1 00:00:05,000 --> 00:00:09,950 so continuing on with classes so what we are going to do is use an example to work with a record 2 00:00:09,950 --> 00:00:15,170 collection to show how classes can be used together and I'm also gonna take this 3 00:00:15,170 --> 00:00:19,310 opportunity to talk about doc strings what they are and what information 4 00:00:19,310 --> 00:00:23,620 should be included in them so lets go ahead and create a new class for this 5 00:00:23,620 --> 00:00:29,080 so I'm gonna come over as I always done new Python file and will 6 00:00:29,080 --> 00:00:38,840 call this one song and we'll start by writing the class so.... 7 00:00:38,840 --> 00:00:40,560 ... 8 00:00:40,560 --> 00:00:44,660 .... 9 00:00:46,700 --> 00:00:52,820 .... 10 00:00:57,580 --> 00:01:01,000 .... 11 00:01:03,960 --> 00:01:23,280 .... 12 00:01:23,280 --> 00:01:38,900 ...so that is our doc string 13 00:01:38,900 --> 00:01:44,300 ...so lets create our __init__ which I said wasn't going to 14 00:01:44,300 --> 00:01:52,070 say the underscore underscore again so.... 15 00:01:52,070 --> 00:01:56,480 .... 16 00:01:56,480 --> 00:02:03,650 ...and lets write the doc string for the init method.... 17 00:02:06,510 --> 00:03:02,600 .... 18 00:03:02,600 --> 00:03:11,900 ....then we need to initialise our data attributes so... 19 00:03:11,900 --> 00:03:22,700 .... 20 00:03:22,700 --> 00:03:29,350 ...so their really shouldn't be much to say at this point about this class and that's 21 00:03:29,350 --> 00:03:33,790 because the doc string should say it all and in fact the doc strings in this class are 22 00:03:33,790 --> 00:03:37,010 probably a little bit to verbose but I just wanted to show you that anyway 23 00:03:37,850 --> 00:03:50,430 ....so their really 24 00:03:50,430 --> 00:03:54,010 shouldn't be too much to say about this class and that's because the doc string 25 00:03:54,010 --> 00:03:57,440 should really say it at all and in fact what I've done I probably have gone overboard 26 00:03:57,440 --> 00:04:02,240 here and made the doc strings a little bit too verbose here so doc strings 27 00:04:02,240 --> 00:04:05,850 themselves can be used to document modules functions classes and methods 28 00:04:05,850 --> 00:04:09,600 and really should provide information that is useful to understanding what the 29 00:04:09,600 --> 00:04:15,340 object does and also how to use it now you should also include other details that 30 00:04:15,340 --> 00:04:20,190 aren't obvious such as what will happen if a value wasn't specified now writing good 31 00:04:20,190 --> 00:04:24,650 documentation is a skill in itself and it can be very difficult to document your own code 32 00:04:24,650 --> 00:04:28,930 and because you know how it works and fits together well you did create it after 33 00:04:28,930 --> 00:04:33,360 all so large companies just out of interest will often employ technical writers to produce 34 00:04:33,360 --> 00:04:38,210 the documentation so someone else who creates a documentation who didn't create the 35 00:04:38,210 --> 00:04:42,140 original program so a description of doc stings and guidelines for writing them 36 00:04:42,140 --> 00:04:45,140 is in pep 257 so if we look at that 37 00:04:46,160 --> 00:04:54,400 paste it on the screen the links are in the Resources section so the pep documents and semantics and 38 00:04:54,400 --> 00:04:58,730 conventions associated with python doc strings well thats actually fairly short 39 00:04:58,730 --> 00:05:02,050 documents as you can see it doesn't really go into a lot of details certainly compared to some 40 00:05:02,050 --> 00:05:06,220 other peps there but I definitely recommend that you read it to specially 41 00:05:06,220 --> 00:05:09,090 as you can skip the final section on handling 42 00:05:09,090 --> 00:05:15,750 doc string indentation that's this bit here so you don't need to worry about that too much getting 43 00:05:15,750 --> 00:05:20,600 back to our code so a doc string should always use triple quotes and