1 00:00:00,030 --> 00:00:05,160 Good, we were able to transform our script which was built using functions, 2 00:00:05,160 --> 00:00:11,219 and we created a class and put those functions inside that class, so they are now 3 00:00:11,219 --> 00:00:18,199 called methods. And while this is a use of a class, it's still not the best use 4 00:00:18,199 --> 00:00:23,820 of object-oriented programming paradigm because we have a database here, 5 00:00:23,820 --> 00:00:29,400 a database object that is being created but we are not treating it as a solid 6 00:00:29,400 --> 00:00:38,010 object, so we are opening it and closing it in every method, so what I'm thinking 7 00:00:38,010 --> 00:00:44,760 is we create a connection when we call the class, so when this function is 8 00:00:44,760 --> 00:00:50,870 executed we create a connection there, and we keep the connection opened, so I 9 00:00:50,870 --> 00:00:57,270 don't include this close method there, but I'll leave the commit method in 10 00:00:57,270 --> 00:01:03,210 case there is no table, and a table will be created and the changes will be 11 00:01:03,210 --> 00:01:08,820 committed to the open connection. Now because we have a connection open there 12 00:01:08,820 --> 00:01:15,570 we don't need to establish a new one in each method and we don't even need to 13 00:01:15,570 --> 00:01:19,590 create a cursor object because the cursor object is being created in here. 14 00:01:19,590 --> 00:01:29,640 So we get rid of those two lines in every single method. So in view we have a 15 00:01:29,640 --> 00:01:35,939 cursor object when we go here and then we execute this statement to that cursor 16 00:01:35,939 --> 00:01:45,810 object, so that is good. Then we go down here, this to as well, 17 00:01:45,810 --> 00:01:48,170 and for the delete method. And update as well. Great. 18 00:01:54,980 --> 00:02:04,640 Now let's see how this goes. Let me make this closer, so we call the 19 00:02:04,640 --> 00:02:11,050 frontend script where we are creating an instance of our object, so far so good. 20 00:02:11,050 --> 00:02:22,420 View all and it says that name cur is not defined in line 17. 21 00:02:22,420 --> 00:02:31,010 So we executed the view method, and what the view method does is it tries to find 22 00:02:31,010 --> 00:02:37,459 a variable, a declared variable inside the function, so the cur variable. 23 00:02:37,459 --> 00:02:43,100 It doesn't find so then it looks if there is a variable outside the function and 24 00:02:43,100 --> 00:02:47,480 it doesn't find it. Actually there is a variable that has been declared in this 25 00:02:47,480 --> 00:02:54,260 init function, but this is a local variable, so this has value only inside 26 00:02:54,260 --> 00:03:03,730 this function, so you cannot access this outside of the init function. But if we 27 00:03:04,299 --> 00:03:12,530 refer to the self argument which is the database object, and see what we get. 28 00:03:12,530 --> 00:03:25,250 Let me close this. Save, and that works and view all, and now we get another type of error. 29 00:03:25,250 --> 00:03:32,060 And it says attribute error, so let me close this, so look carefully here and it says 30 00:03:32,060 --> 00:03:39,410 database object has no attribute cur. And that's in line 17, so this one here. 31 00:03:39,410 --> 00:03:46,549 And the database object, this one is this on here, so self has very placed by 32 00:03:46,549 --> 00:03:54,350 database which is the object created by this class and this is an attribute of 33 00:03:54,350 --> 00:04:02,060 this object and to fix that we need to create an attribute there. 34 00:04:02,060 --> 00:04:08,090 So we say self dot cursor, dot cur equals to connection dot cursor. 35 00:04:08,090 --> 00:04:15,799 the cursor method of the connection object of the sqlite3, a library. 36 00:04:15,799 --> 00:04:26,509 Now if we try again the code, it says name cur is not defined in line 8. So now that 37 00:04:26,509 --> 00:04:31,699 we created an attribute, this is trying to find the variable cur, so the 38 00:04:31,699 --> 00:04:36,350 local variable cur which is not in this function because now this is an 39 00:04:36,350 --> 00:04:43,130 attribute of the class so that means we need to refer to self here as well, so to 40 00:04:43,130 --> 00:04:51,080 the object and then cur execute and so on. So if we try this now view all, we get 41 00:04:51,080 --> 00:04:57,680 another line. Cur is not defined in line 18. So here, that means you need to do the 42 00:04:57,680 --> 00:05:04,880 same and we don't have any curve variable anymore. We have the cur attribute. 