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These are the user uploaded subtitles that are being translated: 1 1 00:00:05,660 --> 00:00:10,950 so continuing on from our previous video so we ended up talking about variables 2 2 00:00:10,950 --> 00:00:15,320 but saying that when a variable is bound to an instance of a class in 3 3 00:00:15,320 --> 00:00:20,130 Python its referred to as a data attribute now their other object orientated 4 4 00:00:20,130 --> 00:00:24,770 terms for the same thing so you might find variable attributes also called 5 5 00:00:24,770 --> 00:00:30,810 fields in languages such as Java or data members in C++ and I'll introduced yet 6 6 00:00:30,810 --> 00:00:32,060 another term shortly 7 7 00:00:32,060 --> 00:00:35,950 one borrowed from the small talk language but I over simplified things 8 8 00:00:35,950 --> 00:00:40,840 here and now we will be talking about different kinds of methods and 9 9 00:00:40,840 --> 00:00:46,270 attributes later but that's the basic jargon relating to classes which you 10 10 00:00:46,270 --> 00:00:50,280 should probably become familiar with now before I summarise these terms 11 11 00:00:50,280 --> 00:00:54,020 there's another way to use a string replacement fields that is useful when 12 12 00:00:54,020 --> 00:00:59,690 dealing with classes so because Kenwood and Hamilton are objects we can specify 13 13 00:00:59,690 --> 00:01:03,960 their attributes in the replacement fields and let's have a look at that 14 14 00:01:03,960 --> 00:01:08,550 to see how we can do that so we are gonna add that on line 20 we type.... 15 15 00:01:14,710 --> 00:01:28,860 .... 16 16 00:01:30,670 --> 00:01:44,070 ...so here because Kenwood and 17 17 00:01:44,070 --> 00:01:47,960 Hamilton are objects we can specify their attributes in the replacement 18 18 00:01:47,960 --> 00:01:52,290 fields so if we run this just to confirm we should see the identical results the 19 19 00:01:52,290 --> 00:02:01,480 print out as you can see their exactly the same but looking at the code I will argue that line 20 is easy to read because 20 20 00:02:01,480 --> 00:02:05,790 you can see immediately which attributes are being used at each point in the 21 21 00:02:05,790 --> 00:02:10,200 string and of course on line 18 you couldn't see that because there wasn't anything in the 22 22 00:02:10,200 --> 00:02:17,680 braces so just to summarize where we are at just gonna paste in that code in their now paste in those 23 23 00:02:17,680 --> 00:02:23,530 comments to be more precise so a class is a template for creating objects all 24 24 00:02:23,530 --> 00:02:26,950 objects created using the same class will have the same characteristics 25 25 00:02:26,950 --> 00:02:34,360 object is an instance of a class instantiate we didn't used the word instantiate but 26 26 00:02:34,360 --> 00:02:39,710 that is how the process of creating an instance of a class method is a function 27 27 00:02:39,710 --> 00:02:44,960 defined in a class and attribute is a variable bound to an instance of a 28 28 00:02:44,960 --> 00:02:50,390 class so as you can see here a class is a template for creating objects the 29 29 00:02:50,390 --> 00:02:53,810 template defines the attributes and methods that all instances of the class 30 30 00:02:53,810 --> 00:02:56,580 will have now although this is not true 31 31 00:02:56,580 --> 00:03:00,950 of other object oriented programming languages in Python a class is also 32 32 00:03:00,950 --> 00:03:06,070 exactly the same thing as a type other languages might refer to Kingwood in 33 33 00:03:06,070 --> 00:03:11,150 our sample program is being of type kettle but they also have 34 34 00:03:11,150 --> 00:03:17,250 types that are not classes but in Python every type is a class which is why we 35 35 00:03:17,250 --> 00:03:21,310 were able to call the add method of our int object at the start of the previous 36 36 00:03:21,310 --> 00:03:22,230 video 37 37 00:03:22,230 --> 00:03:26,530 an object is another name for an instance of a class then once the class 38 38 00:03:26,530 --> 00:03:30,980 has been defined it is the actual instances that are used in a program to 39 39 00:03:30,980 --> 00:03:34,640 store the values of the attributes and perform actions on them and you saw 40 40 00:03:34,640 --> 00:03:39,280 instances being created on line 9 and line 16 on the screen so when 41 41 00:03:39,280 --> 00:03:43,400 creating a new object from a class definition the terms uses instantiating 42 42 00:03:43,400 --> 00:03:48,210 an object and that's just a fancy word for creating an instance of a class but 43 43 00:03:48,210 --> 00:03:53,050 you should be familiar with that term now method is a function as bound to an 44 44 00:03:53,050 --> 00:03:57,010 instance of a class now in the Tkinter videos you might recall we created 45 45 00:03:57,010 --> 