that keeps 44 00:05:20,600 --> 00:05:25,060 your doc strings consistent with everyone else's and also means you can include quotes in the 45 00:05:25,060 --> 00:05:27,830 documentation as well now if 46 00:05:27,830 --> 00:05:33,000 you intend to use escape characters you know such as /t or /n in your doc 47 00:05:33,000 --> 00:05:39,230 string then use r with 3 quotes to make it a raw string and lets just see what I mean so I'm going to create a new 48 00:05:39,230 --> 00:05:53,140 class new file and call this one demo I just wanna show you how this works so.... 49 00:05:53,140 --> 00:06:11,060 .... 50 00:06:11,060 --> 00:06:16,910 ...and will print that out to see if that it's working so if we run that and run demo 51 00:06:16,910 --> 00:06:22,710 string split and tab you can see its gone to the next line as well but I just want to show you how to create this raw string 52 00:06:22,710 --> 00:06:31,990 that I just talk about so... 53 00:06:31,990 --> 00:06:36,540 you notice how IntelliJ automatically added the second one and if I take the contents down 54 00:06:36,540 --> 00:06:41,280 here in the first string so I'm actually copying and pasting that I'm gonna paste that 55 00:06:41,280 --> 00:06:54,090 into their and if we print the raw string and run this is just to see the difference so as 56 00:06:54,090 --> 00:06:57,510 you can see by pasting that in there like that what I've done is we've got 57 00:06:57,510 --> 00:07:06,140 Python to not treat it with those escape characters so the /n on line 1and the /t's we're 58 00:07:06,140 --> 00:07:10,340 treated correctly by Python and treated as the carriage return and also 59 00:07:10,340 --> 00:07:14,230 the tab characters but when we put this in this raw string the /n was 60 00:07:14,230 --> 00:07:19,720 ignored and just was treated as a regular character so raw string saw are useful when you don't want Python to treat 61 00:07:19,720 --> 00:07:24,620 the backslash escaped character in a special way so when the string literal 62 00:07:24,620 --> 00:07:29,100 is defined without the r prefix as you saw these /n causes a 63 00:07:29,100 --> 00:07:34,030 line break and /t causes tabs to be included in the string and that's the output at the bottom left of 64 00:07:34,030 --> 00:07:37,880 the screen so I really just provided this as a convenience other ways you have to enter 65 00:07:37,880 --> 00:07:41,820 the ASCII codes for those characters which not only make the code messy but 66 00:07:41,820 --> 00:07:45,320 they cause problems on different operating systems so if Python didn't 67 00:07:45,320 --> 00:07:49,030 provide a mechanism for escaping certain characters we have to probably create 68 00:07:49,030 --> 00:07:53,820 strings like this is so lets just write some more code so... 69 00:07:54,380 --> 00:08:04,690 .... 70 00:08:05,210 --> 00:08:18,050 .... 71 00:08:18,050 --> 00:08:27,780 ..and we got the same response same output as 72 00:08:27,780 --> 00:08:34,240 we got for a sting when we printed that out and of course that's sort of shows you that by 73 00:08:34,240 --> 00:08:39,000 Python letting us put a /n and a /t in that format it saves us from 74 00:08:39,000 --> 00:08:42,960 having to write a string in this format having to add this characters 75 00:08:42,960 --> 00:08:47,520 via ASCII codes to create a string that way so in other words it's really adding 76 00:08:47,520 --> 00:08:51,030 that as a inconvenience for us a shortcut to put those escape characters and if you 77 00:08:51,030 --> 00:08:54,830 wanted to and again using a raw string with the r in front of it enables us to 78 00:08:54,830 --> 00:08:59,580 effectively ignore those escape characters but it gets worse with 79 00:08:59,580 --> 00:09:03,640 these characters because not only was it annoying to type because obviously a lot 80 00:09:03,640 --> 00:09:07,850 more code than in the first example on line 1 but it's also wrong on 81 00:09:07,850 --> 00:09:12,720 windows and that is because Linux and Mac use