WEBVTT 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.