1 00:00:05,180 --> 00:00:06,000 Before we get back to 2 00:00:06,000 --> 00:00:08,380 subclassing the tkintern listbox, 3 00:00:08,380 --> 00:00:10,590 there's another way of specifying parameters, 4 00:00:10,590 --> 00:00:11,970 that we need to look at 5 00:00:11,970 --> 00:00:15,870 and that's the **kwargs or kw args, and in fact 6 00:00:15,870 --> 00:00:18,620 this is the one that we need to use in our subclass. 7 00:00:18,620 --> 00:00:22,020 Now, it works in the same way as packing a variable number 8 00:00:22,020 --> 00:00:24,320 of elements of arguments into a tuple, 9 00:00:24,320 --> 00:00:26,730 but this time we need to pack a variable number 10 00:00:26,730 --> 00:00:29,390 of named arguments, or keyword arguments, 11 00:00:29,390 --> 00:00:32,200 hence kw args or kwargs. 12 00:00:32,200 --> 00:00:34,960 Now, we've used functions, that have keyword arguments 13 00:00:34,960 --> 00:00:38,890 before and the one we've used most is the print function. 14 00:00:38,890 --> 00:00:41,480 So, I'm gonna clear out all this code 15 00:00:41,480 --> 00:00:44,880 in our **kwargs file and just go back 16 00:00:44,880 --> 00:00:47,630 to a simple print of hello world. 17 00:00:51,140 --> 00:00:53,970 Now, if we Control click on the print function 18 00:00:53,970 --> 00:00:56,700 or command, if you're on a Mac, 19 00:00:56,700 --> 00:00:58,410 you can see there that it specifies 20 00:00:58,410 --> 00:01:01,040 three named, or keyword parameters. 21 00:01:01,040 --> 00:01:04,560 Now this looks slightly different if you're using Python 2, 22 00:01:04,560 --> 00:01:07,590 and the future version of print, so if that's the case, 23 00:01:07,590 --> 00:01:09,970 just follow me in the video, because everything works 24 00:01:09,970 --> 00:01:12,120 the same, it's just your version of print 25 00:01:12,120 --> 00:01:13,890 that's a little bit different. 26 00:01:13,890 --> 00:01:16,550 So we've used this end parameter in the past 27 00:01:16,550 --> 00:01:19,400 to suppress new lines, when we want several things 28 00:01:19,400 --> 00:01:21,630 to print on the same line. 29 00:01:21,630 --> 00:01:23,860 We've also used the file parameter as well. 30 00:01:23,860 --> 00:01:26,290 Over here we've got it currently set to file equals none. 31 00:01:26,290 --> 00:01:29,140 It's again line number 476. 32 00:01:29,140 --> 00:01:31,920 We use that when we write text data to a file on disc. 33 00:01:31,920 --> 00:01:34,540 So, these are keyword arguments 34 00:01:34,540 --> 00:01:37,270 and we wrote our own methods to use them, if you recall 35 00:01:37,270 --> 00:01:42,160 in the object oriented programming section of the course. 36 00:01:42,160 --> 00:01:44,760 Up until now, we've dealt with keyword arguments, 37 00:01:44,760 --> 00:01:46,750 that have default values, 38 00:01:46,750 --> 00:01:49,450 so if we don't provide a sep argument, for example, 39 00:01:49,450 --> 00:01:51,920 it defaults to a space 40 00:01:51,920 --> 00:01:54,910 and similarly, the end argument, you can see here, 41 00:01:54,910 --> 00:01:56,790 it defaults to a new line character 42 00:01:56,790 --> 00:02:00,390 and the file argument, you can see here, defaults to none. 43 00:02:00,390 --> 00:02:04,270 But what happens if we don't wanna provide defaults though? 44 00:02:04,270 --> 00:02:07,350 Now, that might seem like a daft question. 45 00:02:07,350 --> 00:02:09,930 If these arguments are used, then surely they must have 46 00:02:09,930 --> 00:02:12,500 some value, even if that value is none. 47 00:02:12,500 --> 00:02:15,590 Well, the thing is we may not know what those defaults are 48 00:02:15,590 --> 00:02:17,470 and perhaps more importantly, 49 00:02:17,470 --> 00:02:20,760 we often don't want to care what they are. 50 00:02:20,760 --> 00:02:22,940 So let's actually see an example to understand 51 00:02:22,940 --> 00:02:27,130 why we don't necessarily want to specify default values. 