1 00:00:04,540 --> 00:00:05,450 All right so in this video, 2 00:00:05,450 --> 00:00:07,290 we're gonna look at polymorphism 3 00:00:07,290 --> 00:00:10,690 and how it's an important part of the Python philosophy. 4 00:00:10,690 --> 00:00:13,270 Now before I explain what polymorphism is, 5 00:00:13,270 --> 00:00:14,450 let's have a look at some code 6 00:00:14,450 --> 00:00:16,340 and talk about what's going on. 7 00:00:16,340 --> 00:00:18,850 So I'm still in the game project we've been working on. 8 00:00:18,850 --> 00:00:21,610 I'm gonna clear out all the code from our main programme. 9 00:00:21,610 --> 00:00:23,900 I'm just gonna copy it all, delete the lot, 10 00:00:23,900 --> 00:00:27,820 and I'm gonna start by typing in a = 3, 11 00:00:27,820 --> 00:00:30,100 b = 2 in double quotes, 12 00:00:30,100 --> 00:00:31,520 c = 1, 2, 3. 13 00:00:33,650 --> 00:00:37,840 So we've got three variables, a, b, and c. 14 00:00:37,840 --> 00:00:40,410 And I've typed x there and not c. 15 00:00:40,410 --> 00:00:41,940 So a refers to an integer, 16 00:00:41,940 --> 00:00:44,650 b to a string, and c to a topple. 17 00:00:44,650 --> 00:00:47,070 Okay so let's print them out. 18 00:00:52,300 --> 00:00:53,470 Okay let's run 19 00:00:56,530 --> 00:00:58,680 and there's no real surprises at the result 20 00:00:58,680 --> 00:01:00,560 when I run the programme. 21 00:01:00,560 --> 00:01:04,379 So what that's done is pass three total different variables 22 00:01:04,379 --> 00:01:05,930 to the print function 23 00:01:05,930 --> 00:01:08,930 and it's happily printed out a value for each one. 24 00:01:08,930 --> 00:01:11,190 So what's happening here is that each object 25 00:01:11,190 --> 00:01:14,810 is behaving in a similar way when we try to print it. 26 00:01:14,810 --> 00:01:16,260 So in addition to their behaviour 27 00:01:16,260 --> 00:01:18,400 as an int, string, or a topple, 28 00:01:18,400 --> 00:01:21,230 they also have printable behaviour. 29 00:01:23,050 --> 00:01:26,040 So a is an int and we can subtract two ints 30 00:01:26,040 --> 00:01:27,810 to work out their difference. 31 00:01:27,810 --> 00:01:31,460 B is a string and you can't subtract strings in Python 32 00:01:31,460 --> 00:01:34,170 so they are very different types of object, 33 00:01:34,170 --> 00:01:38,340 but they also share printable behaviour and in that respect, 34 00:01:39,500 --> 00:01:42,170 they behave like a different type of object, 35 00:01:42,170 --> 00:01:44,110 an object that's printable. 36 00:01:44,110 --> 00:01:47,110 Now that ability of objects to have different forms 37 00:01:47,110 --> 00:01:51,280 is called polymorphism which just really means many forms. 38 00:01:53,740 --> 00:01:55,670 Now in this particular example, 39 00:01:55,670 --> 00:01:58,340 the polymorphic behaviour of the objects 40 00:01:58,340 --> 00:02:01,080 is implemented using inheritance. 41 00:02:01,080 --> 00:02:05,220 All Python objects inherit from a base class called object, 42 00:02:05,220 --> 00:02:07,890 which defines an __str__ method. 43 00:02:11,020 --> 00:02:13,570 So polymorphism allows the print function 44 00:02:13,570 --> 00:02:15,850 to accept arguments of any type 45 00:02:15,850 --> 00:02:18,940 and it's able to then print them out. 46 00:02:20,410 --> 00:02:23,460 Now even Java programmers wouldn't be surprised at this 47 00:02:23,460 --> 00:02:26,520 because you can do the same thing in Java. 48 00:02:26,520 --> 00:02:29,440 And in fact, I'm gonna do something really strange now 49 00:02:29,440 --> 00:02:33,360 and show you some Java code in a Python course. 