1 00:00:00,000 --> 00:00:05,549 Awesome, we're almost done with classes, but there's one last concept 2 00:00:05,549 --> 00:00:11,280 that I need you to know, and that is inheritance. Inheritance is the process 3 00:00:11,280 --> 00:00:19,140 of creating a new class out or for base class, so a new class that has all the 4 00:00:19,140 --> 00:00:23,820 properties on the methods of a base class but also it has some other methods. 5 00:00:23,820 --> 00:00:31,199 So why do we do that? Well I'll explain that to you through our example, so here 6 00:00:31,199 --> 00:00:37,440 we have this blueprint, so this class and let me delete this you know to confuse 7 00:00:37,440 --> 00:00:42,660 you so that we were just calling the class synthesis, so we have this class 8 00:00:42,660 --> 00:00:47,399 which creates a bank account object for us and then we can withdraw and deposit 9 00:00:47,399 --> 00:00:54,629 money from this bank account. Now what if we want to transfer money from this bank 10 00:00:54,629 --> 00:00:59,359 account? So from our bank account transfer money to another bank account. 11 00:00:59,359 --> 00:01:06,930 Well, we would have to add a transfer method in here and then do something 12 00:01:06,930 --> 00:01:13,140 like just simple like self balance equal to self balance minus the amount, so we 13 00:01:13,140 --> 00:01:20,850 pass in a transfer amount there and that is subtracted from our balance here, but 14 00:01:20,850 --> 00:01:26,430 as far as I know you cannot transfer money from your savings account you can 15 00:01:26,430 --> 00:01:31,020 transfer money from your checking account, but not from your saving account, 16 00:01:31,020 --> 00:01:36,450 so that method wouldn't make sense for a checking account, and this account is 17 00:01:36,450 --> 00:01:44,990 a general background, so what we could do is maybe we could create another class 18 00:01:44,990 --> 00:01:49,229 like we could copy this and create another class and call it something else, 19 00:01:49,229 --> 00:01:56,189 and so we have all these methods there. So that's one solution. The other 20 00:01:56,189 --> 00:02:01,110 solution is to use inheritance, so here is where inheritance comes in handy. 21 00:02:01,110 --> 00:02:07,140 You derive a class out of a base class which is a more general class like this 22 00:02:07,140 --> 00:02:13,410 account class in here, so let's create a new class. This new class is called, 23 00:02:13,410 --> 00:02:16,390 it's referred to as a subclass, and this will be the base class, 24 00:02:16,390 --> 00:02:25,299 again you need to use the same syntax, so you use a class keyword there 25 00:02:25,299 --> 00:02:31,930 to create a class. Let's call this checking, so for checking account, and the 26 00:02:31,930 --> 00:02:38,349 first thing we want to do is to create an init method. With that init method we 27 00:02:38,349 --> 00:02:44,139 will create like this minimal object that I have been talking about. Like in 28 00:02:44,139 --> 00:02:51,909 our base class we also need here we need a balance from our file, so we need that 29 00:02:51,909 --> 00:02:57,340 file path, but instead of writing this code again as I said what you can do is 30 00:02:57,340 --> 00:03:03,099 well first you need to create an init function method, and of course pass self 31 00:03:03,099 --> 00:03:09,939 in there so you always pass the self parameter whether that is base class, 32 00:03:09,939 --> 00:03:20,229 or a sub class, and then in this init method you want to call the init 33 00:03:20,229 --> 00:03:26,169 methods of your account class which means that when you create an object 34 00:03:26,169 --> 00:03:31,449 instance of your checking class this method will be executed, and when this 35 00:03:31,449 --> 00:03:36,879 method is executed, this method executes the init function of your account method 36 00:03:36,879 --> 00:03:42,790 so that you create a list