43 00:05:04,880 --> 00:05:10,400 So now if I run this again I'll get probably an error about the connection 44 00:05:10,400 --> 00:05:20,720 this time. The conn variable. View all. Conn is not defined in line 19, so again 45 00:05:20,720 --> 00:05:25,820 this view function is trying to find the variable conn, connection, so self 46 00:05:25,820 --> 00:05:33,260 commit there and here as well, so you declare the attribute there, and then you commit 47 00:05:33,260 --> 00:05:44,720 the changes down here, and self connection, close, so that looks good now. Rows is equal 48 00:05:44,720 --> 00:05:49,130 to this, so rows is just a local variable. That is created here and then we 49 00:05:49,130 --> 00:05:55,639 returned the fetched rows from this expression, so this method returns 50 00:05:55,639 --> 00:06:05,490 this when it's being called in this script, just here, good. 51 00:06:05,490 --> 00:06:15,270 And let me try what we have at this point. Line seven, name conn is not defined. 52 00:06:15,270 --> 00:06:23,670 Yeah, we need to do the same for these two, and everything looks good 53 00:06:23,670 --> 00:06:30,840 now, so let's try again. Yeah, view all and now we get the data there, but when I 54 00:06:30,840 --> 00:06:37,230 press it for the second time we get yet another error, and so it says cannot 55 00:06:37,230 --> 00:06:44,670 operate on a closed database, so why is that? Well I pressed the view button once and 56 00:06:44,670 --> 00:06:52,460 this method was executed, so the cursor object was able to fetch these data, and 57 00:06:52,460 --> 00:06:58,380 then we closed the connection, I returned the rows, so we are able to get those 58 00:06:58,380 --> 00:07:04,950 three rows there, but then when I pressed the view button for a second time, 59 00:07:04,950 --> 00:07:10,590 the view method faces a close connection. So that's why you get this cannot operate 60 00:07:10,590 --> 00:07:16,970 on a closed database error. So, see what we get. 61 00:07:16,970 --> 00:07:26,160 So view all. It's working well, yep. That means we can go ahead and modify the 62 00:07:26,160 --> 00:07:35,460 other methods, so self cursor and self connection, and remove this close. 63 00:07:35,460 --> 00:07:39,800 Let's think about closing the connection later on, so don't focus on that now. 64 00:07:39,800 --> 00:07:50,130 Just try to make these methods work. Self here, self there, and there, again for 65 00:07:50,130 --> 00:07:58,250 the delete method, and yeah remove the connection, close. 66 00:08:00,310 --> 00:08:08,750 And everything should work fine at this point so let's see. View all. This is 67 00:08:08,750 --> 00:08:17,240 working, let's search for something. "The Sea" and search entry. We got one cursor there. 68 00:08:17,240 --> 00:08:27,979 So cur is not defined in search. Oh yeah, sorry! Again. View all. 69 00:08:27,979 --> 00:08:38,839 View all. "The Sea" search entry and yet another name error. 70 00:08:38,839 --> 00:08:46,339 The name rows is not defined. So by mistake I converted this 71 00:08:46,339 --> 00:08:55,339 local variable to attributes, so rows here, rows there. Okay one more time. 72 00:08:55,339 --> 00:09:10,640 View all. View all. Now it seems to be working. So let's add some data. Add entry there. 73 00:09:10,640 --> 00:09:17,890 And cannot operate on closed database. Why? 74 00:09:17,890 --> 00:09:23,149 Because all in the search method we have a closed database, so we closed the 75 00:09:23,149 --> 00:09:33,020 database there, and I overlooked that, so we don't have any more closed methods 76 00:09:33,020 --> 00:09:41,140 there, no. So let's try one more time then. Sorry about that! 77 00:09:45,670 --> 00:09:54,820 Add entry and now it seems to be working fine, great, so everything looks good 78 00:09:54,820 --> 00:10:00,770 except of we don't have the close method anymore because we can't keep 79 00:10:00,770 --> 00:10:05,959 that method in the methods of our class. Because you saw that when we have that 80 00:10:05,959 --> 00:10:09,270 method there and we end up with this error. 