1 00:00:00,790 --> 00:00:02,200 Hi guys and welcome back! 2 00:00:02,200 --> 00:00:03,490 In this video we're going to learn 3 00:00:03,490 --> 00:00:06,990 about unpacking keyword arguments. 4 00:00:06,990 --> 00:00:08,920 Last video was a little bit confusing. 5 00:00:08,920 --> 00:00:11,700 Now it's going to get a little bit more confusing 6 00:00:11,700 --> 00:00:14,170 as we learn about keyword arguments. 7 00:00:14,170 --> 00:00:15,930 Once these two videos are over though 8 00:00:15,930 --> 00:00:19,361 you're going to have all of the tools necessary in Python 9 00:00:19,361 --> 00:00:22,640 to create any type of function, 10 00:00:22,640 --> 00:00:24,770 and use it in many different ways. 11 00:00:24,770 --> 00:00:26,860 So, up to now we've been learning about 12 00:00:26,860 --> 00:00:28,280 all these fundamental building blocks 13 00:00:28,280 --> 00:00:30,290 and this is one of the last ones. 14 00:00:30,290 --> 00:00:32,750 After this we can learn about object oriented programming 15 00:00:32,750 --> 00:00:35,290 and that's another type of beast altogether now. 16 00:00:35,290 --> 00:00:38,670 Let's start by defining a simple function 17 00:00:38,670 --> 00:00:41,900 that will use this unpacking of keyword arguments. 18 00:00:41,900 --> 00:00:45,685 So I'm gonna do star star, kwargs, now, 19 00:00:45,685 --> 00:00:50,594 args and kwargs, when you use star args and star star kwargs 20 00:00:50,594 --> 00:00:53,200 are the usual names for these things, 21 00:00:53,200 --> 00:00:54,370 but they don't have to be. 22 00:00:54,370 --> 00:00:56,690 You can use whatever variable name you want here 23 00:00:56,690 --> 00:00:59,580 and indeed with star args as well. 24 00:00:59,580 --> 00:01:01,140 We're going to just print them out, 25 00:01:01,140 --> 00:01:03,990 so I want you to take a look at what happens 26 00:01:03,990 --> 00:01:06,880 when I call my named function, and I say something like 27 00:01:06,880 --> 00:01:11,090 name equal Bob and age equal 25. 28 00:01:11,090 --> 00:01:12,800 If I run this code here, 29 00:01:12,800 --> 00:01:16,430 you can see that what we get out is a dictionary. 30 00:01:16,430 --> 00:01:19,100 So, we called our named function 31 00:01:19,100 --> 00:01:21,677 that does this dictionary unpacking 32 00:01:21,677 --> 00:01:26,150 with two keyword arguments, and what happens in Python 33 00:01:26,150 --> 00:01:29,430 is that these keyword arguments all get collected 34 00:01:29,430 --> 00:01:32,700 into this kwargs variable. 35 00:01:32,700 --> 00:01:34,280 That is what the star star does, 36 00:01:34,280 --> 00:01:37,020 it collects keyword arguments. 37 00:01:37,020 --> 00:01:39,400 So, all of these keyword arguments get collected 38 00:01:39,400 --> 00:01:42,810 and put into a dictionary where the dictionary key 39 00:01:42,810 --> 00:01:45,600 is equal ti the name of the keyword argument. 40 00:01:45,600 --> 00:01:49,110 So you end up with a name key and a Bob value 41 00:01:49,110 --> 00:01:53,743 for this one, and an age key and a 25 value for this one. 42 00:01:54,600 --> 00:01:59,290 The same way that you can collect named arguments 43 00:01:59,290 --> 00:02:01,550 into a dictionary you can go the other way round, 44 00:02:01,550 --> 00:02:04,800 you can unpack a dictionary into named arguments. 45 00:02:04,800 --> 00:02:07,710 So I'm going to make a small change to my named function. 46 00:02:07,710 --> 00:02:08,900 I'm going to pass in name 47 00:02:08,900 --> 00:02:11,600 and age and I'm going to print them out, name and age. 48 00:02:12,680 --> 00:02:15,380 And now I'm going to define a details dictionary 49 00:02:15,380 --> 00:02:20,060 which has a name of Bob, and an age of 25. 