minimal object which is the same as this one here, but 37 00:03:42,790 --> 00:03:50,079 then you have some extra methods down here, so a checking account is this plus 38 00:03:50,079 --> 00:03:58,120 some other things, and here you need to pass self and the other parameter that 39 00:03:58,120 --> 00:04:02,919 is necessary for this init function. So here we are passing two parameters and 40 00:04:02,919 --> 00:04:10,900 we should do the same in here, and if I execute this now I'll get an error, so by 41 00:04:10,900 --> 00:04:16,630 intention I want to you to see whatever we get, so let me create an instance of 42 00:04:16,630 --> 00:04:25,439 this function. So checking and yeah, let's try that out. 43 00:04:25,439 --> 00:04:37,599 Here, yeah, that's inside the account folder, so here and then acc.py. 44 00:04:37,599 --> 00:04:46,389 Let's see! So the first thing that this is saying is file path is not defined in 45 00:04:46,389 --> 00:04:51,490 line 21 which is this one here and the reason to that is that we haven't 46 00:04:51,490 --> 00:05:01,599 defined any file path inside this function, so you can pass that inside the 47 00:05:01,599 --> 00:05:07,419 init brackets as a parameter, and then you need to pass it here as well so 48 00:05:07,419 --> 00:05:17,949 let's say account balance dot txt. Good. We don't see any output there because we 49 00:05:17,949 --> 00:05:23,620 are not printing anything out, but what happened basically is that an object 50 00:05:23,620 --> 00:05:28,810 instance was created and this was called, this method was called now this class 51 00:05:28,810 --> 00:05:35,620 was initialized as well. Now if we try to do, so the whole point of inheritance is 52 00:05:35,620 --> 00:05:41,560 to do something like checking so to access methods of the base class without 53 00:05:41,560 --> 00:05:47,219 writing those methods in your subclass. So let's say checking dot deposit and 54 00:05:47,219 --> 00:05:58,000 let's put an amount there, so 10. Now if we execute that we get this attribute 55 00:05:58,000 --> 00:06:02,949 error which says checking object has no attribute deposit so this is not 56 00:06:02,949 --> 00:06:09,219 recognizing this method and the reason to that is that we haven't yet satisfied 57 00:06:09,219 --> 00:06:16,409 the syntax of inheritance and so the key here is to pass an argument to our 58 00:06:16,409 --> 00:06:21,759 checking subclass and that argument is the base class, so the name of the base class. 59 00:06:21,759 --> 00:06:28,479 Say that if you do the same now and maybe print out the altitudes to see 60 00:06:28,479 --> 00:06:34,650 what's happening, so self dot balance. I'm sorry! I meant to say checking 61 00:06:41,110 --> 00:06:47,770 dot balance. Sorry about that! Yeah, now it seems to be working so this 62 00:06:47,770 --> 00:06:53,830 method was recognized and what this did is from 1 100 63 00:06:53,830 --> 00:06:59,530 which was the initial balance it went up to 1 110. 64 00:06:59,530 --> 00:07:04,870 So this printed out the balance instance 65 00:07:04,870 --> 00:07:11,199 variable of the base class, so that's how we deposit some money in our checking 66 00:07:11,199 --> 00:07:17,020 account. Now we want to transfer money, so we need to define a new method here. 67 00:07:17,020 --> 00:07:27,130 Let's call this transfer. Transfer self there, and also a local variable amount 68 00:07:27,130 --> 00:07:32,440 so as a parameter. So this is just a local variable it doesn't have to do 69 00:07:32,440 --> 00:07:38,229 with these ones here, and then we want to update the balance, so we want to 70 00:07:38,229 --> 00:07:43,659 transfer some money to some account. That amount will be subtracted from our 71 00:07:43,659 --> 00:07:54,520 balance, so that means self balance equals to self balance minus amount. 