you are getting is three different tuples. 59 00:03:01,280 --> 00:03:04,250 Let me show you what happens if I say that, 60 00:03:04,250 --> 00:03:07,693 and then I print t there, and then I common that out. 61 00:03:09,560 --> 00:03:14,560 You can see that t is first a tuple of Rolf and 96, 62 00:03:15,110 --> 00:03:17,370 then a tuple of Bob and 80, 63 00:03:17,370 --> 00:03:19,463 and then a tuple of Anne and 100. 64 00:03:20,350 --> 00:03:23,940 So, instead of t, we can access its components 65 00:03:23,940 --> 00:03:27,550 because we can destructure this into two separate variables, 66 00:03:27,550 --> 00:03:29,650 and that's what we were doing earlier on, 67 00:03:29,650 --> 00:03:34,650 when we destructured the tuple into student, and attendance. 68 00:03:35,240 --> 00:03:39,660 Student would be Rolf, and attendance would be 96. 69 00:03:39,660 --> 00:03:41,670 And then the next time you go through the loop, 70 00:03:41,670 --> 00:03:43,210 you'd get the next tuple, 71 00:03:43,210 --> 00:03:46,270 and that would be destructured into its two components, 72 00:03:46,270 --> 00:03:47,690 again, and so on. 73 00:03:47,690 --> 00:03:51,620 So that, is why this works. 74 00:03:51,620 --> 00:03:55,010 Hopefully that makes sense, that is destructuring in use 75 00:03:55,010 --> 00:03:57,610 here in a for-loop, and you're gonna see it quite a lot 76 00:03:57,610 --> 00:03:59,150 when you do something like items, 77 00:03:59,150 --> 00:04:01,390 or when you use something like the enumerate function 78 00:04:01,390 --> 00:04:03,260 that I know we haven't spoke about yet, 79 00:04:03,260 --> 00:04:05,350 it is quite prevalent in Python. 80 00:04:05,350 --> 00:04:08,600 I'm going to link a blog post on destructuring variables 81 00:04:08,600 --> 00:04:10,600 that you can read as well if you'd like. 82 00:04:13,010 --> 00:04:14,750 Let's take a look at another example, 83 00:04:14,750 --> 00:04:18,510 here we have a list of people where instead of a dictionary, 84 00:04:18,510 --> 00:04:22,370 each person has a tuple with three values. 85 00:04:22,370 --> 00:04:25,570 Their name, their age, and their profession. 86 00:04:25,570 --> 00:04:28,230 Because we've got a tuple of three values, 87 00:04:28,230 --> 00:04:32,950 we can do for name, age, and profession in people, 88 00:04:32,950 --> 00:04:35,300 and that is going to, once again, iterate through each 89 00:04:35,300 --> 00:04:38,630 of the tuples, and for each one, destructure it into 90 00:04:38,630 --> 00:04:40,790 its three separate components. 91 00:04:40,790 --> 00:04:44,683 So here, we could print NA string describing this person, 92 00:04:46,440 --> 00:04:48,040 just like that. 93 00:04:48,040 --> 00:04:51,700 Notice that if any one of your tuples is missing a value, 94 00:04:51,700 --> 00:04:54,440 you're going to encounter some problems. 95 00:04:54,440 --> 00:04:57,420 It says, value error, not enough values to unpack. 96 00:04:57,420 --> 00:05:01,840 So you tried to access three values of this tuple, 97 00:05:01,840 --> 00:05:05,380 but there weren't three values, profession was missing, 98 00:05:05,380 --> 00:05:06,660 so you got this error. 99 00:05:06,660 --> 00:05:08,060 So this is something you have to be careful with, 100 00:05:08,060 --> 00:05:10,713 when you're doing this destructuring or unpacking. 101 00:05:12,170 --> 00:05:16,300 Notice that the alternative to doing this is to iterate over 102 00:05:16,300 --> 00:05:20,700 each tuple, and print out each thing, using indices. 103 00:05:20,700 --> 00:05:24,120 So, person zero would be the name, person one 104 00:05:24,120 --> 00:05:27,700 would be the age, and person two would be the profession, 105 00:05:27,700 --> 00:05:30,400 if you don't destructure the variable here. 106 00:05:30,400 --> 00:05:32,460 So, this is clearly much less readable, 107 00:05:32,460 --> 00:05:34,090 it's not as obvious what's going on, 108 00:05:34,090 --> 00:05:36,410 so destructuring these things does help your code 109 00:05:36,410 --> 00:05:37,313 look a bit nicer. 110 00:05:39,480 --> 00:05:41,260 Let's say you got a single tuple, 111 00:05:41,260 --> 00:05:44,610 and you want to extract a person's name and profession, 112 00:05:44,610 --> 00:05:47,013 but you don't care for the age. 113 00:05:47,950 --> 00:05:50,880 In Python, variables can contain and even start 114 00:05:50,880 --> 00:05:54,210 with an underscore, and that is frequently what's used as 115 00:05:54,210 --> 00:05:56,800 a variable when you want to ignore one. 116 00:05:56,800 --> 00:06:01,060 So, you can say name underscore profession equal person, 117 00:06:01,060 --> 00:06:02,760 and what this is going to do is it's going to create 118 00:06:02,760 --> 00:06:05,840 three variables, name, underscore, and profession, 119 00:06:05,840 --> 00:06:09,780 and Bob will become name, 42 will become underscore, 120 00:06:09,780 --> 00:06:11,720 and mechanic will become profession. 