50 00:02:35,680 --> 00:02:38,450 So this slide shows some of the declarations 51 00:02:38,450 --> 00:02:40,540 of the Java print method. 52 00:02:41,720 --> 00:02:45,340 Now all the lines starting with asterisks are comments 53 00:02:45,340 --> 00:02:48,940 so we're interested in the lines that start public void. 54 00:02:48,940 --> 00:02:50,330 So ignore the public, 55 00:02:50,330 --> 00:02:53,300 as we're not interested in scope or visibility in Java. 56 00:02:53,300 --> 00:02:55,670 In statically typed languages like Java, 57 00:02:55,670 --> 00:02:58,440 you have to specify the type of every parameter 58 00:02:58,440 --> 00:03:01,800 and also the type of the result that methods return. 59 00:03:01,800 --> 00:03:03,730 So here, void just means that these methods 60 00:03:03,730 --> 00:03:05,560 don't return anything. 61 00:03:06,730 --> 00:03:08,690 Now the first thing to notice 62 00:03:08,690 --> 00:03:11,730 is that there are several versions of this print statement, 63 00:03:11,730 --> 00:03:15,570 each one taking a different type of parameter. 64 00:03:16,620 --> 00:03:19,570 Now Java's a statically typed language 65 00:03:19,570 --> 00:03:22,790 and that means that the type of everything is checked 66 00:03:22,790 --> 00:03:24,580 when the program's compiled. 67 00:03:24,580 --> 00:03:26,800 So if you try to pass a string 68 00:03:26,800 --> 00:03:29,140 to a method that expects an int, 69 00:03:29,140 --> 00:03:31,290 then the programme won't compile. 70 00:03:31,290 --> 00:03:32,610 Now in Python, 71 00:03:32,610 --> 00:03:35,770 the type of something is only of interest when it's used 72 00:03:35,770 --> 00:03:39,390 and that's because Python's a dynamically typed language. 73 00:03:39,390 --> 00:03:41,270 So as you saw in the previous slide, 74 00:03:41,270 --> 00:03:43,930 we've got quite a few versions of that print method here. 75 00:03:43,930 --> 00:03:48,100 In fact, as we go to the next slide, there's more. 76 00:03:49,220 --> 00:03:50,970 So in this slide we can see versions 77 00:03:50,970 --> 00:03:55,140 that accept a char, an int, a long, a float, and a double. 78 00:03:57,930 --> 00:04:00,260 And here we can see the remaining versions 79 00:04:00,260 --> 00:04:05,230 that take an array of characters, a string, and an object. 80 00:04:05,230 --> 00:04:08,530 Now all the methods call another method, write, 81 00:04:08,530 --> 00:04:09,880 to do the printing, 82 00:04:09,880 --> 00:04:11,620 but what most of them have in common 83 00:04:11,620 --> 00:04:14,590 is that they call the String class' valueOf method 84 00:04:14,590 --> 00:04:18,380 to get the string that should be printed. 85 00:04:18,380 --> 00:04:21,490 Now the char and string versions are slightly different. 86 00:04:21,490 --> 00:04:23,100 A string is already a string 87 00:04:23,100 --> 00:04:26,420 so there's no need to do anything to convert it into one. 88 00:04:26,420 --> 00:04:29,680 And an array of characters is close enough to a string 89 00:04:29,680 --> 00:04:33,520 that there's no conversion needed there either. 90 00:04:33,520 --> 00:04:37,080 But all the other methods use this String.valueOf method 91 00:04:37,080 --> 00:04:41,250 to get the string representation of the thing to be printed. 92 00:04:43,770 --> 00:04:46,780 By the way, this is what's meant by overloading methods. 