1 00:00:00,620 --> 00:00:02,220 Hi guys and welcome back. 2 00:00:02,220 --> 00:00:05,420 In this video we're going to learn about unpacking arguments 3 00:00:05,420 --> 00:00:07,360 and this is a little bit more complicated 4 00:00:07,360 --> 00:00:10,263 than what we've seen so far, so pleased do bear with me. 5 00:00:11,100 --> 00:00:13,660 I'm going to define a multiply function 6 00:00:13,660 --> 00:00:16,680 that takes in any number of arguments. 7 00:00:16,680 --> 00:00:17,513 Here's how. 8 00:00:19,060 --> 00:00:21,900 We create the multiply function as normal, 9 00:00:21,900 --> 00:00:26,123 and in here, we're going to put, star, args. 10 00:00:27,030 --> 00:00:31,040 Now the star is the important part, the args 11 00:00:31,040 --> 00:00:33,580 is just a variable name that will be used 12 00:00:33,580 --> 00:00:36,290 to collect the arguments. 13 00:00:36,290 --> 00:00:38,573 So let's print the args out. 14 00:00:39,730 --> 00:00:41,101 For now it's not going to multiply anything, 15 00:00:41,101 --> 00:00:42,560 it's just going to print them out, 16 00:00:42,560 --> 00:00:44,910 so what's happening here is we've defined a function 17 00:00:44,910 --> 00:00:47,340 and we've said, that the function has 18 00:00:48,780 --> 00:00:53,550 a set of arguments that will be collected into 19 00:00:53,550 --> 00:00:57,720 a tuple of arguments when the function gets called. 20 00:00:57,720 --> 00:01:02,010 So if we do multiply and then we do one, three and five, 21 00:01:02,010 --> 00:01:06,185 notice that here I'm passing different arguments. 22 00:01:06,185 --> 00:01:09,303 But this only has seemingly a single parameter. 23 00:01:09,303 --> 00:01:10,890 So I'm passing three different arguments 24 00:01:10,890 --> 00:01:14,870 and what's gonna happen is they are going to be collected 25 00:01:14,870 --> 00:01:19,870 into this args variable, so I'm gonna save this and run it. 26 00:01:19,950 --> 00:01:22,010 And you can see that what we get printed out 27 00:01:22,010 --> 00:01:25,410 when we run the function is a tuple of my arguments, 28 00:01:25,410 --> 00:01:28,423 so we've got one, three and five printed out as a tuple. 29 00:01:29,370 --> 00:01:32,470 If we wanted to multiply all these numbers together, 30 00:01:32,470 --> 00:01:36,990 we can just iterate over the arguments so for arg in args 31 00:01:36,990 --> 00:01:40,750 we can say total, equal total times arg, 32 00:01:40,750 --> 00:01:45,710 and then return total, then we can print that out. 33 00:01:45,710 --> 00:01:47,330 Now what we're doing here is we're starting off 34 00:01:47,330 --> 00:01:49,710 with a variable which is one and then we're multiplying 35 00:01:49,710 --> 00:01:52,380 that variable each of the arguments, 36 00:01:52,380 --> 00:01:54,330 and then we're returning the final value. 37 00:01:54,330 --> 00:01:59,180 If we run that we get an error because we left the S there 38 00:01:59,180 --> 00:02:01,330 so let's run that again. 39 00:02:01,330 --> 00:02:04,180 There we go we get 15 printed out. 40 00:02:04,180 --> 00:02:08,030 So this is a way collecting multiple arguments 41 00:02:08,030 --> 00:02:11,590 into a single variable when you are calling a function. 42 00:02:11,590 --> 00:02:14,730 And this has a number of uses that you will definitely 43 00:02:14,730 --> 00:02:18,210 be seeing quite a lot as you programme in Python. 44 00:02:18,210 --> 00:02:21,560 Notice that you can call this function with one argument 45 00:02:21,560 --> 00:02:23,980 if you want, and that's totally fine as well, 46 00:02:23,980 --> 00:02:27,170 it just creates a tuple of a single element. 47 00:02:27,170 --> 00:02:29,710 Let's go into another example let's say we have 48 00:02:29,710 --> 00:02:33,190 an add function with two parameters x and y, 49 00:02:33,190 --> 00:02:36,660 and then you have a list of numbers, three and five. 50 00:02:36,660 --> 00:02:40,560 So just as we can use the asterisk to collect arguments 51 00:02:40,560 --> 00:02:44,170 into one parameter, we can go the other way round 52 00:02:44,170 --> 00:02:47,750 and we can use the asterisk to de-structure arguments 53 00:02:47,750 --> 00:02:49,340 into multiple parameters. 