50 00:02:20,060 --> 00:02:23,570 Now I'm going to call my named function with my details. 51 00:02:23,570 --> 00:02:27,780 We know that this won't work because details is a dictionary 52 00:02:27,780 --> 00:02:30,560 and I'm passing it as a positional argument 53 00:02:30,560 --> 00:02:32,890 which means that the details dictionary 54 00:02:32,890 --> 00:02:35,600 will become a value for the name parameter. 55 00:02:35,600 --> 00:02:37,930 But the age parameter will not have a value. 56 00:02:37,930 --> 00:02:41,190 We need to pass that in separately, something like this. 57 00:02:41,190 --> 00:02:42,800 But of course that's not what we want to do. 58 00:02:42,800 --> 00:02:46,130 We don't want to pass in this whole thing as a name. 59 00:02:46,130 --> 00:02:48,000 That would be a bit weird, and if you run this 60 00:02:48,000 --> 00:02:50,710 you'll see that you get the entire dictionary printed out 61 00:02:50,710 --> 00:02:53,790 because that's the name variable, and then 25 printed out 62 00:02:53,790 --> 00:02:55,610 because that's the age variable. 63 00:02:55,610 --> 00:02:58,350 Instead what we want to do is we want to pass in 64 00:02:58,350 --> 00:03:03,350 the name key as the name parameter, and the age key 65 00:03:03,820 --> 00:03:06,010 as the age parameter, and we saw how to do this 66 00:03:06,010 --> 00:03:09,930 in the last video, just by unpacking the dictionary 67 00:03:09,930 --> 00:03:14,030 into two named arguments like that. 68 00:03:14,030 --> 00:03:18,450 When you do that, it treats the key as the name 69 00:03:18,450 --> 00:03:21,870 for the argument, and age as the key for the argument 70 00:03:21,870 --> 00:03:24,440 and assigns to them the value associated with those. 71 00:03:24,440 --> 00:03:28,620 So you do end up now with Bob and 25. 72 00:03:28,620 --> 00:03:31,740 Note that if you had star star kwargs 73 00:03:31,740 --> 00:03:35,033 which is the collection of multiple named arguments, 74 00:03:36,060 --> 00:03:38,820 you can print them out and you can still do this. 75 00:03:38,820 --> 00:03:41,330 What'll happen now is instead of doing 76 00:03:41,330 --> 00:03:45,960 name equal Bob and age equal 25, 77 00:03:45,960 --> 00:03:47,240 you're doing exactly the same thing 78 00:03:47,240 --> 00:03:50,550 but using this dictionary to pass in multiple 79 00:03:50,550 --> 00:03:53,510 keyword arguments and then they are being collected 80 00:03:53,510 --> 00:03:54,940 into this dictionary up here, 81 00:03:54,940 --> 00:03:56,710 and then you're gonna print a dictionary out. 82 00:03:56,710 --> 00:03:59,200 So that is how these two stars work. 83 00:03:59,200 --> 00:04:02,120 They can be used in a function to collect named arguments 84 00:04:02,120 --> 00:04:05,320 into a dictionary, or they can be used in a function call 85 00:04:05,320 --> 00:04:08,743 to unpack a dictionary into keyword arguments. 86 00:04:10,240 --> 00:04:13,140 Let's say we have another function called print nicely 87 00:04:13,140 --> 00:04:17,720 that is going to print out his dictionary into a nice format 88 00:04:17,720 --> 00:04:22,240 so, let's call the named function first 89 00:04:22,240 --> 00:04:23,530 to print the arguments out, 90 00:04:23,530 --> 00:04:25,830 and then we are going to print them nicely. 91 00:04:25,830 --> 00:04:30,610 So we're gonna do for arg, and value, in kwargs dot items. 92 00:04:30,610 --> 00:04:34,760 Then we're gonna print arg and the value. 