93 00:04:46,780 --> 00:04:49,100 You create different versions of the method 94 00:04:49,100 --> 00:04:50,970 that take different parameters 95 00:04:50,970 --> 00:04:53,960 and the compiler decides which one to use 96 00:04:53,960 --> 00:04:57,980 based on the number and type of the parameters passed to it. 97 00:04:57,980 --> 00:05:02,140 Now overloading isn't possible in Python and isn't necessary 98 00:05:02,140 --> 00:05:04,330 and we've seen how to get the same effect 99 00:05:04,330 --> 00:05:06,380 as having different numbers of parameters, 100 00:05:06,380 --> 00:05:09,920 using named parameters with default values. 101 00:05:09,920 --> 00:05:11,410 We automatically get the ability 102 00:05:11,410 --> 00:05:13,580 to pass parameters of different types 103 00:05:13,580 --> 00:05:16,560 because Python doesn't check the type of variables 104 00:05:16,560 --> 00:05:18,730 when it compiles the code. 105 00:05:20,640 --> 00:05:23,590 So in a statically typed language like Java, 106 00:05:23,590 --> 00:05:25,280 if you want to print a string 107 00:05:25,280 --> 00:05:28,230 then the compiler calls a version of the print method 108 00:05:28,230 --> 00:05:30,060 that can print strings. 109 00:05:30,060 --> 00:05:31,670 And if you wanna print an int 110 00:05:31,670 --> 00:05:35,040 then the compiler calls the version that deals with int. 111 00:05:35,040 --> 00:05:38,290 And similarly, with the last method on the slide, 112 00:05:38,290 --> 00:05:39,960 to print an object, 113 00:05:39,960 --> 00:05:43,170 the compiler calls this version that accepts a parameter 114 00:05:43,170 --> 00:05:44,940 whose type is Object. 115 00:05:44,940 --> 00:05:46,570 Now where it gets interesting 116 00:05:46,570 --> 00:05:49,590 and also relevant to us as Python programmers 117 00:05:49,590 --> 00:05:52,200 is what happens in the valueOf method 118 00:05:52,200 --> 00:05:55,370 so let's actually have a look at that. 119 00:05:57,060 --> 00:05:59,640 Once again, the methods have been overloaded 120 00:05:59,640 --> 00:06:02,430 to cope with the various types of the parameters 121 00:06:02,430 --> 00:06:04,010 that can be passed. 122 00:06:05,050 --> 00:06:08,320 Now the interesting thing here is that none of these methods 123 00:06:08,320 --> 00:06:12,750 attempts to work out what the value should be. 124 00:06:12,750 --> 00:06:13,880 In all cases, 125 00:06:13,880 --> 00:06:17,700 the valueOf method delegates the job of producing the string 126 00:06:17,700 --> 00:06:19,530 to the class involved. 127 00:06:21,340 --> 00:06:22,960 So if we wanna print an int, 128 00:06:22,960 --> 00:06:26,080 the method calls the Integer class' toString method. 129 00:06:26,080 --> 00:06:29,280 Similarly for Long, Float, and Double, 130 00:06:29,280 --> 00:06:31,160 the toString method's called 131 00:06:31,160 --> 00:06:34,530 to get the string representations of the object. 132 00:06:34,530 --> 00:06:37,180 And it's the same for the other types too, 133 00:06:37,180 --> 00:06:38,800 they just wouldn't fit on the screen 134 00:06:38,800 --> 00:06:41,980 so the valueOf method delegates the task of deciding 135 00:06:41,980 --> 00:06:44,960 what the string representation of each class should be, 136 00:06:44,960 --> 00:06:46,740 to the class itself. 137 00:06:46,740 --> 00:06:50,280 Now if valueOf just delegates the task to the class, 138 00:06:50,280 --> 00:06:53,730 it doesn't really need to know what class it's dealing with. 139 00:06:53,730 --> 00:06:55,350 In other words, whatever it gets, 140 00:06:55,350 --> 00:06:57,680 it could just call the toString method 141 00:06:57,680 --> 00:06:59,220 and return the result. 142 00:06:59,220 --> 00:07:02,440 And that turns out to be exactly the approach 143 00:07:02,440 --> 00:07:03,940 that Python takes. 