1 00:00:00,269 --> 00:00:07,319 Good, now I'll go ahead and transform this script into an object oriented 2 00:00:07,319 --> 00:00:12,870 programming style, so in other words I'll organize these functions in a more 3 00:00:12,870 --> 00:00:19,740 efficient way and just to refresh your memory what this script does, so we have 4 00:00:19,740 --> 00:00:25,619 a back-end and front-end script, and the front-end what it does is it constructs 5 00:00:25,619 --> 00:00:32,910 the graphical user interface so you know that. And we have this books dot 6 00:00:32,910 --> 00:00:39,420 db database file, and there are some data there and you can search for entries so 7 00:00:39,420 --> 00:00:43,980 for rows in the database, add new and so on. And this frontend.py constructs 8 00:00:43,980 --> 00:00:48,719 the graphical user interface and this backend is responsible for the 9 00:00:48,719 --> 00:00:53,570 database interaction, and so object-oriented programming is about 10 00:00:53,570 --> 00:01:00,859 encapsulating functions inside a class. So class, and let's call this database. 11 00:01:07,820 --> 00:01:16,049 Yeah, that's it basically we still have a lot of things to do but, let's think 12 00:01:16,049 --> 00:01:21,689 thing about what is this database class. Well, whenever you decide to use 13 00:01:21,689 --> 00:01:26,040 object-oriented programming you first need to think about the object that you 14 00:01:26,040 --> 00:01:30,450 want to use which is the object that that you want to apply this methods to 15 00:01:30,450 --> 00:01:38,070 and in our case that would be the database, so then we insert data to the 16 00:01:38,070 --> 00:01:41,970 tables of this database and view data and so on. 17 00:01:41,970 --> 00:01:48,479 Now this object, this object you first need to create a minimal object when you 18 00:01:48,479 --> 00:01:54,060 call the class, and I'll show you how you call a class in a minute, but for now 19 00:01:54,060 --> 00:01:59,040 let's focus on creating the blueprint of the object. So you create the 20 00:01:59,040 --> 00:02:03,840 blueprint of the object which is this one here, and then you create object 21 00:02:03,840 --> 00:02:10,709 instances using that blueprint, and you do that by calling the class. Now when 22 00:02:10,709 --> 00:02:15,840 you call the class you want to have this minimal object, so in this case that 23 00:02:15,840 --> 00:02:21,510 would be to establish a connection to this database, and then create a coursor 24 00:02:21,510 --> 00:02:26,130 object, and maybe check for tables so that you make sure that your database 25 00:02:26,130 --> 00:02:33,540 has a table, and so that it allows you to do later apply methods to this table of 26 00:02:33,540 --> 00:02:39,330 database, otherwise the database would be useless, so it needs to have 27 00:02:39,330 --> 00:02:44,760 a table and an established connection. That means you have to figure out a way 28 00:02:44,760 --> 00:02:52,890 that when you call the class these lines here are executed. And Python has 29 00:02:52,890 --> 00:02:59,510 reserved a special syntax for that. So the first function of your script you 30 00:02:59,510 --> 00:03:07,260 need to call it init with double underscore in front and on the back. 31 00:03:07,260 --> 00:03:11,790 So that's an init function which means initialize an object. In other 32 00:03:11,790 --> 00:03:16,400 programming languages that is called a constructor so, it constructs the object. 33 00:03:16,400 --> 00:03:23,340 So when you call a class, this function is executed but not the other ones. 34 00:03:23,340 --> 00:03:27,450 The other ones are executed if you refer to them like in this case that would be 35 00:03:27,450 --> 00:03:31,860 something like a database dot insert, and then you pass the parameters, the 36 00:03:31,860 --> 00:03:37,260 arguments there. Now we have quite a few things to add here in script for this to 37 00:03:37,260 --> 00:03:44,730 work, but I'd prefer to do this on a trial and error base, so we'll execute it, we'll 38 00:03:44,730 --> 00:03:49,980 see whatever we get so that you understand how things are working there, so what 39 00:03:49,980 --> 00:03:55,620 I'll do I'll call this class as it is now, so we don't need this function. This 40 00:03:55,620 --> 00:04:01,799 function is gone now because I replaced that with init, so