1 1 00:00:00,520 --> 00:00:01,940 Hi guys and welcome back. 2 2 00:00:01,940 --> 00:00:04,600 In this video, we're going to talk about static methods 3 3 00:00:04,600 --> 00:00:06,140 and class methods. 4 4 00:00:06,140 --> 00:00:08,990 These can be a little bit confusing so let's get started. 5 5 00:00:08,990 --> 00:00:12,550 Here, we've got a class that I have called ClassTest. 6 6 00:00:12,550 --> 00:00:15,040 This is just to show you how these things work. 7 7 00:00:15,040 --> 00:00:18,160 And then we have defined a method. 8 8 00:00:18,160 --> 00:00:21,630 We know it's a method because it is a function, 9 9 00:00:21,630 --> 00:00:23,280 which starts with a def keyword, 10 10 00:00:23,280 --> 00:00:27,040 and it has self as a first parameter. 11 11 00:00:27,040 --> 00:00:30,950 All functions inside the class that use the object 12 12 00:00:30,950 --> 00:00:33,380 as the first parameter, 13 13 00:00:33,380 --> 00:00:35,673 are called instance methods. 14 14 00:00:36,770 --> 00:00:38,860 And this instance_method, 15 15 00:00:38,860 --> 00:00:41,030 prints that you've called the instance_method. 16 16 00:00:41,030 --> 00:00:44,210 How would you call this method if you wanted to call it? 17 17 00:00:44,210 --> 00:00:46,640 You have two options. 18 18 00:00:46,640 --> 00:00:50,610 Okay, the first option is to do something like, 19 19 00:00:50,610 --> 00:00:55,610 test = ClassTest, and create a new object of type ClassTest. 20 20 00:00:56,780 --> 00:00:59,780 Notice that we don't have an init method in this class, 21 21 00:00:59,780 --> 00:01:00,810 but that's okay. 22 22 00:01:00,810 --> 00:01:02,105 If you don't have an init method, 23 23 00:01:02,105 --> 00:01:03,071 that just means that you 24 24 00:01:03,071 --> 00:01:05,380 don't have any initialization to do, 25 25 00:01:05,380 --> 00:01:08,470 but the self is still created 26 26 00:01:08,470 --> 00:01:10,310 and you still get a new object. 27 27 00:01:10,310 --> 00:01:13,050 And make it equal to your variable. 28 28 00:01:13,050 --> 00:01:14,210 Here, we're creating a new object 29 29 00:01:14,210 --> 00:01:18,170 and you can do test.instance_method, or as we know it, 30 30 00:01:18,170 --> 00:01:20,770 you can ClassTest.instance_method 31 31 00:01:20,770 --> 00:01:22,630 with test as the argument. 32 32 00:01:22,630 --> 00:01:24,610 Both of these things are exactly the same. 33 33 00:01:24,610 --> 00:01:27,670 If you run this code, you'll get that you've called 34 34 00:01:27,670 --> 00:01:30,430 the instance_method of this object. 35 35 00:01:30,430 --> 00:01:33,770 Notice that we are printing the object self here, 36 36 00:01:33,770 --> 00:01:35,630 and as we saw earlier on, 37 37 00:01:35,630 --> 00:01:38,590 you get back this ugly print out. 38 38 00:01:38,590 --> 00:01:40,470 If you wanted it to be a little bit nicer, 39 39 00:01:40,470 --> 00:01:44,050 you would have to implement your str or repr methods, 40 40 00:01:44,050 --> 00:01:45,050 one of the two. 41 41 00:01:45,050 --> 00:01:49,150 But for now, we have called the instance_method two times. 42 42 00:01:49,150 --> 00:01:52,800 It is called an instance_method because you call it 43 43 00:01:52,800 --> 00:01:55,430 on a class instance. 44 44 00:01:55,430 --> 00:01:58,600 The name instance, it means the same thing, an object. 45 45 00:01:58,600 --> 00:02:00,980 What we're doing here is we're creating an object 46 46 00:02:00,980 --> 00:02:01,880 of type ClassTest. 47 47 00:02:03,220 --> 00:02:04,530 You can also say that you're creating 48 48 00:02:04,530 --> 00:02:06,410 an instance of ClassTest. 49 49 00:02:06,410 --> 00:02:08,170 They mean the same thing. 