1 00:00:05,460 --> 00:00:09,600 so as I mentioned the album classes uses a list to store the songs and provides a 2 00:00:09,600 --> 00:00:14,750 method the add song method that we've create for adding more songs or 3 00:00:14,750 --> 00:00:18,880 adding songs to the list and it's also got a field for the artist and we 4 00:00:18,880 --> 00:00:22,080 got an error at the moment and that's because we haven't created that class so we are going 5 00:00:22,080 --> 00:00:27,700 to be doing that in this video now having an artists object stored in the album object can cause 6 00:00:27,700 --> 00:00:33,050 problems because the artist class will also hold the list of the artists publish albums but 7 00:00:33,050 --> 00:00:37,180 I'm gonna talk more about that when we've seen the artist class now one thing worth 8 00:00:37,180 --> 00:00:41,500 mentioning that before we get to that is that the error in the doc string and that is up 9 00:00:41,500 --> 00:00:46,640 here you can see that if we flag is it says unresolved reference album_name so this 10 00:00:46,640 --> 00:00:50,260 really highlights the fact that the doc strings aren't just ordinary strings but they 11 00:00:50,260 --> 00:00:54,690 really do have a special significance looking at the init method as you can 12 00:00:54,690 --> 00:01:00,120 see here on line 30 or 30 downwards we can see that the data attributes of the 13 00:01:00,120 --> 00:01:06,710 class a name year artist and tracks but I incorrectly referred to the album_name 14 00:01:06,710 --> 00:01:12,340 attribute in the doc string so IntelliJ has flag a warning unresolved reference album_ 15 00:01:12,340 --> 00:01:16,770 name so it's unusual for the content of strings to be check for 16 00:01:16,770 --> 00:01:21,350 syntax errors but doc strings are used by a number of tools including IntelliJ 17 00:01:21,350 --> 00:01:26,460 itself now IntelliJ uses the doc strings of the built-in modules to produce the 18 00:01:26,460 --> 00:01:30,630 intellisense tooltips that appear when you're using Python built-in 19 00:01:30,630 --> 00:01:34,500 functions and classes but as the built-in themselves are actually written in 20 00:01:34,500 --> 00:01:40,570 C IntelliJ can't examine the real source built-ins.py is a stub provider so that you 21 00:01:40,570 --> 00:01:44,540 can checked that various declarations and we have talked about that before now its easy 22 00:01:44,540 --> 00:01:47,930 discussed this with built-in with built-ins modules on the screens so I add 23 00:01:47,930 --> 00:01:53,160 a call to the code down here on line 54 to do that so we can see what we are talking 24 00:01:53,160 --> 00:01:56,160 about so type... 25 00:01:58,110 --> 00:02:05,080 again we get that same warning here unexpected argument so we can command or 26 00:02:05,080 --> 00:02:11,009 control-click on album you get access to the documentation that way but also 27 00:02:11,009 --> 00:02:15,560 going back and closed it down or come down here again we click on Help 28 00:02:15,560 --> 00:02:20,200 commander or control click on Help we can see the definitions in the built-in.py 29 00:02:20,200 --> 00:02:24,680 you can see that there's a comment field next to it so that the 30 00:02:24,680 --> 00:02:28,450 real signatures are unknown and has been restored from the doc string but the 31 00:02:28,450 --> 00:02:32,320 doc string doesn't list the functions arguments as a result that's the reason 32 00:02:32,320 --> 00:02:36,200 why IntelliJ doesn't know that it can take a parameter and to give the warning 33 00:02:36,200 --> 00:02:42,940 as such so close it down and go back to song.py and specifically we'll fix this 34 00:02:42,940 --> 00:02:47,890 error with the album_name so I'm going to change that to what it should 35 00:02:47,890 --> 00:02:51,710 have been called with name and we should find that error disappears because of course name 36 00:02:51,710 --> 00:02:54,020 is an attribute defined in 37 00:02:54,020 --> 00:02:58,950 our init method you can see the error then goes away and also delete this help album which 38 00:02:58,950 --> 00:03:03,570 we no longer need and I what will do is instead will start typing the new 39 00:03:03,570 --> 00:03:06,360 class so lets type the artist class 40 00:03:06,360 --> 00:03:19,120 out so...and I'm going to make a space again so we can see what we are doing here so... 41 00:03:19,120 --> 00:03:23,110 .... 