you know you don't call 41 00:04:01,799 --> 00:04:08,850 functions anymore like we did here, so for instance here we are calling 42 00:04:08,850 --> 00:04:14,280 functions from the backend module so which is like a module, you are importing 43 00:04:14,280 --> 00:04:25,419 this as a module. And in this case now what we do is from the backend import 44 00:04:25,419 --> 00:04:34,030 database, so we import the database class from the backend script so that makes 45 00:04:34,030 --> 00:04:39,520 available the database class for us so the database blueprint. So this is just 46 00:04:39,520 --> 00:04:43,810 a blueprint. Now we need to create an object out of this blueprint, so let's 47 00:04:43,810 --> 00:04:50,770 call that object database and that would be equal to database, so the class 48 00:04:50,770 --> 00:04:58,659 and so let's, for now let's not pass any arguments there, and then we need to replace 49 00:04:58,659 --> 00:05:05,590 these references, so we don't do back-end anymore, we do database dot view. So we 50 00:05:05,590 --> 00:05:14,110 are referring to the view function of the database object which is an instance 51 00:05:14,110 --> 00:05:16,830 of this class. Great, so we go ahead and replace 52 00:05:21,580 --> 00:05:30,629 these other ones with database, so back-end replace that, replac, replace. 53 00:05:30,629 --> 00:05:37,770 Yeah, that's it. This is the script name. And it should be like that. Escape here. 54 00:05:37,770 --> 00:05:45,490 Great, now I'll go ahead and execute that, so don't expect this to work but we have 55 00:05:45,490 --> 00:05:51,879 to look closely to the error trying to understand what it says. So trace back 56 00:05:51,879 --> 00:06:01,389 file frontend dot py line 4, here so in it takes 0 positional argument, but one was given. 57 00:06:01,389 --> 00:06:08,800 And what's happening here now is that when you call the class this, class will 58 00:06:08,800 --> 00:06:15,490 create an object instance, and store it in this variable, and then it sends this 59 00:06:15,490 --> 00:06:24,250 object instance to this function. So let me split this on the right so that we 60 00:06:24,250 --> 00:06:30,550 have both the scripts in here, so the frontend and the backend, so basically 61 00:06:30,550 --> 00:06:37,060 this database object instance is being sent to this init object even 62 00:06:37,060 --> 00:06:42,400 though this is an empty bracket. So that's like a hidden syntax there and 63 00:06:42,400 --> 00:06:49,639 to fix this you need to perform a trick here, so it says init takes zero 64 00:06:49,639 --> 00:06:55,129 positional arguments because it actually it has an empty bracket here, so it's 65 00:06:55,129 --> 00:06:59,870 like a normal function, and functions if you don't put a parameter there, but then 66 00:06:59,870 --> 00:07:05,330 you pass a parameter when you call it, the function will say I don't take any 67 00:07:05,330 --> 00:07:09,949 positional arguments or any mandatory arguments but one argument was given to 68 00:07:09,949 --> 00:07:17,259 me so I'll throw an error. Now to fix that you need to pass a parameter there and 69 00:07:17,259 --> 00:07:24,169 everyone passes self there, so you can pass any word, that that is still work, 70 00:07:24,169 --> 00:07:28,999 but as a convention so that so that your code is readable by other people you 71 00:07:28,999 --> 00:07:35,300 need to pass self. So now your init function takes in a parameter and when 72 00:07:35,300 --> 00:07:41,089 you call this class, this class will send object instance, and that object instance 73 00:07:41,089 --> 00:07:46,639 will go inside this parameter. So this parameter will be replaced by the object 74 00:07:46,639 --> 00:07:53,839 instance argument. Great, if you still don't understand that, I'm sure you'll do 75 00:07:53,839 --> 00:08:01,250 so later as you write more classes. Good. Now I'll go ahead and save this and this, 76 00:08:01,250 --> 00:08:08,509 and try out the script again, and great. So that means that the class was 77 00:08:08,509 --> 00:08:15,229 successfully initialized, and it created on the object basis for us. Let me 78 00:08:15,229 --> 00:08:20,960 close this, and maybe add something there. This is not necessary, but just to 79 00:08:20,960 --> 00:08:26,180 show you that if you want to add more parameters to your init