72 00:07:54,520 --> 00:08:08,699 Now if we try that new method there, so we are at this point. Let's transfer 73 00:08:08,699 --> 00:08:24,490 110, yeah, and I was expecting to have one thousand and one hundred here but I got 74 00:08:24,490 --> 00:08:29,789 this number and the reason to that is because we didn't commit the changes 75 00:08:29,789 --> 00:08:36,130 earlier when we deposited some money so when we executed the script now an 76 00:08:36,130 --> 00:08:40,570 object instance was created from the beginning and it got to this number here 77 00:08:40,570 --> 00:08:45,580 which wasn't updated to this because we didn't commit the changes earlier. 78 00:08:45,580 --> 00:08:51,820 But you get the idea, so from this balanced subtracting this one here we 79 00:08:51,820 --> 00:08:56,830 get this now. That means that the transfer method is working, but we just 80 00:08:56,830 --> 00:09:05,010 need to apply the commit method there. So again we can commit those changes. 81 00:09:05,010 --> 00:09:12,040 Yeah, let's see what we get now, so from 1 100 I want to transfer 100 and now I 82 00:09:12,040 --> 00:09:20,410 expect to have this updated to 1 000. Let's see, Yeah, 1 000 printed out here, 83 00:09:20,410 --> 00:09:26,920 and 1,000 a year too, so that is working great. Also another thing I want 84 00:09:26,920 --> 00:09:33,220 you to know about inheritance is that you can add your own instance variables 85 00:09:33,220 --> 00:09:39,220 for your subclass. Like in this case our only instance variable is balance which 86 00:09:39,220 --> 00:09:45,550 has been inherited from the account base class, but we can add our own instance 87 00:09:45,550 --> 00:09:52,030 variables, so let's suppose this transfer also comes with some fee. So when you do 88 00:09:52,030 --> 00:09:58,390 a transfer your bank charges you some money. And you want to give that fee as a parameter, 89 00:09:58,390 --> 00:10:05,950 so let's keep things simple and say when you transfer money, 90 00:10:05,950 --> 00:10:12,010 so balance minus amount minus the fee, so let's not apply 91 00:10:12,010 --> 00:10:16,690 percentages there. Let's say, let's suppose we have a fee of $1 per transfer. 92 00:10:16,690 --> 00:10:22,750 Now if we apply the transfer method again we'll probably get an error 93 00:10:22,750 --> 00:10:28,870 because this fee is just a local variable here, so what you need to apply 94 00:10:28,870 --> 00:10:38,440 now is you need to transform this fee to an instance variable which is self dot 95 00:10:38,440 --> 00:10:48,280 fee equals to fee, and that means if you pass now a fee here, so let's say 1, $1 or 96 00:10:48,280 --> 00:10:55,510 any other currency, you pass that here. This will go here, and that will go 97 00:10:55,510 --> 00:11:01,360 here and this will be equal to the fee that you're passing you here. 98 00:11:01,360 --> 00:11:08,080 And then we can make use of this instance variable, and use it in this transfer 99 00:11:08,080 --> 00:11:17,130 method. Great, now we are at 1,000 there and lets me let me execute this. 100 00:11:18,480 --> 00:11:27,790 Yeah, so 899, and yeah, the same here. Which means 1000 minus 100 minus 101 00:11:27,790 --> 00:11:33,250 the fee you get this number, and yeah I hope you were able to see how 102 00:11:33,250 --> 00:11:39,610 inheritance comes in handy to create subclasses out of a base class. That's it 103 00:11:39,610 --> 00:11:44,110 for this lecture, and we have one more lecture to go, and in the next lecture 104 00:11:44,110 --> 00:11:50,560 what I'll do is I will clarify some terms of object-oriented programming, so 105 00:11:50,560 --> 00:11:55,200 that will be a short lecture and that will help you later on when you refer to 106 00:11:55,200 --> 00:12:01,810 documentation because there are quite a few concepts out there in OOP and I 107 00:12:01,810 --> 00:12:06,339 don't want you to get confused with those, so I think that's an important 108 00:12:06,339 --> 00:12:07,230 lecture, and yeah, I'll talk to you later.