42 00:03:28,070 --> 00:03:50,040 .... 43 00:03:50,040 --> 00:04:23,170 ..... 44 00:04:23,170 --> 00:04:34,910 ....ok we also want to document the method so.... 45 00:04:34,910 --> 00:04:37,910 ..... 46 00:04:39,070 --> 00:05:22,030 ..... 47 00:05:22,030 --> 00:05:52,190 .... 48 00:05:52,190 --> 00:06:07,250 ...again this is a good idea to put that type of documentation in if 49 00:06:07,250 --> 00:06:12,550 that is the case remember when you do make the change to go back and revise 50 00:06:12,550 --> 00:06:22,950 the documentation so all we are doing here is..... 51 00:06:22,950 --> 00:06:26,270 so theirs not really any checking their.... 52 00:06:26,270 --> 00:06:32,710 so the artist class has got 2 data attributes the string holding the artist's 53 00:06:32,710 --> 00:06:37,870 name and a list containing the album objects and its a standard way to organize your 54 00:06:37,870 --> 00:06:41,090 record collection now with the albums by particular group stored together 55 00:06:41,090 --> 00:06:46,000 compilation albums or the songs by various artists do complicate things in 56 00:06:46,000 --> 00:06:49,730 this design attempts to deal with that by allowing an artist to be specified 57 00:06:49,730 --> 00:06:54,480 for each song now the major problem with this design currently is that an artist 58 00:06:54,480 --> 00:06:59,540 object will have a reference to an album and that album object will also have a 59 00:06:59,540 --> 00:07:03,380 reference to the artist now the problems that that can cause are to do with 60 00:07:03,380 --> 00:07:07,890 garbage collection so when objects are no longer used in your Python code they 61 00:07:07,890 --> 00:07:13,200 still take up memory until they're ultimately destroyed now many more modern 62 00:07:13,200 --> 00:07:17,310 programming languages deal with this by a system known as garbage collection 63 00:07:17,310 --> 00:07:21,960 and that means that the runtime system keeps track of variables and reclaims their 64 00:07:21,960 --> 00:07:26,560 memory when they are no longer being used in the program anymore and that generally means even 65 00:07:26,560 --> 00:07:30,210 when they go out of scope or when there is no longer anything else referring to them 66 00:07:30,210 --> 00:07:36,300 as an example now if where to create an album object for say adells 25 67 00:07:36,300 --> 00:07:41,640 it would have a adell as its artist attribute however the adellt artist object would have 25 68 00:07:41,640 --> 00:07:46,180 in its list of albums so when the program is no longer using these the 69 00:07:46,180 --> 00:07:48,560 garbage collector might see that there's still a reference to both 70 00:07:48,560 --> 00:07:52,850 objects each from the other one and then ultimately not reclaim the memory 71 00:07:52,850 --> 00:07:57,880 for either so you can see how would then would stall both of those entries sort of 72 00:07:57,880 --> 00:08:03,230 permanently well until your program finishes and won't claim 73 00:08:03,230 --> 00:08:07,230 the amount of memory space with that said the Python 3 garbage collector is quite 74 00:08:07,230 --> 00:08:12,390 advanced and can actually cope quite well with these situations but circular 75 00:08:12,390 --> 00:08:18,210 object references like the one like this that I just mentioned are still best avoided and 76 00:08:18,210 --> 00:08:21,780 another problem with circular references like this is that it can seriously 77 00:08:21,780 --> 00:08:26,640 complicates saving the objects to file or to a database so sticking with the 78 00:08:26,640 --> 00:08:31,220 adell for a moment when we try to write a adelts artist object then we will go through all the 79 00:08:31,220 --> 00:08:35,450 attributes and save them into disk when we get to the album's object the program 80 00:08:35,450 --> 00:08:38,200 will go through the album attributes and save them to disk 81 00:08:38,200 --> 00:08:41,600 in the process it would find an artist attribute that would then need to be 82 00:08:41,600 --> 00:08:45,390 saved so it will go through all the attributes and save them to disk in the 83 00:08:45,390 --> 00:08:49,140 process finding a collection of albums each containing a reference to an artist 84 00:08:49,140 --> 00:08:55,060 object that contains a collection of objects and so on so Python pickle module 85 00:08:55,060 --> 00:08:59,870 can often cope with this type of listing but there are limits and it 86 00:08:59,870 --> 00:09:04,350 can also get quite confused he can get quite confused and I think you can too as the 87 00:09:04,350 --> 00:09:09,680 program in this scenario and in actual fact I have made it even worse here and 88 00:09:09,680 --> 00:09:15,880 I've done that by also storing the artist as an attribute of each song in the album's tracklist so lets just open up 89 00:09:15,880 --> 00:09:21,410 an image to see this in more detail so let's just have a look at the class 90 00:09:21,410 --> 00:09:26,950 structure for what we've come up with you can see the diagram on the screen so this is a 91 00:09:26,950 --> 00:09:30,590 little bit complicated and there's really no good reason for storing the 92 00:09:30,590 --> 00:09:36,630 artist object in the song class for example the artists name would be enough 93 00:09:36,630 --> 00:09:39,830 to allow the actual artist object to be retrieved 94 00:09:40,700 --> 00:09:45,990 if more details were required so we are gonna take a look at that later and just to be clear 95 00:09:45,990 --> 00:09:49,420 I'm not saying that you should never allow circular references in your code 96 00:09:49,420 --> 00:09:54,840 but if you do allow them then it should be for you know a really good reason with 97 00:09:54,840 --> 00:09:59,540 that said you should also be aware of what's going on and also of the potential 98 00:09:59,540 --> 00:10:04,430 problems that may arise as a result of using them so we're going to remove the 99 00:10:04,430 --> 00:10:10,940 circular references a little later but we are gonna leave them in for now so that we can see the program does work as 100 00:10:10,940 --> 00:10:16,520 its currently set up so we are going to go back to the code and now that our classes are 101 00:10:16,520 --> 00:10:20,930 defined let's move on and start doing something with them know rather than 102 00:10:20,930 --> 00:10:26,240 doing loads of typing and also because using classes to pass data is a very good 103 00:10:26,240 --> 00:10:31,340 way to do it I'm gonna read in the songs information from a text file now we've got 104 00:10:31,340 --> 00:10:34,830 a file albums.zip that can be downloaded from the Resources section 105 00:10:34,830 --> 00:10:40,270 of this video so download that file then extract the file and then copy that file 106 00:10:40,270 --> 00:10:43,410 to the same directory as your Python project 107 00:10:43,410 --> 00:10:47,720 file in IntelliJ and again you can do that too easily determine where you are by just 108 00:10:47,720 --> 00:10:49,680 right clicking the project here 109 00:10:49,680 --> 00:10:54,510 and coming down to reveal in finder which may be show files or something like that on a 110 00:10:54,510 --> 00:10:59,640 different platform click on that and it takes us into the folder so I'm gonna put the 111 00:10:59,640 --> 00:11:05,780 actual files in that folder there where my other py files are and I'm just gonna navigate 112 00:11:05,780 --> 00:11:07,800 to the file location 113 00:11:07,800 --> 00:11:12,960 where I've already downloaded the files the album.zip and unzip and copy it in