50 50 00:02:08,170 --> 00:02:10,653 That is why these are called instance methods. 51 51 00:02:11,750 --> 00:02:16,200 Instance methods need the object in order to call them. 52 52 00:02:16,200 --> 00:02:20,290 The next type is called a class method. 53 53 00:02:20,290 --> 00:02:23,280 Here, you can define another method. 54 54 00:02:23,280 --> 00:02:28,280 But now instead of the instance, or the object, self, 55 55 00:02:28,360 --> 00:02:31,000 it actually takes a different parameter 56 56 00:02:31,000 --> 00:02:33,610 that we usually in Python, call cls, 57 57 00:02:33,610 --> 00:02:35,550 but you can call this parameter whatever you want, 58 58 00:02:35,550 --> 00:02:37,470 the value will still be the same no matter 59 59 00:02:37,470 --> 00:02:39,520 what you call this parameter. 60 60 00:02:39,520 --> 00:02:42,740 What this parameter will be in a class method, 61 61 00:02:42,740 --> 00:02:45,870 that is when you put this at @classmethod on top 62 62 00:02:45,870 --> 00:02:50,573 of the method definition, is this will be the class itself. 63 63 00:02:51,930 --> 00:02:54,510 Let me show you how this works. 64 64 00:02:54,510 --> 00:02:57,380 Called class_method of {cls}. 65 65 00:02:57,380 --> 00:02:59,740 I'll just print the parameter there, 66 66 00:02:59,740 --> 00:03:02,130 and then, because it is a class method, 67 67 00:03:02,130 --> 00:03:03,370 it's not an instance method, 68 68 00:03:03,370 --> 00:03:05,480 you no longer need an instance. 69 69 00:03:05,480 --> 00:03:06,800 That means that you can just do, 70 70 00:03:06,800 --> 00:03:10,960 ClassTest.class_method, and that's it, 71 71 00:03:10,960 --> 00:03:13,040 you don't have to pass in anything in there 72 72 00:03:13,040 --> 00:03:16,080 because we no longer need the object. 73 73 00:03:16,080 --> 00:03:17,980 Python is smart enough that when you do 74 74 00:03:17,980 --> 00:03:21,820 ClassTest.class_method and you don't pass anything in there, 75 75 00:03:21,820 --> 00:03:25,940 Python will actually pass ClassTest in there. 76 76 00:03:25,940 --> 00:03:29,920 So, actually cls is ClassTest, 77 77 00:03:29,920 --> 00:03:32,443 because that is what you called the method on. 78 78 00:03:33,520 --> 00:03:34,620 That is how this works. 79 79 00:03:34,620 --> 00:03:36,650 When you do ClassTest.class_method, 80 80 00:03:36,650 --> 00:03:40,040 given that you have put this @classmethod on top. 81 81 00:03:40,040 --> 00:03:41,710 We're going to learn what this is later on. 82 82 00:03:41,710 --> 00:03:43,450 Don't worry about it too much. 83 83 00:03:43,450 --> 00:03:47,600 Then Python will pass in the class as the argument 84 84 00:03:47,600 --> 00:03:48,793 to this parameter. 85 85 00:03:49,740 --> 00:03:52,110 If we save and play, 86 86 00:03:52,110 --> 00:03:53,910 you'll see that now you get that you've called 87 87 00:03:53,910 --> 00:03:58,120 the class_method of this class called ClassTest. 88 88 00:03:58,120 --> 00:04:00,760 Don't worry too much about the __main__, 89 89 00:04:00,760 --> 00:04:03,370 we will learn about that later on if it becomes relevant. 90 90 00:04:03,370 --> 00:04:06,160 It's largely irrelevant to most Python programmes. 91 91 00:04:06,160 --> 00:04:08,430 This just means that you've called the class_method 92 92 00:04:08,430 --> 00:04:09,690 of this class. 93 93 00:04:09,690 --> 00:04:11,850 We're going to find used cases for class_methods 94 94 00:04:11,850 --> 00:04:14,220 in just a little bit later on in this video. 95 95 00:04:14,220 --> 00:04:17,510 Finally, the last option is the @staticmethod. 