93 00:04:34,760 --> 00:04:37,270 Notice that kwargs is a dictionary 94 00:04:37,270 --> 00:04:39,850 so you can use the items method on it 95 00:04:39,850 --> 00:04:42,660 so that you can iterate over two of these things. 96 00:04:42,660 --> 00:04:45,200 Notice that here we're using a whole bunch of things 97 00:04:45,200 --> 00:04:46,410 that we've learned so far, 98 00:04:46,410 --> 00:04:49,660 including normal data types like dictionaries. 99 00:04:49,660 --> 00:04:53,323 We are calling functions, we are de-structuring the tuple 100 00:04:53,323 --> 00:04:55,510 that that returns into two values. 101 00:04:55,510 --> 00:04:59,490 We are even unpacking and repacking named arguments. 102 00:04:59,490 --> 00:05:01,030 So there's a lot of stuff going on here 103 00:05:01,030 --> 00:05:03,390 and I know it's a little bit confusing. 104 00:05:03,390 --> 00:05:05,440 Now with this function we can do print nicely 105 00:05:05,440 --> 00:05:09,040 and we can pass name is Bob and age is 25 106 00:05:09,040 --> 00:05:12,360 and what's gonna happen is we're going to collect 107 00:05:12,360 --> 00:05:15,920 these named arguments into this dictionary. 108 00:05:15,920 --> 00:05:18,290 Then we're gonna call the named function, 109 00:05:18,290 --> 00:05:20,290 but we're going to unpack that dictionary 110 00:05:20,290 --> 00:05:22,040 into named arguments so we're gonna pass in 111 00:05:22,040 --> 00:05:24,440 name equal Bob and age equal 25. 112 00:05:24,440 --> 00:05:28,020 Those are then gonna be collected into this dictionary, 113 00:05:28,020 --> 00:05:29,080 and they're gonna be printed out, 114 00:05:29,080 --> 00:05:30,747 so we're gonna print a dictionary. 115 00:05:30,747 --> 00:05:32,680 And then we're gonna come back out here 116 00:05:32,680 --> 00:05:34,630 and we're going to iterate over the items, 117 00:05:34,630 --> 00:05:38,060 getting arg and value for each item in the dictionary 118 00:05:38,060 --> 00:05:41,650 which is gonna give us name and Bob and then age and 25. 119 00:05:41,650 --> 00:05:43,360 And finally we're going to print those out 120 00:05:43,360 --> 00:05:44,650 in a bit of a nicer way. 121 00:05:44,650 --> 00:05:46,170 So we're gonna print the dictionary first, 122 00:05:46,170 --> 00:05:47,623 and then two more lines. 123 00:05:48,500 --> 00:05:50,713 And that is indeed what we get back. 124 00:05:53,070 --> 00:05:54,700 You can use both actually, 125 00:05:54,700 --> 00:05:58,740 star args and star star kwargs if you want. 126 00:05:58,740 --> 00:06:00,920 And nothing changes, you're gonna collect 127 00:06:00,920 --> 00:06:03,340 all the positional arguments into args 128 00:06:03,340 --> 00:06:06,100 and all the named arguments into kwargs. 129 00:06:06,100 --> 00:06:08,120 So you can do something like both. 130 00:06:08,120 --> 00:06:12,330 One, three, five, name equal Bob, and age equal 25. 131 00:06:12,330 --> 00:06:14,740 One, three and five will be collected into args 132 00:06:14,740 --> 00:06:16,560 because they are positional arguments. 133 00:06:16,560 --> 00:06:18,330 Name equal Bob and age equal 25 134 00:06:18,330 --> 00:06:20,480 will be collected into kwargs. 135 00:06:20,480 --> 00:06:22,403 So if we run that, you can see that you get 136 00:06:22,403 --> 00:06:26,073 a tuple of arguments, and then a dictionary of arguments. 137 00:06:27,250 --> 00:06:30,350 This syntax here, args and kwargs with the stars 138 00:06:30,350 --> 00:06:35,350 is normally used to accept an unlimited number of arguments. 