00:04:01,430 instances of the back label and canvas classes and we used their grid and pack 46 46 00:04:01,430 --> 00:04:07,000 methods to place them on the screen and finally an attribute is a variable that 47 47 00:04:07,000 --> 00:04:11,570 is bound to an instance of a class so the kettle variable and kenwood can keep 48 48 00:04:11,570 --> 00:04:15,990 track of the values of its making price without affecting the attributes 49 49 00:04:15,990 --> 00:04:22,180 of the Hamilton object which has got its own values for those attributes so as I 50 50 00:04:22,180 --> 00:04:26,060 hinted at earlier this is a slight over-simplification in the last two 51 51 00:04:26,060 --> 00:04:29,960 definitions and i will be explaining a bit more about that a little bit later on in the 52 52 00:04:29,960 --> 00:04:33,920 section but we have to walk here before we can run so those definitions will give 53 53 00:04:33,920 --> 00:04:37,910 us something to work with while we get the hang of classes and object oriented 54 54 00:04:37,910 --> 00:04:42,940 programming in general so continuing on now with our kettle class at the moment 55 55 00:04:42,940 --> 00:04:47,880 it has 2 data attributes make and price and a single method called init 56 56 00:04:47,880 --> 00:04:51,060 __init__ which you can see is 57 57 00:04:51,060 --> 00:04:55,790 showing on line three and if you look at that lit looks like it's sets the values for 58 58 00:04:55,790 --> 00:04:58,790 the attributes but also looks a little strange 59 59 00:04:59,360 --> 00:05:04,110 now as I explain what's going on here I'm gonna be referring to quite a few things whos names 60 60 00:05:04,110 --> 00:05:08,920 start with and end with 2 underscores and it's going to get a lil bit tedious 61 61 00:05:08,920 --> 00:05:12,410 if I keep saying __ all the time so I'm just going to use the name 62 62 00:05:12,410 --> 00:05:16,070 of the object and you can see on the screen that its got underscores around 63 63 00:05:16,070 --> 00:05:19,750 it but I will review this use of underscores and how it relates to 64 64 00:05:19,750 --> 00:05:23,420 classes later on but just keep in mind that time when we refer to something for 65 65 00:05:23,420 --> 00:05:26,810 example on line 3 as init I'm referring to __init__ 66 66 00:05:26,810 --> 00:05:32,010 now we looked at most of the users of underscore around 67 67 00:05:32,010 --> 00:05:34,000 variable names in the previous section 68 68 00:05:34,000 --> 00:05:38,910 and theirs a little bit more to add to that once we get more into classes now this 69 69 00:05:38,910 --> 00:05:39,640 class 70 70 00:05:39,640 --> 00:05:44,310 obviously doesn't do a great deal and theirs not really a lot going on and it's 71 71 00:05:44,310 --> 00:05:47,700 not really a lot you can do with a kettle other than switching it on and wait but 72 72 00:05:47,700 --> 00:05:52,910 I will add a method to do that before looking at more complicated classes now the Python 73 73 00:05:52,910 --> 00:05:55,200 style guide pep8 by the way 74 74 00:05:55,200 --> 00:05:59,290 recommends just single blank line between methods inside a class which is 75 75 00:05:59,290 --> 00:06:03,810 different to the recommendation and there should be two blank lines between 76 76 00:06:03,810 --> 00:06:09,030 top level functions in a module so if you use 2 blanks lines then IntelliJ will show 77 77 00:06:09,030 --> 00:06:12,750 warning and personally I like to leave 2 blank lines after class definitions 78 78 00:06:12,750 --> 00:06:17,290 but this is not covered in the style guide and you won't get a warning unless you leave more than two blank 79 79 00:06:17,290 --> 00:06:18,630 lines and 80 80 00:06:18,630 --> 00:06:21,630 I'm gonna add a second method now 81 81 00:06:23,280 --> 00:06:34,570 and if we start this....and notice as I press the left bracket 82 82 00:06:34,570 --> 00:06:39,790 IntelliJ automatically added the self parameter so as I mentioned functions that are bound to a 83 83 00:06:39,790 --> 00:06:44,530 class called methods and main difference between a method and a function is the 84 84 00:06:44,530 --> 00:06:49,390 presence of this self parameter that's been added automatically just go ahead and 85 85 00:06:49,390 --> 00:07:01,660 finish this method....and just a reminder if I go to and add that as a 86 86 00:07:01,660 --> 00:07:06,570 second add 2 lines between methods we get the warning from IntelliJ 2 87 87 00:07:06,570 --> 00:07:07,780 blanks lines so if your in a 88 88 00:07:07,780 --> 00:07:14,330 class only have one line between methods to remove that warning and you might be 89 89 00:07:14,330 --> 00:07:18,200 wondering what is the self parameter well so there's nothing really special 90 90 00:07:18,200 --> 00:07:23,200 about the name self or the parameter and you have called it anything you wanted avoiding 91 