54 00:02:49,340 --> 00:02:53,480 So you can do add, star nums, and what this'll do 55 00:02:53,480 --> 00:02:58,240 is it will use three for x and five for y 56 00:02:58,240 --> 00:03:01,550 so it is de-structuring the nums variable 57 00:03:01,550 --> 00:03:04,060 and splitting it out into multiple values 58 00:03:04,060 --> 00:03:07,367 such that one value can go to one parameter. 59 00:03:07,367 --> 00:03:10,170 Notice that if you do just nums here, 60 00:03:10,170 --> 00:03:13,820 you're going to pass three and five, the list itself 61 00:03:13,820 --> 00:03:17,310 as the value for x and y is not going to have a value 62 00:03:17,310 --> 00:03:18,950 so you're going to get an error. 63 00:03:18,950 --> 00:03:23,950 So by doing this it passes one value for each parameter. 64 00:03:23,980 --> 00:03:26,570 Of course you do need to have the same number of values 65 00:03:26,570 --> 00:03:29,210 in this list, as parameters in this function, 66 00:03:29,210 --> 00:03:30,610 otherwise you're going to have an error 67 00:03:30,610 --> 00:03:32,030 where you're gonna have too many parameters 68 00:03:32,030 --> 00:03:33,654 or too many arguments. 69 00:03:33,654 --> 00:03:37,130 Let's print this out so you can check that it works. 70 00:03:37,130 --> 00:03:39,270 There you have it, eight comes out. 71 00:03:39,270 --> 00:03:43,160 Now this can also work with named arguments 72 00:03:43,160 --> 00:03:45,690 so remember you can call the add function 73 00:03:45,690 --> 00:03:49,530 by doing x equals three and y equal five. 74 00:03:49,530 --> 00:03:53,270 So you can do something like that using these asterisks. 75 00:03:53,270 --> 00:03:55,840 Let's say that you've got your nums and these are now 76 00:03:55,840 --> 00:04:00,840 a dictionary with a value for x, and a value for y. 77 00:04:00,940 --> 00:04:04,740 If you wanted to use this dictionary as the argument 78 00:04:04,740 --> 00:04:09,420 to these functions, you would have to do nums x 79 00:04:09,420 --> 00:04:13,700 and nums y, like that, which is kind of ugly 80 00:04:13,700 --> 00:04:15,770 and also difficult to read, you've started to get a lot 81 00:04:15,770 --> 00:04:17,470 of brackets and square brackets and so on. 82 00:04:17,470 --> 00:04:19,940 If you wanted to be even more specific you can do 83 00:04:19,940 --> 00:04:24,580 x equals nums x and y equal nums y. 84 00:04:24,580 --> 00:04:27,740 And here is where the magic happens. 85 00:04:27,740 --> 00:04:29,630 You can see that the parameter name 86 00:04:30,660 --> 00:04:35,170 is the same as the key in the dictionary that we are using 87 00:04:35,170 --> 00:04:40,170 as a value for that parameter, so x is the key x 88 00:04:40,470 --> 00:04:43,220 and y is the key y. 89 00:04:43,220 --> 00:04:47,200 Instead of doing this entire thing, we can tell Python 90 00:04:47,200 --> 00:04:52,200 to pass in this dictionary as named arguments, 91 00:04:52,630 --> 00:04:54,970 where the argument will be equal to the value 92 00:04:54,970 --> 00:04:58,750 associated with that same name for a key. 93 00:04:58,750 --> 00:05:02,240 So essentially doing exactly this, but just do 94 00:05:02,240 --> 00:05:05,940 start star, nums, and that will do the same thing. 95 00:05:05,940 --> 00:05:08,240 So because we're using start star, it will say 96 00:05:08,240 --> 00:05:11,020 okay you've got a dictionary with two keys, 97 00:05:11,020 --> 00:05:13,160 what I'm gonna do is I'm gonna pass in each key 98 00:05:13,160 --> 00:05:16,300 as a named argument, and the value is going to be 99 00:05:16,300 --> 00:05:18,590 that associated with the key, so it'll say 100 00:05:18,590 --> 00:05:21,760 x equals nums x and y equals nums y. 101 00:05:21,760 --> 00:05:24,220 I know that's quite complicated thing, 102 00:05:24,220 --> 00:05:26,760 but it simplifies your code so much 103 00:05:26,760 --> 00:05:29,370 when you can use dictionaries like that. 104 00:05:29,370 --> 00:05:31,960 Let's go back to our multiply function 105 00:05:31,960 --> 00:05:35,750 that we had earlier on with this collected set of arguments. 