121 00:06:11,720 --> 00:06:16,720 But, the underscore on its own is so bad a variable name, 122 00:06:17,170 --> 00:06:18,900 that nobody would wanna use it, 123 00:06:18,900 --> 00:06:22,020 unless it was not meant to be used. 124 00:06:22,020 --> 00:06:25,120 So that is what the Python community has decided, 125 00:06:25,120 --> 00:06:27,550 that if you use an underscore variable on its own, 126 00:06:27,550 --> 00:06:30,003 that is because you don't care about this variable, 127 00:06:30,003 --> 00:06:32,150 this variable is meant to be ignored. 128 00:06:32,150 --> 00:06:35,740 So then, it's just as good to print name and profession out, 129 00:06:35,740 --> 00:06:38,663 if you wanted to just print the person's details out. 130 00:06:40,520 --> 00:06:43,210 Let's say you've got a list of five elements, 131 00:06:43,210 --> 00:06:44,720 one, two, three, four, and five, 132 00:06:44,720 --> 00:06:47,860 and you wanted to separate this into two lists. 133 00:06:47,860 --> 00:06:51,760 The first element, and all the other elements. 134 00:06:51,760 --> 00:06:56,240 Well, you can do head comma star tail, 135 00:06:56,240 --> 00:06:58,540 and that is the syntax for collecting. 136 00:06:58,540 --> 00:07:01,410 So, what you are doing, is you are collecting all of the 137 00:07:01,410 --> 00:07:04,760 destructured values into this variable. 138 00:07:04,760 --> 00:07:07,590 All of the values that don't match any of the other 139 00:07:07,590 --> 00:07:10,420 destructured values, which in this case you've got the head. 140 00:07:10,420 --> 00:07:13,290 So the first value in here will become the head, 141 00:07:13,290 --> 00:07:16,170 and then this syntax here will collect the other values, 142 00:07:16,170 --> 00:07:18,530 and will put them all into tail. 143 00:07:18,530 --> 00:07:21,010 So if you print head, and you print tail, 144 00:07:21,010 --> 00:07:24,540 then you'll see what happens is you get one, as the first 145 00:07:24,540 --> 00:07:28,460 value, and two, three, four, and five as the second value. 146 00:07:28,460 --> 00:07:31,760 Notice that you only need to do this when you want to put 147 00:07:31,760 --> 00:07:34,440 all the values into one place, you never need to use it 148 00:07:34,440 --> 00:07:36,983 again like that, you don't have to do that. 149 00:07:37,840 --> 00:07:39,870 Instead of that, you can move this of course, 150 00:07:39,870 --> 00:07:42,010 and put the star at the front if you like, 151 00:07:42,010 --> 00:07:43,280 and then this is going to collect 152 00:07:43,280 --> 00:07:44,950 as many values as possible, 153 00:07:44,950 --> 00:07:47,020 but leaving room for the tail, 154 00:07:47,020 --> 00:07:48,510 which is gonna be the last one. 155 00:07:48,510 --> 00:07:50,340 So, head will be one, two, three, four, 156 00:07:50,340 --> 00:07:53,840 and tail will be five, just like that. 157 00:07:53,840 --> 00:07:57,890 The asterisk can also be used here, we're not going 158 00:07:57,890 --> 00:07:59,190 to learn about that just now, 159 00:07:59,190 --> 00:08:00,960 we're going to learn about it in a few videos times, 160 00:08:00,960 --> 00:08:02,230 it's a little bit more advanced, 161 00:08:02,230 --> 00:08:05,870 so I wanna get into some of the other topics before 162 00:08:05,870 --> 00:08:07,620 we talk about that one. 163 00:08:07,620 --> 00:08:10,110 So, please do bear with me, and know that that will work 164 00:08:10,110 --> 00:08:12,470 if you do that, and it will print out something slightly 165 00:08:12,470 --> 00:08:15,280 different, and not printing the list anymore, 166 00:08:15,280 --> 00:08:17,040 now which brings you the separate values, 167 00:08:17,040 --> 00:08:19,640 and that is valid Python and you will use it 168 00:08:19,640 --> 00:08:21,680 quite a lot as you programme in Python, 169 00:08:21,680 --> 00:08:24,180 but again we're going to learn about it in a later video. 170 00:08:24,180 --> 00:08:27,000 So, please, bear with me, I hope this video's been useful, 171 00:08:27,000 --> 00:08:28,650 and I'll see you in the next one.