96 96 00:04:18,426 --> 00:04:21,804 And this defines a static method that I'm going to call 97 97 00:04:21,804 --> 00:04:22,760 static_method. 98 98 00:04:22,760 --> 00:04:24,340 Again, you can call it whatever you want 99 99 00:04:24,340 --> 00:04:28,580 and now I'm not going to put anything inside the brackets. 100 100 00:04:28,580 --> 00:04:31,993 Here, we're going to do "Called static_method." 101 101 00:04:32,950 --> 00:04:35,880 In a static method, that is a method 102 102 00:04:35,880 --> 00:04:39,370 that doesn't have a parameter cls or self, 103 103 00:04:39,370 --> 00:04:42,370 and that has @staticmethod on top. 104 104 00:04:42,370 --> 00:04:45,710 These methods don't get anything when you call them. 105 105 00:04:45,710 --> 00:04:48,210 The class method gets the class normally 106 106 00:04:48,210 --> 00:04:50,090 and you can print it out or use it. 107 107 00:04:50,090 --> 00:04:52,900 The instance_methods gets the instance or the object 108 108 00:04:52,900 --> 00:04:55,360 and you can print it out or use it in them. 109 109 00:04:55,360 --> 00:04:58,560 But static methods don't get either of those things. 110 110 00:04:58,560 --> 00:05:00,710 And you can call them in the same way 111 111 00:05:00,710 --> 00:05:03,720 just by doing static_method inside ClassTest 112 112 00:05:03,720 --> 00:05:07,330 or ClassTest.static_method but Python will not put 113 113 00:05:07,330 --> 00:05:09,040 the ClassTest in the brackets. 114 114 00:05:09,040 --> 00:05:11,140 It will just call that method. 115 115 00:05:11,140 --> 00:05:13,480 This really isn't a method. 116 116 00:05:13,480 --> 00:05:16,710 It's just a function that you've placed inside a class. 117 117 00:05:16,710 --> 00:05:18,840 We should still call it a method but it doesn't have 118 118 00:05:18,840 --> 00:05:21,630 any information about the class or the object 119 119 00:05:21,630 --> 00:05:24,560 so it's really just its own separate function 120 120 00:05:24,560 --> 00:05:26,800 that lives inside the class. 121 121 00:05:26,800 --> 00:05:29,400 If you want a function inside the class 122 122 00:05:29,400 --> 00:05:31,410 that doesn't use the class for anything, 123 123 00:05:31,410 --> 00:05:35,630 or the instance, you can decorate it with @staticmethod. 124 124 00:05:35,630 --> 00:05:38,600 If you want a method that uses the class for something, 125 125 00:05:38,600 --> 00:05:41,500 then you can decorate it @classmethod. 126 126 00:05:41,500 --> 00:05:42,960 And if you want to use the object, 127 127 00:05:42,960 --> 00:05:45,820 or if you wanna create any other type of instance_method, 128 128 00:05:45,820 --> 00:05:47,940 then just don't put anything on top. 129 129 00:05:47,940 --> 00:05:50,240 No @classmethod or @staticmethod 130 130 00:05:50,240 --> 00:05:52,610 and make sure to include your parameter there. 131 131 00:05:52,610 --> 00:05:55,010 Again, if you call this like that and you press play, 132 132 00:05:55,010 --> 00:05:57,720 you just get Called static_method. 133 133 00:05:57,720 --> 00:05:59,683 So, what are these used for? 134 134 00:06:00,530 --> 00:06:02,880 Instance methods are used for most things. 135 135 00:06:02,880 --> 00:06:05,800 When you wanna produce an action that uses the data 136 136 00:06:05,800 --> 00:06:08,340 inside the object that you created earlier on for example, 137 137 00:06:08,340 --> 00:06:11,230 that is when instance methods would get used. 138 138 00:06:11,230 --> 00:06:13,620 Also, if you wanna call a method to modify 139 139 00:06:13,620 --> 00:06:16,110 some sort of data inside self or the object, 140 140 00:06:16,110 --> 00:06:17,840 then you would also use an instance method. 