144 00:07:07,230 --> 00:07:08,060 Now I'm not gonna go 145 00:07:08,060 --> 00:07:10,970 into the pros and cons of Java versus Python, 146 00:07:10,970 --> 00:07:14,970 nor of statically versus dynamically typed languages. 147 00:07:14,970 --> 00:07:17,600 I enjoy programming in both Java and Python 148 00:07:17,600 --> 00:07:19,990 and each have their own uses. 149 00:07:19,990 --> 00:07:23,300 What's important is the different philosophies adopted 150 00:07:23,300 --> 00:07:25,400 by the two languages. 151 00:07:25,400 --> 00:07:29,370 Now in Java, you have to specify exactly what type of object 152 00:07:29,370 --> 00:07:30,630 you'll be dealing with. 153 00:07:30,630 --> 00:07:32,950 Otherwise, the code won't even compile. 154 00:07:32,950 --> 00:07:34,790 And that means that every method 155 00:07:34,790 --> 00:07:38,960 knows exactly what it can do with the arguments it receives. 156 00:07:41,570 --> 00:07:44,170 Now Python takes a completely different approach. 157 00:07:44,170 --> 00:07:46,940 It doesn't care about what something is, 158 00:07:46,940 --> 00:07:50,250 it's only interested in how each thing behaves. 159 00:07:50,250 --> 00:07:53,170 So Python focuses on what something does, 160 00:07:53,170 --> 00:07:56,420 without worrying about what type it is. 161 00:07:59,350 --> 00:08:00,840 Now in both languages, 162 00:08:00,840 --> 00:08:05,420 classes inherit from the topmost base class called object 163 00:08:05,420 --> 00:08:08,770 and this base class defines a basic implementation 164 00:08:08,770 --> 00:08:12,030 of the toString method, in the case of Java, 165 00:08:12,030 --> 00:08:14,780 and the __str__ method in Python. 166 00:08:17,060 --> 00:08:19,960 Now the default implementation isn't very pretty, 167 00:08:19,960 --> 00:08:23,080 it just returns the name of the class and a hash code 168 00:08:23,080 --> 00:08:26,380 or the address in memory where the object lives. 169 00:08:26,380 --> 00:08:27,880 Now in both languages, 170 00:08:27,880 --> 00:08:30,430 the polymorphic behaviour of the classes, 171 00:08:30,430 --> 00:08:33,010 in the case of the toString and str methods, 172 00:08:33,010 --> 00:08:35,760 is implemented using inheritance. 173 00:08:38,110 --> 00:08:40,580 So polymorphism basically means 174 00:08:40,580 --> 00:08:44,350 that objects can be more than one thing at the same time. 175 00:08:44,350 --> 00:08:45,860 And then when int is a number, 176 00:08:45,860 --> 00:08:48,740 so it's something you can use to perform arithmetic, 177 00:08:48,740 --> 00:08:51,520 but it's also something that can be printed. 178 00:08:51,520 --> 00:08:55,210 So inheritance is one way to implement polymorphism. 179 00:08:55,210 --> 00:08:58,110 So in this example of making things printable, 180 00:08:58,110 --> 00:09:00,040 every object can be printed, 181 00:09:00,040 --> 00:09:03,780 as well as used for whatever else it does. 182 00:09:03,780 --> 00:09:06,220 Now this is possible because every object 183 00:09:06,220 --> 00:09:09,640 automatically inherits the __str__ method 184 00:09:11,460 --> 00:09:14,620 from it's object base class in Python. 185 00:09:16,730 --> 00:09:18,750 Now inheritance isn't the only way 186 00:09:18,750 --> 00:09:20,820 to implement polymorphism. 187 00:09:20,820 --> 00:09:22,900 Consider a ball and an orange. 