a line feed character which is the ASCII 82 00:09:12,720 --> 00:09:17,870 code for ten as you can see on line 7 character 10 to start a new line but 83 00:09:17,870 --> 00:09:22,420 Windows users uses 2 characters a carriage return followed by line feed so 84 00:09:22,420 --> 00:09:26,750 to do the equivalent in Windows its character 13 + character 10 or ASCII 85 00:09:26,750 --> 00:09:32,300 symbol 13 and ASCII 10 now reasons why Windows does this are historical and really 86 00:09:32,300 --> 00:09:35,680 dates back to the time before screens when computers were program using 87 00:09:35,680 --> 00:09:40,540 teletype terminals so it was necessary to return the carriage to the 88 00:09:40,540 --> 00:09:45,200 left hand side and in feed down one line and that explains 89 00:09:45,200 --> 00:09:50,240 Windows Control carriage return line feed combination that's the 90 00:09:50,240 --> 00:09:55,950 character 13 and character 10 now UNIX drivers started automatically using the carriage 91 00:09:55,950 --> 00:09:59,790 returns so it wasn't necessary to have it in the file now when you use a backslash 92 00:09:59,790 --> 00:10:04,300 n Pyton takes care of this for you and the same code will work on all three 93 00:10:04,300 --> 00:10:07,820 operating systems in other words it knows by you just putting a /n 94 00:10:07,820 --> 00:10:14,240 then it needs to essentially put a character 13 and a character 10 and therefore on 95 00:10:14,240 --> 00:10:15,110 Windows 96 00:10:15,110 --> 00:10:21,130 and to leave it just a character 10 for Mac and Linux enough of the history well 97 00:10:21,130 --> 00:10:25,870 almost that last bit of history may be an interesting but probably of very 98 00:10:25,870 --> 00:10:29,300 little use but there's a bit more history that is important I'm getting to 99 00:10:29,300 --> 00:10:34,920 that now these raw strings literals that we've used on line 4 their introduced 100 00:10:34,920 --> 00:10:39,500 in Python 2 mainly for using regular expressions now we will be covering regular 101 00:10:39,500 --> 00:10:43,670 expressions later in the course for now all we really need to know about regular 102 00:10:43,670 --> 00:10:48,020 expressions is that the strings that contain a lot of back / characters now 103 00:10:48,020 --> 00:10:52,530 because the backslashes used to escape the next character in a string if you 104 00:10:52,530 --> 00:10:56,210 actually want a backslash then you have to double it up and I'll show you what I mean so 105 00:10:56,780 --> 00:11:04,900 lets continue on here so we're going to type.... 106 00:11:04,900 --> 00:11:18,800 ....and see what happens 107 00:11:20,710 --> 00:11:24,300 notice as what happened their so this is a backslash followed by some texts that's 108 00:11:24,300 --> 00:11:29,580 because the way that we've type that in the / f has been treaded or the f that is after 109 00:11:29,580 --> 00:11:34,250 the / has been treated as a control character and we can't see that on 110 00:11:34,250 --> 00:11:37,200 the screen but the bottom line is the f hasn't been treated as a regular f 111 00:11:37,200 --> 00:11:40,720 in that case so if we want to use a backslash character would have to do 112 00:11:40,720 --> 00:11:43,290 something like this so.... 113 00:11:43,290 --> 00:11:47,260 .... 114 00:11:49,570 --> 00:12:00,090 ....and if we run that 115 00:12:00,090 --> 00:12:03,940 you could see we got it now appearing so / is now being shown because we've 116 00:12:03,940 --> 00:12:07,910 used to backslashes and the f is now showing and incidentally in the previous 117 00:12:07,910 --> 00:12:12,960 example a strange character may appear on Linux and Windows it 118 00:12:12,960 --> 00:12:17,560 doesn't appear as you can see under Mac right here theirs probably a strange 119 00:12:17,560 --> 00:12:21,040 characters so if your seeing that looks a little bit like a or just a 120 00:12:21,040 --> 00:12:26,690 strange character the reason for that is Python interpreting /f as an ASCII 6 121 