106 00:05:35,750 --> 00:05:38,310 And I'm going to create anther function called apply 107 00:05:38,310 --> 00:05:42,463 and now we're going to say start args, comma operator. 108 00:05:43,340 --> 00:05:46,030 So this is a special bit of syntax in Python 109 00:05:46,030 --> 00:05:49,930 and what it means is collect all the positional arguments 110 00:05:49,930 --> 00:05:54,510 into this tuple args, and also have 111 00:05:54,510 --> 00:05:57,780 a named argument at the end. 112 00:05:57,780 --> 00:06:02,710 This creates a compulsory or required named argument. 113 00:06:02,710 --> 00:06:04,640 So when you're calling this function you're gonna pass in 114 00:06:04,640 --> 00:06:07,610 as many arguments as you want and they will all be collected 115 00:06:07,610 --> 00:06:11,120 into args and at the end you must pass in 116 00:06:11,120 --> 00:06:13,350 a named argument for this one. 117 00:06:13,350 --> 00:06:16,160 So let me just give it a body there, and we're gonna say 118 00:06:16,160 --> 00:06:18,620 if the operator is equal to the asterisk, 119 00:06:18,620 --> 00:06:21,130 we're going to return multiply of args, 120 00:06:21,130 --> 00:06:23,940 otherwise if the operator is equal to plus, 121 00:06:23,940 --> 00:06:25,620 we're going to return the sum of args 122 00:06:25,620 --> 00:06:27,660 and finally we're going to return no valid operator 123 00:06:27,660 --> 00:06:28,810 if it's neither of them. 124 00:06:28,810 --> 00:06:30,790 Now I have a bug in this function 125 00:06:30,790 --> 00:06:33,300 and you may notice what it is based on 126 00:06:33,300 --> 00:06:35,260 what we've seen already, but I know that you don't have 127 00:06:35,260 --> 00:06:38,430 a lot of practise with this yet so it is possible 128 00:06:38,430 --> 00:06:40,650 to not notice it, that's fine. 129 00:06:40,650 --> 00:06:44,890 We're going to print apply of one three six and seven, 130 00:06:44,890 --> 00:06:47,660 and the operator is going to be plus. 131 00:06:47,660 --> 00:06:49,780 So this is how this function must be called. 132 00:06:49,780 --> 00:06:52,290 You can pass in as many arguments as you want. 133 00:06:52,290 --> 00:06:54,043 They will be collected by args. 134 00:06:54,043 --> 00:06:57,710 Finally you must pass in a named argument 135 00:06:57,710 --> 00:06:59,180 for this one there. 136 00:06:59,180 --> 00:07:02,030 If you just pass in plus, Python's gonna try to 137 00:07:02,030 --> 00:07:05,438 collect it with args, so you're gonna miss this parameter. 138 00:07:05,438 --> 00:07:09,560 Indeed if we run this, you'll see that you get an error. 139 00:07:09,560 --> 00:07:12,590 Missing one required keyword only argument. 140 00:07:12,590 --> 00:07:15,310 So you do need the operator in there. 141 00:07:15,310 --> 00:07:18,700 So if we run this, it'll all work, you get 17 back 142 00:07:18,700 --> 00:07:21,230 because that's the addition of all of these numbers. 143 00:07:21,230 --> 00:07:24,833 However if you change this to a asterisk, 144 00:07:25,880 --> 00:07:30,570 then you get back a tuple, so why this tuple? 145 00:07:30,570 --> 00:07:33,850 Why not the multiplication of all the numbers? 146 00:07:33,850 --> 00:07:37,030 Is there a bug in our multiply function? 147 00:07:37,030 --> 00:07:39,520 Well you might be tempted to go through 148 00:07:39,520 --> 00:07:41,860 and see what the multiply function is doing. 149 00:07:41,860 --> 00:07:45,040 Multiply function starts with a variable called total 150 00:07:45,040 --> 00:07:47,220 that has the value one, and then it goes through 151 00:07:47,220 --> 00:07:50,560 the arguments and makes total equal to 152 00:07:50,560 --> 00:07:53,010 total multiplied by the arg. 153 00:07:53,010 --> 00:07:56,500 So how on earth is it managing to end up 154 00:07:56,500 --> 00:07:58,580 with a tuple being returned? 