141 141 00:06:17,840 --> 00:06:21,930 Class methods are used often as factories 142 142 00:06:21,930 --> 00:06:24,130 and I'm going to show you what that is in a second. 143 143 00:06:24,130 --> 00:06:27,570 And finally, static_methods are used to just place a method 144 144 00:06:27,570 --> 00:06:28,670 inside a class. 145 145 00:06:28,670 --> 00:06:31,270 Because you feel like it belongs there for some reason. 146 146 00:06:31,270 --> 00:06:33,330 For you as a developer, you wanna put that method in there 147 147 00:06:33,330 --> 00:06:36,920 because it makes sense logically for code organisation 148 148 00:06:36,920 --> 00:06:39,410 or something like that, then you can use that. 149 149 00:06:39,410 --> 00:06:41,040 I'll say though that most of the time, 150 150 00:06:41,040 --> 00:06:43,610 you'll be using class methods and instance methods. 151 151 00:06:43,610 --> 00:06:46,010 You won't be using static methods all that much. 152 152 00:06:46,900 --> 00:06:49,570 Let's start with our factory example 153 153 00:06:49,570 --> 00:06:52,440 that is going to show you how you might use a class method. 154 154 00:06:52,440 --> 00:06:54,610 This example is gonna be a little bit tricky 155 155 00:06:54,610 --> 00:06:56,010 so please bear with me for a second 156 156 00:06:56,010 --> 00:06:58,930 while I explain first what I've done here. 157 157 00:06:58,930 --> 00:07:01,290 In a class, as well as methods, 158 158 00:07:01,290 --> 00:07:03,670 you can also put variables 159 159 00:07:03,670 --> 00:07:06,610 and these become class properties. 160 160 00:07:06,610 --> 00:07:10,270 If you wanted to print out this tuple here, 161 161 00:07:10,270 --> 00:07:13,680 all you have to do is access the TYPES tuple 162 162 00:07:13,680 --> 00:07:15,660 inside the Book class. 163 163 00:07:15,660 --> 00:07:18,690 You would do Book.TYPES, 164 164 00:07:18,690 --> 00:07:20,930 and that is how you access this tuple there. 165 165 00:07:20,930 --> 00:07:22,090 You can print it out 166 166 00:07:22,090 --> 00:07:24,793 and you can see that you get hardcover and paperback. 167 167 00:07:25,750 --> 00:07:29,130 We have some data stored inside the class, 168 168 00:07:29,130 --> 00:07:30,400 which is this tuple here, 169 169 00:07:30,400 --> 00:07:33,290 and a reason why you might put some data inside a class, 170 170 00:07:33,290 --> 00:07:35,650 as opposed to putting it outside the class, 171 171 00:07:35,650 --> 00:07:37,650 is because you feel like it belongs there 172 172 00:07:37,650 --> 00:07:40,030 or only the Book class is going to use it, 173 173 00:07:40,030 --> 00:07:41,820 or maybe there is some sort of reason, 174 174 00:07:41,820 --> 00:07:44,410 logical reason to put that data in there. 175 175 00:07:44,410 --> 00:07:46,950 Here I'm creating a class that talks about Books. 176 176 00:07:46,950 --> 00:07:50,053 So, it makes sense to have book types inside there. 177 177 00:07:51,040 --> 00:07:53,690 Then we're going to create an init method for this class. 178 178 00:07:53,690 --> 00:07:55,660 Remember, because this is an instance method 179 179 00:07:55,660 --> 00:07:57,920 it creates an instance and gives it back to you, 180 180 00:07:57,920 --> 00:07:59,720 you need the self parameter there. 181 181 00:07:59,720 --> 00:08:01,830 You can call self whatever you want, once again, 182 182 00:08:01,830 --> 00:08:04,130 but the convention in Python is to call it self. 183 183 00:08:04,130 --> 00:08:06,610 And then we're going to be creating objects 184 184 00:08:06,610 --> 00:08:09,280 of type Book that have three things. 185 185 00:08:09,280 --> 00:08:12,570 A name, a book type, and a weight. 