52 00:02:27,130 --> 00:02:30,420 So to do that, I'm going to just close the builtins.py, 53 00:02:30,420 --> 00:02:32,190 go back to our **dorarg 54 00:02:32,190 --> 00:02:35,820 and I'm going to use an exercise from a previous video, 55 00:02:35,820 --> 00:02:38,550 one that prints the words backwards. 56 00:02:38,550 --> 00:02:40,920 So I'm going to just type it in quickly, 57 00:02:40,920 --> 00:02:42,750 so def print_backwards 58 00:02:45,470 --> 00:02:46,390 and **args, 59 00:02:49,690 --> 00:02:51,110 for word in args, 60 00:02:53,790 --> 00:02:56,790 like so, ::-1 in square brackets, :, 61 00:02:57,760 --> 00:03:00,260 then we're gonna do print word 62 00:03:01,970 --> 00:03:04,350 and in square brackets again, 63 00:03:04,350 --> 00:03:05,180 ::-1 64 00:03:07,130 --> 00:03:08,620 comma end= 65 00:03:08,620 --> 00:03:11,150 and in double quotes, sorry, in single quotes, 66 00:03:11,150 --> 00:03:14,100 a space up to the end=, and that's it for our method 67 00:03:14,100 --> 00:03:15,130 and in terms of calling it, 68 00:03:15,130 --> 00:03:17,480 let's just write print_backwards, 69 00:03:17,480 --> 00:03:20,180 I'm gonna pass the parameters 70 00:03:20,180 --> 00:03:21,360 hello 71 00:03:21,360 --> 00:03:22,190 planet 72 00:03:23,080 --> 00:03:23,910 earth, 73 00:03:26,100 --> 00:03:27,620 take 74 00:03:27,620 --> 00:03:29,020 me 75 00:03:29,020 --> 00:03:30,380 to 76 00:03:30,380 --> 00:03:31,210 your 77 00:03:32,410 --> 00:03:33,240 leader. 78 00:03:35,010 --> 00:03:37,910 Alright, so this is just basically a modified version 79 00:03:37,910 --> 00:03:40,770 of the print function, so I think, 80 00:03:40,770 --> 00:03:42,170 well, actually we'll just run this first 81 00:03:42,170 --> 00:03:44,280 to make sure that it in fact does work 82 00:03:44,280 --> 00:03:46,860 and you can see that we've got it working quite nicely. 83 00:03:46,860 --> 00:03:49,520 Hello planet earth, take me to your leader, 84 00:03:49,520 --> 00:03:51,450 you can see that's all showing backwards, 85 00:03:51,450 --> 00:03:53,540 which is what we tried to do. 86 00:03:53,540 --> 00:03:55,670 So what we've done here is basically 87 00:03:55,670 --> 00:03:57,910 a modified version of the print function. 88 00:03:57,910 --> 00:04:00,160 So that said, I think it would be nice 89 00:04:00,160 --> 00:04:02,870 if it behaved the same as the real print function. 90 00:04:02,870 --> 00:04:05,450 For example, we should print it to a file, 91 00:04:05,450 --> 00:04:07,590 if that's what the user wants to do with it. 92 00:04:07,590 --> 00:04:10,890 So that's actually easy, we just add a named parameter, 93 00:04:10,890 --> 00:04:13,720 then pass that on to the builtin function. 94 00:04:13,720 --> 00:04:16,300 So to do that, we come up here after the args. 95 00:04:16,300 --> 00:04:20,459 We've defined the *args, we'll put file=None, 96 00:04:21,630 --> 00:04:24,850 then for the print line, after the end=, 97 00:04:24,850 --> 00:04:29,260 two single quotes and a space, we put file=file. 98 00:04:29,260 --> 00:04:30,890 So firstly, we can try to run an as is 99 00:04:30,890 --> 00:04:32,360 to make sure it still works. 100 00:04:32,360 --> 00:04:34,600 No change to functionality there, 101 00:04:34,600 --> 00:04:36,520 but now that we've introduced file, 102 00:04:36,520 --> 00:04:38,660 we can actually print to a file if we want to. 103 00:04:38,660 --> 00:04:42,130 So to do that, we can then change the code here 104 00:04:42,130 --> 00:04:44,710 and put with open backwards.txt 105 00:04:48,980 --> 00:04:52,480 and mode w in single quotes as backwards:, 106 00:04:54,300 --> 00:04:58,150 and then on the end of the line there 107 00:04:58,150 --> 00:05:01,810 we can put comma file=backwards. 