81 00:10:09,270 --> 00:10:16,560 Or you can leave the code without a close method, but you'd end up with some 82 00:10:16,560 --> 00:10:21,080 problems later because if you have some methods that have not been committed, 83 00:10:21,080 --> 00:10:26,310 then one when another program connects to your database, then these data 84 00:10:26,310 --> 00:10:30,810 will be committed with the next operation, so it messes something up 85 00:10:30,810 --> 00:10:37,980 sometimes, so as a rule you should close the connection, and to do that there is 86 00:10:37,980 --> 00:10:45,080 another special method that you can add to your class like this one here, but 87 00:10:45,080 --> 00:10:54,510 this other one is for destructing your object. That is called del, so for delete. 88 00:10:54,510 --> 00:11:02,839 And this also gets the object instance as a parameter, so this method is applied, 89 00:11:02,839 --> 00:11:09,510 it's executed when you call an instance of the class like we are doing in the 90 00:11:09,510 --> 00:11:16,940 frontend script, just here, while this one here it is executed when this 91 00:11:16,940 --> 00:11:22,860 instance is deleted from the script which happens to be when the script is 92 00:11:22,860 --> 00:11:30,029 exited, so before the script exits this method will be executed. And what we want 93 00:11:30,029 --> 00:11:38,640 to do here is self dot connection dot close. So that should do the trick. Now if 94 00:11:38,640 --> 00:11:43,100 we try out this, we should get the same results, so I added this by mistake but 95 00:11:47,520 --> 00:11:54,570 anyway, so "The Sea". You can search for that. And you get the rows with that particular 96 00:11:54,570 --> 00:12:01,680 value, so this is my solution and there's no standard way to write 97 00:12:01,680 --> 00:12:06,959 a script so there may be other alternatives to solve this problem, 98 00:12:06,959 --> 00:12:13,850 so to build this application so you may also choose to add a method there or 99 00:12:13,850 --> 00:12:19,140 here in the graphical user interface so you could do something like you add 100 00:12:19,140 --> 00:12:23,279 a button like commit button, and then here you add another 101 00:12:23,279 --> 00:12:31,170 method called commit, and then you remove probably all these commit methods from these 102 00:12:31,170 --> 00:12:37,800 methods, from the methods of your SQLite operations, and then you put that method, 103 00:12:37,800 --> 00:12:45,509 so one method commit inside that commit method of your class. So when the user 104 00:12:45,509 --> 00:12:49,319 presses commit, then you commit the changes to the database, so that would be 105 00:12:49,319 --> 00:12:56,610 another solution, but this is what we came up with in this particular case, so 106 00:12:56,610 --> 00:13:01,290 I hope you understand a few things now. This is rarely used, so normally you 107 00:13:01,290 --> 00:13:05,910 only have to use init and then the other methods, but this is appropriate 108 00:13:05,910 --> 00:13:11,639 when you want to apply something just before exiting the program, so that's 109 00:13:11,639 --> 00:13:17,699 great and in the next lecture I will write another class, so we will think 110 00:13:17,699 --> 00:13:23,670 about how to model a problem which is about creating some sort of bank account 111 00:13:23,670 --> 00:13:30,180 object where we store a balance of money, Then we withdraw and deposited money 112 00:13:30,180 --> 00:13:38,189 to this balance. Now as an exercise you can convert this script, so the tkinter 113 00:13:38,189 --> 00:13:46,860 part of the program, the frontend into an object-oriented form so you put 114 00:13:46,860 --> 00:13:53,160 these inside a class and try to come up with a good solution. I'll also provide 115 00:13:53,160 --> 00:13:58,079 you a solution to this, but before that I would like you to try out yourself, so you 116 00:13:58,079 --> 00:14:03,329 practice object-oriented programming a little bit. Yeah, that's good. With that 117 00:14:03,329 --> 00:14:04,860 I'll see you in the next lecture.