function you do 80 00:08:26,180 --> 00:08:36,349 something like db, and then replace this with db, and then here you pass the 81 00:08:36,349 --> 00:08:42,229 argument that will be passed to this db parameter, so that would be books 82 00:08:42,229 --> 00:08:50,269 dot db, the file path to our database. Great, and if you agree do this again 83 00:08:50,269 --> 00:08:56,280 you get the same result. So what's happening here is that Python 84 00:08:56,280 --> 00:09:02,010 is first sending the database object instance to the self parameter and then 85 00:09:02,010 --> 00:09:08,850 it sends the path of the file to the db parameter, and that db parameter 86 00:09:08,850 --> 00:09:17,580 sends that file path to the db parameter over the connect method. Great, now let's 87 00:09:17,580 --> 00:09:23,070 see how these methods will work, so we have the view all button which if we 88 00:09:23,070 --> 00:09:30,150 press it what will happen is the view function of the database class, actually 89 00:09:30,150 --> 00:09:36,990 the view method of the database class will be executed, so this one here. Let's 90 00:09:36,990 --> 00:09:44,730 see if we were able to get it. Well an error. Let me close this. So it says view takes 91 00:09:44,730 --> 00:09:51,120 zero positional arguments, but one was given, so that again suggests that when 92 00:09:51,120 --> 00:09:56,700 you call a method of class, the class is sending the object instance to that 93 00:09:56,700 --> 00:10:01,290 method as well just as we did with the init function. That means we need to do 94 00:10:01,290 --> 00:10:04,980 the trick again. So just pass self there, and you try out 95 00:10:04,980 --> 00:10:13,710 again, see whatever we get this time. Well, no error things are working obviously. 96 00:10:13,710 --> 00:10:20,610 So that's great, however if you try the other methods now like insert, you'll probably get 97 00:10:20,610 --> 00:10:24,750 an error, so let me try that. So View all is working fine, but if we insert 98 00:10:24,750 --> 00:10:30,650 something let's say "Hey There" and… And yeah, add entry. So insert takes four 99 00:10:40,720 --> 00:10:46,839 positional arguments, but five were given. So insert and these values are being 100 00:10:46,839 --> 00:10:51,639 passed to these parameters, but also we are passing the object instance, so let's 101 00:10:51,639 --> 00:11:05,040 go ahead and add self there, and I'll do the same for search as well, delete and update. 102 00:11:05,040 --> 00:11:18,779 And yeah, that's what we have. Looks good. Let's try out again, so view all and 103 00:11:22,949 --> 00:11:30,459 add entry, and yeah this seems like it's working. View all, and here we have "Hey There". 104 00:11:30,459 --> 00:11:40,360 Close, so we were able to create a class and put all the functions inside 105 00:11:40,360 --> 00:11:46,120 the class, so we have an organized program now, and so that's 106 00:11:46,120 --> 00:11:50,740 object-oriented programming, but object-oriented programming is not 107 00:11:50,740 --> 00:11:55,959 simply about putting some functions inside a class. Object oriented 108 00:11:55,959 --> 00:12:01,930 programming is about good design, so this script it's not probably the best 109 00:12:01,930 --> 00:12:08,589 design that we can come up with. Why is that? Because we're not making use of the 110 00:12:08,589 --> 00:12:15,730 real advantages that we have by using classes, so what I mean is that here we 111 00:12:15,730 --> 00:12:21,750 are creating these methods and in each method we're executing some closed 112 00:12:21,750 --> 00:12:28,750 functions, and also creating a connection in each of the methods. So we can reduce 113 00:12:28,750 --> 00:12:35,769 this code further more buried by removing the repeated actions, so in a 114 00:12:35,769 --> 00:12:42,730 class of what you need to think about is a way of constructing something at first, 115 00:12:42,730 --> 00:12:45,910 so in the init function, and then you refer to that something 116 00:12:45,910 --> 00:12:50,110 in the other functions, in the other methods, so we're talking about 117 00:12:50,110 --> 00:12:53,920 methods here. So even though our script is working fine 118 00:12:53,920 --> 00:13:00,070 in the next lecture we will go ahead and improve the design of it. 119 00:13:00,070 --> 00:13:00,280 So I'll see you there.