155 00:07:58,580 --> 00:08:02,260 Well if you multiply by a tuple, 156 00:08:02,260 --> 00:08:04,650 you actually end up with that tuple. 157 00:08:04,650 --> 00:08:09,650 So that means that args must be a tuple of tuples 158 00:08:10,260 --> 00:08:15,260 such that the first argument, the first arg in the tuple 159 00:08:15,272 --> 00:08:18,680 is itself a tuple, such that you're multiplying 160 00:08:18,680 --> 00:08:23,130 the tuple by one, and ending up with the tuple itself 161 00:08:23,130 --> 00:08:26,600 and then returning that, so the best way to do this 162 00:08:26,600 --> 00:08:28,410 is to use a debugger, but we haven't learned 163 00:08:28,410 --> 00:08:29,790 how to use a debugger yet. 164 00:08:29,790 --> 00:08:31,860 So we're gonna print the argumetns and you'll see that 165 00:08:31,860 --> 00:08:33,830 you're gonna get something a little bit weird. 166 00:08:33,830 --> 00:08:37,650 When you print the args, you get a tuple of tuples. 167 00:08:37,650 --> 00:08:39,870 And then when you iterate over the args, 168 00:08:39,870 --> 00:08:42,520 arg is the inner tuple. 169 00:08:42,520 --> 00:08:45,951 You're multiplying that by one, making it equal to the total 170 00:08:45,951 --> 00:08:49,570 such that total is the tuple, and then because that's 171 00:08:49,570 --> 00:08:53,150 the only value in args, that is the end of the for loop 172 00:08:53,150 --> 00:08:54,910 and you return the tuple. 173 00:08:54,910 --> 00:08:59,000 The reason for that is because we did not pass in 174 00:08:59,000 --> 00:09:01,580 individual arguments to the multiply function, 175 00:09:01,580 --> 00:09:02,963 we passed in a tuple. 176 00:09:04,010 --> 00:09:06,540 We want to put a star here. 177 00:09:06,540 --> 00:09:08,310 The reason for that is because when we call 178 00:09:08,310 --> 00:09:11,540 the apply function we are passing in four different 179 00:09:11,540 --> 00:09:13,430 arguments that are being collected 180 00:09:13,430 --> 00:09:16,850 into one tuple, by this star. 181 00:09:16,850 --> 00:09:19,330 But when we call the multiply function, 182 00:09:19,330 --> 00:09:22,360 we wanna pass in four different arguments 183 00:09:22,360 --> 00:09:25,630 so that they can be collected by args, 184 00:09:25,630 --> 00:09:29,410 inside the multiplier function and then used in here. 185 00:09:29,410 --> 00:09:32,460 But before, we were passing the args tuple itself, 186 00:09:32,460 --> 00:09:34,590 the collected tuple in this function. 187 00:09:34,590 --> 00:09:38,350 What we wanna do is we wanna unpack it into four values 188 00:09:38,350 --> 00:09:41,590 and pass them as individual values to this function 189 00:09:41,590 --> 00:09:44,920 so that they can be re-collected by args here. 190 00:09:44,920 --> 00:09:47,530 The other option is to remove the star 191 00:09:47,530 --> 00:09:49,970 so that you are passing a tuple directly 192 00:09:49,970 --> 00:09:52,940 to the multiplier function, and then moving this star 193 00:09:52,940 --> 00:09:56,150 so that the multiply function won't try to collect 194 00:09:56,150 --> 00:09:59,520 all its positional arguments into another tuple. 195 00:09:59,520 --> 00:10:02,740 However I do like passing multiple different arguments 196 00:10:02,740 --> 00:10:04,500 so I'm going to keep this stars there, 197 00:10:04,500 --> 00:10:05,820 but that's a choice for you to make, 198 00:10:05,820 --> 00:10:09,160 you can either pass arguments, or you can pass a tuple. 199 00:10:09,160 --> 00:10:11,830 That is a very common problem and quite a difficult 200 00:10:11,830 --> 00:10:13,890 to diagnose problem as well. 201 00:10:13,890 --> 00:10:15,690 So I just wanted to go through it so that 202 00:10:15,690 --> 00:10:17,400 if you encounter that in the future, 203 00:10:17,400 --> 00:10:18,510 you'll know what's going on. 204 00:10:18,510 --> 00:10:19,880 Thanks for joining me in this video. 205 00:10:19,880 --> 00:10:22,070 Hope it's been useful and not too confusing. 206 00:10:22,070 --> 00:10:23,720 And I'll see you in the next one.