186 186 00:08:12,570 --> 00:08:14,850 So, we will do self.name = name, 187 187 00:08:14,850 --> 00:08:16,183 Self.book_type = book_type, 188 188 00:08:17,610 --> 00:08:19,470 and self.weight = weight. 189 189 00:08:19,470 --> 00:08:21,460 This is a very common thing in Python 190 190 00:08:21,460 --> 00:08:23,640 to create an object with three different values 191 191 00:08:23,640 --> 00:08:26,280 that you then store each one inside a property 192 192 00:08:26,280 --> 00:08:27,970 of the object itself. 193 193 00:08:27,970 --> 00:08:30,070 Just to recap, if you were to create a variable 194 194 00:08:30,070 --> 00:08:32,260 such as Book, and make it an object, 195 195 00:08:32,260 --> 00:08:35,410 then you will need to pass in some information, 196 196 00:08:35,410 --> 00:08:39,230 let's say, "Harry Potter", "hardcover", and 1500, 197 197 00:08:39,230 --> 00:08:42,200 then you can print book.name 198 198 00:08:42,200 --> 00:08:43,640 and what that's going to do is it's going 199 199 00:08:43,640 --> 00:08:47,210 to pass in "Harry Potter" as the value for this parameter. 200 200 00:08:47,210 --> 00:08:50,040 Then that is going to be assigned to the name property 201 201 00:08:50,040 --> 00:08:52,400 of the self object, which is essentially 202 202 00:08:52,400 --> 00:08:54,730 an empty container at this stage, 203 203 00:08:54,730 --> 00:08:56,810 and then we're going to print it out down here. 204 204 00:08:56,810 --> 00:08:58,340 So if we press play, 205 205 00:08:58,340 --> 00:08:59,820 you'll see that "Harry Potter" comes out. 206 206 00:08:59,820 --> 00:09:01,560 That's how the data is flowing through 207 207 00:09:01,560 --> 00:09:03,730 this system at this stage. 208 208 00:09:03,730 --> 00:09:05,990 Then I'm also going to create a repr method 209 209 00:09:05,990 --> 00:09:07,040 so that it's a little bit easier 210 210 00:09:07,040 --> 00:09:08,660 to work with these books. 211 211 00:09:08,660 --> 00:09:12,430 And we are going to return a Book with this name. 212 212 00:09:12,430 --> 00:09:13,800 It is this Book type 213 213 00:09:13,800 --> 00:09:18,590 and it's weighing {self.weight}g>. 214 214 00:09:18,590 --> 00:09:22,170 This is a representation of the book that would allow us, 215 215 00:09:22,170 --> 00:09:25,050 with all the data included inside, 216 216 00:09:25,050 --> 00:09:27,440 to recreate a book object if we wanted to. 217 217 00:09:27,440 --> 00:09:29,920 Remember, that is the purpose of the repr method. 218 218 00:09:29,920 --> 00:09:32,090 If you wanted to just print the book out nicely 219 219 00:09:32,090 --> 00:09:33,420 for users to read, 220 220 00:09:33,420 --> 00:09:36,500 then you could use the str method instead. 221 221 00:09:36,500 --> 00:09:37,930 Now instead of putting in book.name, 222 222 00:09:37,930 --> 00:09:40,780 we're just going to print book and I will press play 223 223 00:09:40,780 --> 00:09:42,390 and show you what the output is. 224 224 00:09:42,390 --> 00:09:43,747 Here's the book, "Harry Potter." 225 225 00:09:43,747 --> 00:09:47,190 It's a hardcover, weighing 1500 grammes. 226 226 00:09:47,190 --> 00:09:51,030 Now we come into the factories. 227 227 00:09:51,030 --> 00:09:52,810 Here's where we're gonna use class_methods. 228 228 00:09:52,810 --> 00:09:56,660 I wanna avoid, when I'm creating a new book object, 229 229 00:09:56,660 --> 00:09:58,400 when I'm calling the init method, 230 230 00:09:58,400 --> 00:10:01,910 I want to avoid passing in this "hardcover" string 231 231 00:10:01,910 --> 00:10:04,290 because I only want to be able to create books 232 232 00:10:04,290 --> 00:10:06,760 that are either hardcover or paperback, 233 233 00:10:06,760 --> 00:10:08,810 I wanna make sure that when I create a book, 234 234 00:10:08,810 --> 00:10:11,220 I use one of these two types. 