139 00:06:35,960 --> 00:06:38,080 So you can pass in anything you want to this function, 140 00:06:38,080 --> 00:06:40,860 any positional arguments and any named arguments 141 00:06:40,860 --> 00:06:44,740 and it will accept them, and that is normally used 142 00:06:44,740 --> 00:06:47,300 so that some of those arguments or all of those arguments 143 00:06:47,300 --> 00:06:49,780 can be passed onto another function. 144 00:06:49,780 --> 00:06:54,610 Here's an example of something that is done quite frequently 145 00:06:54,610 --> 00:06:56,340 and we haven't written this code, 146 00:06:56,340 --> 00:06:58,860 don't worry about how it's implemented. 147 00:06:58,860 --> 00:07:02,910 But you have a post function that takes in 148 00:07:02,910 --> 00:07:06,830 a URL, some data, some variable called json, 149 00:07:06,830 --> 00:07:10,040 and then any number of keyword arguments. 150 00:07:10,040 --> 00:07:13,420 And it returns just calling another function. 151 00:07:13,420 --> 00:07:17,200 And it passes the URL, the data, the json, 152 00:07:17,200 --> 00:07:19,350 and all of the keyword arguments. 153 00:07:19,350 --> 00:07:22,150 So it essentially seems to do the same thing, 154 00:07:22,150 --> 00:07:25,920 but it also adds an extra argument. 155 00:07:25,920 --> 00:07:28,830 So this is very common pattern to do in Python. 156 00:07:28,830 --> 00:07:32,480 To have a function that seemingly just calls another 157 00:07:32,480 --> 00:07:34,910 but adds something more to it. 158 00:07:34,910 --> 00:07:38,170 And that is used to simplify calling this initial function 159 00:07:38,170 --> 00:07:41,890 so that you don't have to pass in this argument as well. 160 00:07:41,890 --> 00:07:44,850 So just an example of the kwargs being used 161 00:07:44,850 --> 00:07:48,380 to essentially pass arguments from one function to another 162 00:07:48,380 --> 00:07:51,653 until they get used, so that's very common thing to do. 163 00:07:52,850 --> 00:07:55,010 A final example, if you define a function 164 00:07:55,010 --> 00:08:00,010 that used this unpacking, you need to call it with a mapping 165 00:08:00,480 --> 00:08:03,120 if you're going to pass in this star star. 166 00:08:03,120 --> 00:08:05,860 So if you do star star Bob, you're gonna get an error 167 00:08:05,860 --> 00:08:07,230 because this is not a dictionary. 168 00:08:07,230 --> 00:08:09,940 Similarly with star star none, you're going to get an error 169 00:08:09,940 --> 00:08:11,590 because that's not a dictionary. 170 00:08:11,590 --> 00:08:14,080 So if you're working with variables in your code 171 00:08:14,080 --> 00:08:16,580 and then eventually you're passing them unpacked 172 00:08:16,580 --> 00:08:18,990 to a function then you must make sure 173 00:08:18,990 --> 00:08:20,280 that they are dictionaries. 174 00:08:20,280 --> 00:08:22,680 If they are anything else then you're gonna get an error 175 00:08:22,680 --> 00:08:25,130 when this line runs, som that's something that you'll see 176 00:08:25,130 --> 00:08:27,040 quite frequently in Python code. 177 00:08:27,040 --> 00:08:30,140 The variable has an unexpected value or something like that 178 00:08:30,140 --> 00:08:32,370 and students sometimes struggle to determine 179 00:08:32,370 --> 00:08:34,260 why you get an error. 180 00:08:34,260 --> 00:08:36,660 That's because sometimes the variables that we use 181 00:08:36,660 --> 00:08:39,260 may not be what we expect. 182 00:08:39,260 --> 00:08:41,800 That's it for this video, thanks for joining me in this one. 183 00:08:41,800 --> 00:08:44,700 I hope we've enjoyed it, and I'll see you in the next one.