108 00:05:01,810 --> 00:05:05,870 So now if we do that, and we need to indent that 109 00:05:05,870 --> 00:05:07,650 and if we run that, we should find 110 00:05:07,650 --> 00:05:09,750 backwards.txt gets created. 111 00:05:09,750 --> 00:05:11,240 You can see, it appeared up here 112 00:05:11,240 --> 00:05:13,300 and if we just double-click that, we're now seeing 113 00:05:13,300 --> 00:05:16,300 hello planet earth take me to your leader showing. 114 00:05:16,300 --> 00:05:18,260 Alright, so that's working well. 115 00:05:18,260 --> 00:05:20,060 I had an extra space there, by the way, 116 00:05:20,060 --> 00:05:21,370 I'll get rid of that in case you were wondering 117 00:05:21,370 --> 00:05:23,720 why there was an extra space there. 118 00:05:23,720 --> 00:05:25,830 That was easy, but the print function 119 00:05:25,830 --> 00:05:28,430 only defines three keyword parameters, 120 00:05:28,430 --> 00:05:30,210 so we'd only have to code it 121 00:05:30,210 --> 00:05:32,170 for three in our function definition. 122 00:05:32,170 --> 00:05:33,890 In fact, we'd only have to code it for two, 123 00:05:33,890 --> 00:05:36,670 because we force the end to be a space, 124 00:05:36,670 --> 00:05:39,680 so that all the words printed are printed on the same line, 125 00:05:39,680 --> 00:05:42,370 so that's our definition there, you can see end calling, 126 00:05:42,370 --> 00:05:44,810 define the end too up into space. 127 00:05:44,810 --> 00:05:47,100 Now in the case of the listbox class, for example, 128 00:05:47,100 --> 00:05:49,000 there are a lot of named arguments, 129 00:05:49,000 --> 00:05:51,960 that we can provide when creating a new listbox, 130 00:05:51,960 --> 00:05:55,210 things like background, font, the highlight colour, 131 00:05:55,210 --> 00:05:57,010 and the highlight thickness. 132 00:05:57,010 --> 00:05:59,220 There are about 22 different parameters, 133 00:05:59,220 --> 00:06:01,590 that we'd actually have to match in our nip method, 134 00:06:01,590 --> 00:06:03,980 if we wanna provide all the same options. 135 00:06:03,980 --> 00:06:05,260 Now that's bad enough, but what if say, 136 00:06:05,260 --> 00:06:07,930 the next version of the listbox has a new option? 137 00:06:07,930 --> 00:06:09,800 Our class would know nothing about it, 138 00:06:09,800 --> 00:06:12,070 so users of our subclass then wouldn't be able 139 00:06:12,070 --> 00:06:14,770 to take advantage of these latest features. 140 00:06:14,770 --> 00:06:17,100 Now fortunately, there's an easier way 141 00:06:17,100 --> 00:06:21,900 and this is where the **kw args or kwargs actually comes in. 142 00:06:21,900 --> 00:06:23,820 Now we've seen that the asterix can be used 143 00:06:23,820 --> 00:06:25,650 to unpack a tuple or a list, 144 00:06:25,650 --> 00:06:28,890 but the double asterix operator, **, 145 00:06:28,890 --> 00:06:31,450 unpacks a dictionary in the same way. 146 00:06:31,450 --> 00:06:34,410 Now a dictionary is used because keyword arguments 147 00:06:34,410 --> 00:06:37,560 are specified as a keyword and a value 148 00:06:37,560 --> 00:06:40,700 and that's exactly what you have in the dictionary, 149 00:06:40,700 --> 00:06:42,260 a key and a value. 150 00:06:42,260 --> 00:06:43,770 So what all this means is that we can scoop up 151 00:06:43,770 --> 00:06:46,060 all the keyword arguments and pass the whole lot 152 00:06:46,060 --> 00:06:48,570 on to the print function. 