their 114 00:11:12,960 --> 00:11:16,280 so I'm gonna go to that folder to my downloads folder 115 00:11:17,830 --> 00:11:23,950 ok I got albums.zip their so I'm going to unzip that theirs the albums.txt file that we 116 00:11:23,950 --> 00:11:29,800 are going to import I'm going to take a copy of that go back to the path where my project files are and 117 00:11:29,800 --> 00:11:33,620 pasted that in there and then when I go back to the code in IntelliJ 118 00:11:34,330 --> 00:11:40,950 we got albums.txt just appeared so the file if we have a look at it you can see 119 00:11:40,950 --> 00:11:46,960 it's a tab separate list of songs with each song records starting on a new line now 120 00:11:46,960 --> 00:11:52,080 a record here consists of the artists name the album name the year it was released as 121 00:11:52,080 --> 00:11:57,650 well as the song title now Python will make light work of parsing this kind of data all we 122 00:11:57,650 --> 00:12:02,360 really have to do is to split each line at the tab character and convert the result to a 123 00:12:02,360 --> 00:12:06,930 tuple and it's also necessary to remove the end of line marker from each 124 00:12:06,930 --> 00:12:11,140 line and fortunately there is again trivial to do in Python so let's 125 00:12:11,140 --> 00:12:15,830 create a function to load this data up and call it only if the program is executed 126 00:12:15,830 --> 00:12:22,530 as a script so going back to our code again I want to close this text file so here in our 127 00:12:22,530 --> 00:12:27,270 artists class we are not going to put in the artist class we are going to start typing it as 128 00:12:27,270 --> 00:12:37,370 a separate function outside of the class so.... 129 00:12:37,370 --> 00:12:39,780 ..... 130 00:12:39,780 --> 00:12:45,490 .... 131 00:12:53,640 --> 00:13:08,010 ..... 132 00:13:08,010 --> 00:13:17,370 ... 133 00:13:17,370 --> 00:13:21,740 .... 134 00:13:24,870 --> 00:13:46,830 .... 135 00:13:47,510 --> 00:13:53,890 ..... 136 00:13:53,890 --> 00:14:01,700 ...so that should give us our 4 fields and as you can see Python makes light work of something that 137 00:14:01,700 --> 00:14:05,890 would take a lot more code in other languages..... 138 00:14:05,890 --> 00:14:11,940 ... 139 00:14:11,940 --> 00:14:26,310 ....and that's our method so lets go back 140 00:14:26,310 --> 00:14:31,110 and confirm this works again we are only running this 141 00:14:31,940 --> 00:14:37,630 if the programs executed as scripts so we are going to do a test here so... 142 00:14:37,630 --> 00:14:47,260 .... 143 00:14:47,260 --> 00:14:54,010 ...alright lets try running the program 144 00:14:54,010 --> 00:14:57,440 and we should see that its displays the details for all the songs from the 145 00:14:58,030 --> 00:15:04,940 text file so run song and it seems to have worked ok we've got a list of the 146 00:15:04,940 --> 00:15:10,380 artists field the album the year as well as the song title itself so that's 147 00:15:10,380 --> 00:15:16,890 good that is working well.....so you can 148 00:15:16,890 --> 00:15:23,590 see there or you could see their that the fields variables seem to be getting the correct values ok so we got 149 00:15:23,590 --> 00:15:26,590 that working.... 150 00:15:28,610 --> 00:15:32,420 so now we got this working I'm gonna finish the video here in the next video we're going to 151 00:15:32,420 --> 00:15:37,890 move on and enhance a little bit more so if we are gonna start putting the data into the classes 152 00:15:37,890 --> 00:15:41,340 following naturally from the data so in other words as each rows are read 153 00:15:41,940 --> 00:15:45,550 we are gonna create a song object then add it to the album and when a new album is 154 00:15:45,550 --> 00:15:51,640 found in the data the current album is going to be store in the artist album list and a new album objects is going to be created 155 00:15:51,640 --> 00:15:54,900 with that current rows details so lets work on that in the next video