1 00:00:00,000 --> 00:00:04,799 Hello again, and in this lecture I would like to make sure you know the 2 00:00:04,799 --> 00:00:10,320 terminology of object-oriented programming in Python, so now we have the 3 00:00:10,320 --> 00:00:17,580 class which is this blueprint here or you can call it a prototype that defines 4 00:00:17,580 --> 00:00:23,609 the characteristics of an object that is about to be created, so that's the class. 5 00:00:23,609 --> 00:00:29,640 And then we have the object instance, or simply the object, so this here is 6 00:00:29,640 --> 00:00:36,870 an object, so a checking account object that stores data in this specific 7 00:00:36,870 --> 00:00:43,860 file path and it has this attribute so it has this fee. Then we have instance 8 00:00:43,860 --> 00:00:50,309 variables, and these are variables that are defined inside the methods of the 9 00:00:50,309 --> 00:00:57,059 class like file path here, and these are accessible by the instance, so by the object 10 00:00:57,059 --> 00:01:03,120 instance that is created by that class. Then we also have something called class 11 00:01:03,120 --> 00:01:08,820 variable, so instance variables, and class variables. Now we didn't create a class 12 00:01:08,820 --> 00:01:13,680 variable in any of the previous lectures so I'll go ahead and create one now. 13 00:01:13,680 --> 00:01:18,240 Let's create one for our checking account, so that would be something like 14 00:01:18,240 --> 00:01:30,090 let's say type equals to checking, so a class variable is declared outside of 15 00:01:30,090 --> 00:01:36,360 the methods of that class, and class variables are shared by all the 16 00:01:36,360 --> 00:01:42,240 instances of a class, so instance variables are shared by only the object instance. 17 00:01:42,240 --> 00:01:48,360 For instance let me explain that. Here this is one of our object instances, 18 00:01:48,360 --> 00:01:54,390 so we only have one object instance here, but we could have more, so I'll copy this 19 00:01:54,390 --> 00:02:00,290 and create another object instance here. And I'll call this, let's say this is 20 00:02:00,290 --> 00:02:12,640 Jack's checking account, so I'll replace that, and that, and that. This is 21 00:02:12,640 --> 00:02:28,250 John's checking account, and they also have their personal storage there, so let's 22 00:02:28,250 --> 00:02:40,370 say Jack dot txt and John dot txt. So Jack has 100 dollars, and John has 23 00:02:40,370 --> 00:02:47,680 200 dollars. Good, so we need to change that, Jack's, Jack, 24 00:02:47,680 --> 00:03:05,780 and John, and if we execute this script now, we will get two values 25 00:03:05,780 --> 00:03:15,470 printed out, so minus 1 for this statement here because Jack had, so he had $100 26 00:03:15,470 --> 00:03:21,890 there, but when he transferred 100 and then minus the fee of $1 and then he 27 00:03:21,890 --> 00:03:29,709 ended up with minus 1 dollar there. He owns the bank some money, and then 28 00:03:29,709 --> 00:03:37,220 John had 200 so he has 99 now. So what I'm trying to say is that these 29 00:03:37,220 --> 00:03:46,840 variables belong only to their object, but if you try now Jack's checking dot 30 00:03:46,840 --> 00:04:03,739 type, so you want to print that, print John's checking dot type. You'll see 31 00:04:03,739 --> 00:04:09,980 that this a class variable is shared by all the instances of that class. 32 00:04:09,980 --> 00:04:14,600 So that's the difference between class and instance variables. Class variables 33 00:04:14,600 --> 00:04:19,640 are rarely used, so you'll really have to pass them in here. 34 00:04:19,640 --> 00:04:27,970 Another concept now, doc strings and a doc stirring is usually passed 35 00:04:27,970 --> 00:04:33,910 as soon as you ride the class keyword, so just down the class keyword and that 36 00:04:33,910 --> 00:04:41,259 provides the explanation of your class. Let's say this class generates checking 37 00:04:41,259 --> 00:04:53,400 account object, and then you close that with three quotes, so what that does is 38 00:04:53,400 --> 00:05:01,449 you know if we print that out, so we grab one of our object instances, let's say 39 00:05:01,449 --> 00:05:13,889 John's checking dot double underscore doc and double under score again, so print that. 40 00:05:19,199 --> 00:05:24,520 So as you see a doc string is used to provide some information about that 41 00:05:24,520 --> 00:05:29,680 class, so it's good to define this string when you're writing classes so that 42 00:05:29,680 --> 00:05:35,740 others if they use your class and they can apply this operation here and they can 43 00:05:35,740 --> 00:05:40,630 see what your class is about. This is particularly useful when you're 44 00:05:40,630 --> 00:05:46,000 importing classes from modules, so when you don't actually see the code of your 45 00:05:46,000 --> 00:05:52,780 class, and you can do this and you can, you have access to the doc string of the 46 00:05:52,780 --> 00:05:59,340 class, and then we have the concept of data members which actually is referred 47 00:05:59,340 --> 00:06:06,970 to class variables or instance variables. So this is a data member, and this also is 48 00:06:06,970 --> 00:06:13,680 a data member as well. Then we have the constructor which was this init function. 49 00:06:13,680 --> 00:06:20,199 And as you know the init function, so the constructor constructs your class. Then 50 00:06:20,199 --> 00:06:26,259 you have class methods that you apply to your object instance, so transfer is a 51 00:06:26,259 --> 00:06:31,570 class method, so this is also a method but it's a special method, so it's 52 00:06:31,570 --> 00:06:36,400 a constructor, and instantiation if you hear about instantiation that is the 53 00:06:36,400 --> 00:06:42,550 process of creating object instances, so instances of a class like 54 00:06:42,550 --> 00:06:49,960 this is instantiation of the class, and this one here as well, and you know 55 00:06:49,960 --> 00:06:55,390 inheritance now. Inheritance is when you create a subclass out of a base class. 56 00:06:55,390 --> 00:07:02,800 And this subclass shares the methods of the base class plus it has its own 57 00:07:02,800 --> 00:07:09,520 methods that are specific to that subclass. Sometimes you also hear about 58 00:07:09,520 --> 00:07:17,410 attributes and attributes are these one here, so when you access these instance 59 00:07:17,410 --> 00:07:24,220 variables, or even, so class variables, or even instance variables, you can say 60 00:07:24,220 --> 00:07:30,220 you're accessing attributes of your class instance, so of your object. And yeah, 61 00:07:30,220 --> 00:07:35,830 that's about the vocabulary of object-oriented programming in Python. 62 00:07:35,830 --> 00:07:40,960 I really believe I have given you a solid understanding of object-oriented 63 00:07:40,960 --> 00:07:47,080 programming, and creating classes and objects and so on. If you still find 64 00:07:47,080 --> 00:07:51,490 something confusing you can either rewatch your videos 65 00:07:51,490 --> 00:07:55,210 because I know object-oriented programming is a bit tricky, but you can 66 00:07:55,210 --> 00:08:00,790 even ask questions, and I'll make my best to explain things that are not yet 67 00:08:00,790 --> 00:08:07,060 clear to you, and I hope you find this useful, and I'll talk you in the next 68 00:08:07,060 --> 00:08:11,410 lectures, so we're done with this section, and yeah, see you later.