153 00:06:48,570 --> 00:06:50,240 So let's have a go at doing that. 154 00:06:50,240 --> 00:06:53,130 We're gonna come back here and remove the file=None, 155 00:06:53,130 --> 00:06:57,300 and instead what we're gonna do is put **kwargs or kw args, 156 00:06:58,740 --> 00:07:02,170 like so, and then we come down here 157 00:07:02,170 --> 00:07:05,150 and after the end, instead of file=file, 158 00:07:05,150 --> 00:07:06,570 we replace that to, or with rather, 159 00:07:06,570 --> 00:07:08,740 **kwargs or kw args again, 160 00:07:10,260 --> 00:07:12,580 and just to be sure, 161 00:07:12,580 --> 00:07:14,670 we'll just delete the backwards.txt file 162 00:07:14,670 --> 00:07:18,830 to confirm it is being created with this new code. 163 00:07:19,870 --> 00:07:24,330 So if you run this, we can see that it still worked 164 00:07:24,330 --> 00:07:26,200 and backwards.txt got created, and in fact 165 00:07:26,200 --> 00:07:28,420 there's no space there like there was before, 166 00:07:28,420 --> 00:07:31,410 so it confirmation that has worked nicely. 167 00:07:31,410 --> 00:07:34,430 So here what we've done is on line 1, 168 00:07:34,430 --> 00:07:38,430 is we've defined our function to accept **kwargs 169 00:07:38,430 --> 00:07:40,100 and whatever appears in there 170 00:07:40,100 --> 00:07:42,320 is just passed onto the print function 171 00:07:42,320 --> 00:07:44,340 and you can see that happening on line 3. 172 00:07:44,340 --> 00:07:45,990 So that is pretty cool. 173 00:07:45,990 --> 00:07:48,210 The calling code is free to add as many 174 00:07:48,210 --> 00:07:50,880 keyword arguments as print will accept 175 00:07:50,880 --> 00:07:52,760 and we don't have to worry about what they are, 176 00:07:52,760 --> 00:07:55,800 nor do we need to worry what their default values are. 177 00:07:55,800 --> 00:07:58,830 Now once again, we could call a parameter anything, 178 00:07:58,830 --> 00:08:01,290 but the convention here is to use 179 00:08:01,290 --> 00:08:03,920 kw args or kwargs as the name. 180 00:08:03,920 --> 00:08:06,720 Now let's check what's in kwargs, 181 00:08:06,720 --> 00:08:09,370 by printing it out before you do any processing. 182 00:08:09,370 --> 00:08:11,710 So we can add a line two to do that 183 00:08:11,710 --> 00:08:14,460 then we'll just do a print kwargs 184 00:08:15,800 --> 00:08:18,490 and if we run this, you can see there 185 00:08:18,490 --> 00:08:20,390 that kwarg is just a dictionary, 186 00:08:20,390 --> 00:08:22,170 it's got an entry with the key file 187 00:08:22,170 --> 00:08:25,290 and the value is our file object. 188 00:08:25,290 --> 00:08:27,390 We can see the name backwards, like txt there 189 00:08:27,390 --> 00:08:30,330 and mode equals w in the printed representation 190 00:08:30,330 --> 00:08:32,490 of the file down here in the Run window. 191 00:08:32,490 --> 00:08:34,640 Now if the print function changes in the future, 192 00:08:34,640 --> 00:08:36,789 to have additional keyword arguments 193 00:08:36,789 --> 00:08:39,299 our code will still work fine. 194 00:08:39,299 --> 00:08:40,760 The user can specify them 195 00:08:40,760 --> 00:08:43,070 when calling our print_backwards function, 196 00:08:43,070 --> 00:08:46,000 and we just pass them on to the builtin print. 197 00:08:46,000 --> 00:08:48,670 That means that we can handle functions in classes, 198 00:08:48,670 --> 00:08:50,160 that take keyword arguments, 199 00:08:50,160 --> 00:08:52,000 that we don't have to know or care about 200 00:08:52,000 --> 00:08:54,800 and that obviously makes life a lot easier. 201 00:08:54,800 --> 00:08:57,010 However with that said, there's a slight problem 202 00:08:57,010 --> 00:08:58,830 with this code and it's easy to fix 203 00:08:58,830 --> 00:09:01,130 and that's gonna be your next challenge. 204 00:09:01,130 --> 00:09:03,310 But I'll end the video here. 205 00:09:03,310 --> 00:09:04,670 In the next video, we'll have a look 206 00:09:04,670 --> 00:09:06,140 and see what the problem is first 207 00:09:06,140 --> 00:09:07,490 and then we'll get you to look at it, 208 00:09:07,490 --> 00:09:09,450 by way of a challenge to solve that problem. 209 00:09:09,450 --> 00:09:12,280 So I'll see you in the next video.