00:12:26,690 --> 00:12:31,400 which is an echo used by device to acknowledge receiving of a transmission in 122 00:12:31,400 --> 00:12:35,090 the early days so you may or may not see that but the point was that it was 123 00:12:35,090 --> 00:12:38,620 basically not showing a regular f but by putting two backslashes we can see 124 00:12:38,620 --> 00:12:43,490 that its working correctly so again the point of this raw string literals 125 00:12:43,490 --> 00:12:47,070 are introduced to remove the need to double up backslashes in a string like 126 00:12:47,070 --> 00:12:52,780 we've had to do their on line 13 so why now you probably asking why is the 127 00:12:52,780 --> 00:12:55,560 fact that raw stings that were introduced to deal with regular expressions 128 00:12:55,560 --> 00:12:59,600 important well although regular expressions used a lot of backslashes 129 00:12:59,600 --> 00:13:04,500 you can't have one at the end of a regular expressions string now as a result the 130 00:13:04,500 --> 00:13:09,030 Python raw string process it but doesn't suppress escaping if the backslashes the 131 00:13:09,030 --> 00:13:14,320 last character in the string and to show you what I mean I'm going to show by just type in a bit of code we can get it 132 00:13:14,320 --> 00:13:20,520 to show an error that you may not think should be an error so.... 133 00:13:22,130 --> 00:13:32,370 ....so Python treats the final backslashes as escape character 134 00:13:32,370 --> 00:13:36,920 and its escaping the final close quote so consequently the string isn't 135 00:13:36,920 --> 00:13:37,960 terminated 136 00:13:37,960 --> 00:13:43,200 so hence we are getting an error if that makes sense so if you want to end a string with a backslash you have to 137 00:13:43,200 --> 00:13:47,210 escape it even when using raw string literal in other words we have to put a 138 00:13:47,210 --> 00:13:51,390 double backslash in there and that fixes the problem now raw string 139 00:13:51,390 --> 00:13:55,690 literals are sometimes used to specify file paths in Windows which used the 140 00:13:55,690 --> 00:13:58,860 backslash character rather than a forward slash two separate the directory 141 00:13:58,860 --> 00:14:03,120 names but it's not actually necessary to do this because once again Python takes 142 00:14:03,120 --> 00:14:07,560 care of it for us but if you do and your Python ends up with a backslash such a 143 00:14:07,560 --> 00:14:11,840 a c:/ for example then you need to double up that final backslash 144 00:14:11,840 --> 00:14:16,260 so I'll be referring to these raw type literals because there's really no such 145 00:14:16,260 --> 00:14:20,610 thing as a raw string type in Python their all just string which is really 146 00:14:20,610 --> 00:14:24,760 telling Python to alter how it interprets them and that's why I had to 147 00:14:24,760 --> 00:14:28,890 copy and paste the strings rather than somehow convert them into a raw string when 148 00:14:28,890 --> 00:14:32,430 initializing the raw variables there's no such conversion because there's no 149 00:14:32,430 --> 00:14:34,550 raw type to convert to 150 00:14:34,550 --> 00:14:39,660 their are prefix that we're putting you can see on line 16 really only applies to string 151 00:14:39,660 --> 00:14:44,780 literals to tell Python on how to interpret so pep 257 that we saw earlier in 152 00:14:44,780 --> 00:14:49,190 this video was written along time ago and the comment about using a u prefix 153 00:14:49,190 --> 00:14:55,260 doesn't apply to Python 3 if you go back you can read up on prefix all strings in Python 154 00:14:55,260 --> 00:14:59,790 3 are already Unicode that's what I was referring to so I'm going to end the video 155 00:14:59,790 --> 00:15:05,320 here now in the next video we are going to switch back to the doc string and talk more about a how you 156 00:15:05,320 --> 00:15:09,830 got about documenting your classes and methods and functions etc see you in the next 157 00:15:09,830 --> 00:15:10,060 video