91 00:07:23,200 --> 00:07:28,140 name conflicts with other objects of course but unlike to key word this in Java for 92 92 00:07:28,140 --> 00:07:32,910 example self is just the name of a parameter the important thing is the 93 93 00:07:32,910 --> 00:07:36,910 presence of the parameter not what its called so having said that the 94 94 00:07:36,910 --> 00:07:40,950 convention is that it's called self and although this is not enforced in any way 95 95 00:07:40,950 --> 00:07:45,010 it's not a good idea to change the convention is so well established that 96 96 00:07:45,010 --> 00:07:47,160 tools may expect to be called self and may not 97 97 00:07:47,160 --> 00:07:52,420 work if you use another name in fact as we have just seen IntelliJ automatically use the 98 98 00:07:52,420 --> 00:07:53,830 name self for it 99 99 00:07:53,830 --> 00:07:57,820 when we are defining the method just now so what is self 100 100 00:07:57,820 --> 00:08:02,780 well the answer is that it's a reference to the instance of the class now if you 101 101 00:08:02,780 --> 00:08:07,670 used to C++ or Java then you can think of it as equivalent to this and it's 102 102 00:08:07,670 --> 00:08:11,730 used to refer to instance variables and you could see that we're doing that 103 103 00:08:11,730 --> 00:08:18,290 on lines four five and six and again on line 9 and although we've use many methods in the early 104 104 00:08:18,290 --> 00:08:21,910 lectures in this course we've never had to specify an argument for the self 105 105 00:08:21,910 --> 00:08:26,160 parameter and this is because Python supplies the value automatically when 106 106 00:08:26,160 --> 00:08:29,820 calling the method now I will talked about this a little bit more in a minute but first 107 107 00:08:29,820 --> 00:08:41,970 let's call the method and check to see if it is a kettle or not so we type.... 108 108 00:08:41,970 --> 00:08:53,830 ....and notice that again when I typed that.... 109 109 00:08:53,830 --> 00:08:59,340 ....you can see the definition shows the self parameter but when I press enter it automatically 110 110 00:08:59,340 --> 00:09:03,240 there's no parameter passed in the brackets and if we just confirm that.... 111 111 00:09:03,240 --> 00:09:15,020 ...and if we run this first time we ran it on line 33 was false 112 112 00:09:15,020 --> 00:09:19,930 that's because up here on line six we defined it as false second time after 113 113 00:09:19,930 --> 00:09:25,470 calling Hamilton.switch_on the variable the attribute in this case for 114 114 00:09:25,470 --> 00:09:31,470 our hamilton instance was set to true in other words the value of the hamiltons instance on 115 115 00:09:31,470 --> 00:09:35,190 started off as false because that's what it was initialized to in the constructor 116 116 00:09:35,190 --> 00:09:40,440 and calling the switch-on method on line 34 set to true which is the value of 117 117 00:09:40,440 --> 00:09:45,960 course we are getting when we print out the value for the second time on line 35 now the 118 118 00:09:45,960 --> 00:09:50,770 term constructor by the way refers to a special method that is executed when an 119 119 00:09:50,770 --> 00:09:55,590 instance of a class is created or constructed in Python this is the init 120 120 00:09:55,590 --> 00:09:58,180 method that you can see defined on line 3 121 121 00:09:58,180 --> 00:10:03,150 and as you saw when I typed in line 34 we don't provide a value for the 122 122 00:10:03,150 --> 00:10:07,000 self parameter when calling the switch_on method when you call a method 123 123 00:10:07,000 --> 00:10:08,300 on an instance 124 124 00:10:08,300 --> 00:10:12,560 Python automatically provides a reference to the instance as the first parameter to 125 125 00:10:12,560 --> 00:10:17,110 the method now we can see this in a bit more detail by not using and instance but 126 126 00:10:17,110 --> 00:10:21,860 calling the method the class itself instead so we come down here and put this 127 127 00:10:21,860 --> 00:10:30,780 on lines 37 we can type.... 128 128 00:10:30,780 --> 00:10:43,030 ...so if we run that you can see that we got the value true 129 129 00:10:43,030 --> 00:10:46,950 and of course Hamilton initially when we created the instance of the 130 130 00:10:46,950 --> 00:10:52,460 object was then constructor made it the value false that by calling this method in an 131 131 00:10:52,460 --> 00:10:57,080 alternative ways you can see they're using the class definition initially you 132 132 00:10:57,080 --> 00:11:00,960 can see what's happening there and the parameters has been passed so instead of 133 133 00:11:00,960 --> 00:11:05,420 using an instance of kettle to call a method we've use the class itself because 134 134 00:11:05,420 --> 00:11:09,830 there is no instance here the value for self has to be specified and failing to do so 135 135 