235 235 00:10:11,220 --> 00:10:15,090 Right now, I can pass in comic book if I wanted to here 236 236 00:10:15,090 --> 00:10:17,840 and that would be totally fine because again, 237 237 00:10:17,840 --> 00:10:20,990 the data, this string here, becomes the value of book_type. 238 238 00:10:20,990 --> 00:10:23,470 Book_type gets assigned to self.book_type, 239 239 00:10:23,470 --> 00:10:24,580 and that gets used down here. 240 240 00:10:24,580 --> 00:10:26,770 There is no check for this, 241 241 00:10:26,770 --> 00:10:28,703 nothing is using this at the moment. 242 242 00:10:29,610 --> 00:10:32,920 What I'm going to do is I'm going to create a method 243 243 00:10:32,920 --> 00:10:36,360 that will take in the name and the weight 244 244 00:10:36,360 --> 00:10:40,340 and will create a new book object of type hardcover. 245 245 00:10:40,340 --> 00:10:44,063 I'm going to call this @classmethod hardcover, 246 246 00:10:45,450 --> 00:10:46,940 and again, because it is a class method, 247 247 00:10:46,940 --> 00:10:49,170 it takes in the class as the first parameter, 248 248 00:10:49,170 --> 00:10:51,080 usually called that cls in Python. 249 249 00:10:51,080 --> 00:10:53,220 And that is going to take in the name 250 250 00:10:53,220 --> 00:10:55,370 and the page weight, 251 251 00:10:55,370 --> 00:10:59,500 and what it's going to do is it is going to return a book 252 252 00:10:59,500 --> 00:11:02,310 with the name as the first argument, 253 253 00:11:02,310 --> 00:11:04,030 which then goes into the init method. 254 254 00:11:04,030 --> 00:11:09,030 Book.TYPES[0] as the second argument. 255 255 00:11:09,680 --> 00:11:12,180 Remember, Book.TYPES gives us this tuple. 256 256 00:11:12,180 --> 00:11:15,700 Book.TYPES[0] gives us the hardcover. 257 257 00:11:15,700 --> 00:11:18,957 And finally, the page_weight + 100 258 258 00:11:19,901 --> 00:11:21,940 for the weight of the book. 259 259 00:11:21,940 --> 00:11:23,690 I'm adding 100 because it's a hardcover 260 260 00:11:23,690 --> 00:11:25,290 so it's probably going to be a little heavier 261 261 00:11:25,290 --> 00:11:26,810 than a paperback. 262 262 00:11:26,810 --> 00:11:27,870 Hopefully this makes sense. 263 263 00:11:27,870 --> 00:11:30,710 It is a little bit confusing because you are using 264 264 00:11:30,710 --> 00:11:35,080 the class inside a method defined inside the class. 265 265 00:11:35,080 --> 00:11:36,750 Which I know is a little bit weird, 266 266 00:11:36,750 --> 00:11:40,700 but that is how you can create a new object 267 267 00:11:40,700 --> 00:11:42,233 inside of a class. 268 268 00:11:42,233 --> 00:11:44,630 This is something you can totally do. 269 269 00:11:44,630 --> 00:11:46,060 And because it is a class method, 270 270 00:11:46,060 --> 00:11:48,860 you no longer need to create your own object first. 271 271 00:11:48,860 --> 00:11:49,860 All you have to do now is say, 272 272 00:11:49,860 --> 00:11:53,640 Book.hardcover and pass in here "Harry Potter" 273 273 00:11:53,640 --> 00:11:56,370 and the page weight which was 1500. 274 274 00:11:56,370 --> 00:11:59,650 Notice that this hardcover method takes in the name 275 275 00:11:59,650 --> 00:12:02,360 and the page weight but it does not take in a type. 276 276 00:12:02,360 --> 00:12:04,730 The type is added inside the method, 277 277 00:12:04,730 --> 00:12:07,520 you don't have to pass it in as an argument. 