188 00:09:22,900 --> 00:09:24,690 They both have a similar shape 189 00:09:24,690 --> 00:09:26,350 so we could juggle with balls 190 00:09:26,350 --> 00:09:28,490 and we could also juggle with oranges, 191 00:09:28,490 --> 00:09:30,370 if we can juggle at all of course, 192 00:09:30,370 --> 00:09:32,810 but they don't inherit their round shape 193 00:09:32,810 --> 00:09:34,910 from any common ancestor. 194 00:09:34,910 --> 00:09:37,800 So a ball and an orange are totally unrelated, 195 00:09:37,800 --> 00:09:40,180 but they can still share some properties 196 00:09:40,180 --> 00:09:43,680 without inheriting them from a base class. 197 00:09:45,090 --> 00:09:49,310 So to a juggler, balls and oranges have similar behaviour. 198 00:09:49,310 --> 00:09:50,710 To a hungry person, 199 00:09:50,710 --> 00:09:54,390 an orange probably has more in common with a loaf of bread 200 00:09:54,390 --> 00:09:56,240 because both can be eaten. 201 00:09:56,240 --> 00:09:59,600 So to stretch the example one more time before I move on, 202 00:09:59,600 --> 00:10:01,420 a gardener may be more interested 203 00:10:01,420 --> 00:10:03,490 in what happens when we plant the orange 204 00:10:03,490 --> 00:10:06,470 and might consider it to have more in common with an acorn, 205 00:10:06,470 --> 00:10:08,840 both can be used to grow a tree. 206 00:10:08,840 --> 00:10:12,170 Okay so going back to our print example. 207 00:10:13,830 --> 00:10:15,600 The print function in Python 208 00:10:15,600 --> 00:10:18,040 doesn't have to cater for the possibility 209 00:10:18,040 --> 00:10:20,190 that the thing it's being asked to print 210 00:10:20,190 --> 00:10:22,820 may not have a suitable method to return a string. 211 00:10:22,820 --> 00:10:25,900 And that's because every class automatically inherits 212 00:10:25,900 --> 00:10:28,780 the method from the object base class. 213 00:10:28,780 --> 00:10:29,870 So that's fine here, 214 00:10:29,870 --> 00:10:31,230 the print function 215 00:10:31,230 --> 00:10:33,880 can cope with anything that's passed to it 216 00:10:33,880 --> 00:10:35,350 because whatever it is, 217 00:10:35,350 --> 00:10:39,520 it's guaranteed to have a string method, str method. 218 00:10:40,850 --> 00:10:43,460 But what bout our enemies in Python? 219 00:10:43,460 --> 00:10:44,480 If we wrote a function 220 00:10:44,480 --> 00:10:46,660 that accepted an Enemy as a parameter 221 00:10:46,660 --> 00:10:49,810 and called the take_damage method of its parameter, 222 00:10:49,810 --> 00:10:52,910 there's no real guarantee that the object we passed to it 223 00:10:52,910 --> 00:10:56,430 would have a take_damage method in Python. 224 00:10:56,430 --> 00:10:59,340 Now we could pass a Player instance to it, for example, 225 00:10:59,340 --> 00:11:03,510 and we haven't given our Player class that method. 226 00:11:04,520 --> 00:11:06,610 Now in Java, that wouldn't be a problem. 227 00:11:06,610 --> 00:11:08,460 We'd have to specify Enemy, say, 228 00:11:08,460 --> 00:11:10,200 as the type of the parameter 229 00:11:10,200 --> 00:11:11,610 and the compiler would then check 230 00:11:11,610 --> 00:11:15,650 that anything we passed did inherit from the Enemy class. 231 00:11:15,650 --> 00:11:18,750 Now in Python though, there's no such checking. 232 00:11:18,750 --> 00:11:21,270 Python isn't interested in the type of objects, 233 00:11:21,270 --> 00:11:23,440 it's only interested in their behaviour 234 00:11:23,440 --> 00:11:25,520 at the time they're used. 235 00:11:26,630 --> 00:11:28,870 Okay so let's end the video here. 236 00:11:28,870 --> 00:11:31,660 In the next video, let's see how all this relates 237 00:11:31,660 --> 00:11:32,960 to Python code. 238 00:11:32,960 --> 00:11:35,020 See you in the next video.