00:11:09,830 --> 00:11:15,280 would give an error so the two forms have exactly the same effect and as you can see 136 136 00:11:15,280 --> 00:11:19,960 when we ran the program kettle switch on ended up returning true so we can see 137 137 00:11:19,960 --> 00:11:24,500 that it was updated correctly so lets go back to the definition again I just want to 138 138 00:11:24,500 --> 00:11:32,330 go back and delete this out...just to confirm that we may have miss this the first and when I add the bracket not the parameter 139 139 00:11:32,330 --> 00:11:36,110 showing up there as self so its asking for that parameter in this case and I can 140 140 00:11:36,110 --> 00:11:40,960 change it too kenwood but again just to confirm if we do the 141 141 00:11:40,960 --> 00:11:49,860 alternative syntax......so when I do that 142 142 00:11:49,860 --> 00:11:54,140 noting that the self parameter is showing at the top of the pop up but when 143 143 00:11:54,140 --> 00:11:59,260 I press enter notice how it didn't ask for the parameter in this case because it knows 144 144 00:11:59,260 --> 00:12:02,210 we've already got an instance that we're working on 145 145 00:12:02,210 --> 00:12:06,160 so the implementation of classes in Python can create and some unexpected 146 146 00:12:06,160 --> 00:12:07,550 behavior 147 147 00:12:07,550 --> 00:12:11,780 partly because it's a mix of modular three and C++ with some small talk thrown 148 148 00:12:11,780 --> 00:12:15,900 in for good measure and once again if you are learning object oriented programming for 149 149 00:12:15,900 --> 00:12:19,780 the first time using Python then you'll probably be quite happy with the way 150 150 00:12:19,780 --> 00:12:24,140 things work that's not to say you won't have to concentrate but there shouldn't be 151 151 00:12:24,140 --> 00:12:31,070 any surprises but for C++ and Java programmers it might be a good idea to put what you already know to the back 152 152 00:12:31,070 --> 00:12:35,310 of your mind and be prepared to learn a new way of thinking about things when it 153 153 00:12:35,310 --> 00:12:38,410 comes to classes and object-oriented programming in Python 154 154 00:12:39,410 --> 00:12:43,440 now just like everything else in Python classes are dynamic and it can be modified 155 155 00:12:43,440 --> 00:12:47,630 after their created now to see this in action and reinforce my previous comment 156 156 00:12:47,630 --> 00:12:51,690 about surprises for C++ and Java programmers I'm going to add a measure 157 157 00:12:51,690 --> 00:12:56,430 for the keyword kettles power and kilowatts let's go ahead and do that so 158 158 00:12:56,430 --> 00:13:03,730 come down here to line 41 a bit more space so we can see..... 159 159 00:13:03,730 --> 00:13:22,070 .... 160 160 00:13:27,970 --> 00:13:32,450 so this case now the Kenwood instance of the kettle class now has another 161 161 00:13:32,450 --> 00:13:38,470 data attribute called power with the value 1.5 now remember that variables 162 162 00:13:38,470 --> 00:13:42,310 in Python come into existence the first time they're assigned a value and the 163 163 00:13:42,310 --> 00:13:46,750 same thing is true for instance variables which is the small talk name for data 164 164 00:13:46,750 --> 00:13:52,140 attributes so here I created a data attribute for the Kenwood object called power 165 165 00:13:52,140 --> 00:13:58,940 and I gave it a value of 1.5 so the term instance variable is useful because it describes 166 166 00:13:58,940 --> 00:14:04,510 power as a variable that is bound to an instance of the kettle class so as a 167 167 00:14:04,510 --> 00:14:10,750 result the following code I'm gonna add shouldn't be surprising..... 168 168 00:14:10,750 --> 00:14:26,290 ...and if we run this we get an error message attribute kettle object has not attribute power so if I close that 169 169 00:14:26,290 --> 00:14:31,200 down again and that's because the Hamilton instance doesn't have a power attribute 170 170 00:14:31,200 --> 00:14:36,170 because if you're not created one by assigning a value to it on line 43 we added 171 171 00:14:36,170 --> 00:14:40,940 an instance variable for the Kenwood instance of the kettle class but of course we didn't 172 172 00:14:40,940 --> 00:14:44,710 do that for the Hamilton instance and that's why we got an error when we run 173 173 00:14:44,710 --> 00:14:45,510 the program 174 174 00:14:45,510 --> 00:14:50,280 alright so I'm going to finish this video here now in the next video we're going to talk more 175 175 00:14:50,280 --> 00:14:53,860 about objects and find a bit more about instance variables 176 176 00:14:54,360 --> 00:15:00,010 and continue on generally speaking in our discussion and in our journey finding out more 177 177 00:15:00,010 --> 00:15:03,050 about object oriented programming in Python see you in the next video 21119