278 278 00:12:07,520 --> 00:12:09,240 If I save and run that, 279 279 00:12:09,240 --> 00:12:11,130 you can see that now you get a book "Harry Potter," 280 280 00:12:11,130 --> 00:12:13,770 which is a hardcover, and weighs 1600 grammes. 281 281 00:12:13,770 --> 00:12:17,230 Let's do the same thing for the paperback. 282 282 00:12:17,230 --> 00:12:18,860 Only now instead of Book.TYPES[0], 283 283 00:12:18,860 --> 00:12:20,330 I'm going to do Book.TYPES[1] 284 284 00:12:20,330 --> 00:12:22,853 and I'm going to remove this + 100. 285 285 00:12:23,740 --> 00:12:26,070 So now, if we wanted to create a softcover book, 286 286 00:12:26,070 --> 00:12:27,630 or a paperback, you can say something like, 287 287 00:12:27,630 --> 00:12:30,147 light = Book.paperback("Python 101"), 288 288 00:12:31,220 --> 00:12:33,360 and you can say for example, 600. 289 289 00:12:33,360 --> 00:12:35,890 If you then print the light book as well, 290 290 00:12:35,890 --> 00:12:38,460 you'll see that you get two books printed out now. 291 291 00:12:38,460 --> 00:12:41,290 Book "Harry Potter," which is hardcover, weighing 1600 grammes 292 292 00:12:41,290 --> 00:12:43,440 and Book "Python 101," which is a paperback, 293 293 00:12:43,440 --> 00:12:45,190 weighing 600 grammes. 294 294 00:12:45,190 --> 00:12:49,390 This is a very, very common way of using class methods 295 295 00:12:49,390 --> 00:12:51,700 because you have access to the class itself 296 296 00:12:51,700 --> 00:12:54,790 inside the method, that means that it is a perfect place 297 297 00:12:54,790 --> 00:12:59,500 to be creating new objects by using that class. 298 298 00:12:59,500 --> 00:13:01,850 With that said though, notice that I'm using Book here 299 299 00:13:01,850 --> 00:13:03,810 and I'm not using cls anywhere. 300 300 00:13:03,810 --> 00:13:07,580 Book is the class as we saw earlier on. 301 301 00:13:07,580 --> 00:13:10,300 And cls is also the class. 302 302 00:13:10,300 --> 00:13:11,950 They are interchangeable. 303 303 00:13:11,950 --> 00:13:15,590 Instead of Book, you can put cls in there. 304 304 00:13:15,590 --> 00:13:19,230 And instead of Book down here, you can put cls in there. 305 305 00:13:19,230 --> 00:13:20,700 You should do that. 306 306 00:13:20,700 --> 00:13:24,490 Use cls instead of Book and that will come in handy 307 307 00:13:24,490 --> 00:13:26,510 when you look at inheritance, 308 308 00:13:26,510 --> 00:13:28,330 which we will look at later on. 309 309 00:13:28,330 --> 00:13:30,420 Just because using cls gives you a little bit more 310 310 00:13:30,420 --> 00:13:32,650 flexibility down the line. 311 311 00:13:32,650 --> 00:13:35,670 Do use cls there since you've got access to it 312 312 00:13:35,670 --> 00:13:37,960 and if you were creating a static method 313 313 00:13:37,960 --> 00:13:39,050 instead of a class method, 314 314 00:13:39,050 --> 00:13:40,820 you would have to use book class 315 315 00:13:40,820 --> 00:13:42,680 because you wouldn't have cls. 316 316 00:13:42,680 --> 00:13:45,150 If you want to use cls use a class method, 317 317 00:13:45,150 --> 00:13:48,450 use the paramter, and you can create new objects 318 318 00:13:48,450 --> 00:13:50,810 and return them or use them however you like. 319 319 00:13:50,810 --> 00:13:52,460 Hopefully this makes sense. 320 320 00:13:52,460 --> 00:13:54,360 I know that it's a little bit confusing 321 321 00:13:54,360 --> 00:13:56,400 looking at class methods and static methods 322 322 00:13:56,400 --> 00:13:57,930 and it's been a bit of a long video 323 323 00:13:57,930 --> 00:13:59,630 but hopefully it was helpful. 324 324 00:13:59,630 --> 00:14:02,280 Hope you enjoyed it and I'll see you in the next one.