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These are the user uploaded subtitles that are being translated: 1 1 00:00:05,540 --> 00:00:10,630 so just as a recap from the previous video when we run this we do get an error 2 2 00:00:10,630 --> 00:00:16,470 and looking at the error we can see objects has no attribute power and the 3 3 00:00:16,470 --> 00:00:21,460 error is on line 45 which you can see on the screen 4 4 00:00:21,460 --> 00:00:26,690 and the reason for that is the Hamilton instance doesn't have a power 5 5 00:00:26,690 --> 00:00:31,260 attribute because we haven't created one by assigning a value to it like we did for 6 6 00:00:31,260 --> 00:00:36,780 the Kenwood instance on line 43 and this is the dynamic nature of Python that 7 7 00:00:36,780 --> 00:00:40,800 allows this kind of behaviour and you can easily end up with instances that are 8 8 00:00:40,800 --> 00:00:45,160 created from the same class template but which ultimately have 9 9 00:00:45,160 --> 00:00:50,410 different attributes and it can be useful feature but can also cause 10 10 00:00:50,410 --> 00:00:54,710 problems if you make a typing error when trying to assign a value to an existing 11 11 00:00:54,710 --> 00:00:59,800 data attribute for argument sake so it's worth paying careful attention to the 12 12 00:00:59,800 --> 00:01:04,370 IntelliJ tool tips that will appear after you type a do and to make sure that 13 13 00:01:04,370 --> 00:01:09,400 you're assigning it to attribute that you really expecting it to be so 14 14 00:01:09,400 --> 00:01:13,840 going back to the analogy of a class definition being like house plan that 15 15 00:01:13,840 --> 00:01:18,320 can be used to create houses in the real world theirs nothing to stop an extension 16 16 00:01:18,320 --> 00:01:21,700 being added to a house out after its built and that's pretty much what is happening 17 17 00:01:21,700 --> 00:01:26,820 here now of course subclassing which is where a new class is created from an 18 18 00:01:26,820 --> 00:01:32,010 existing one may be preferable to adding attributes to instances and will be 19 19 00:01:32,010 --> 00:01:36,360 looking at sub classes a little later there are ways to prevent this kind of 20 20 00:01:36,360 --> 00:01:41,200 behaviour and you can create classes in such a way that additional 21 21 00:01:41,200 --> 00:01:45,800 attributes can't be added to instances forcing classes to be sub class extra 22 22 00:01:45,800 --> 00:01:52,070 functionality is required but Python allow you to take either approach in now their is one aspect of 23 23 00:01:52,070 --> 00:01:55,930 classes that I want to mention before we start practicing all this by creating 24 24 00:01:55,930 --> 00:02:02,340 some more useful classes now I introduced to small talk term instance variable in 25 25 00:02:02,340 --> 00:02:06,870 the previous video and that's because it includes the words variable whereas data 26 26 00:02:06,870 --> 00:02:11,840 attribute does not and this is important because methods are also attributes of 27 27 00:02:11,840 --> 00:02:17,410 classes and you find the term attribute used to refer to both in the documentation 28 28 00:02:17,410 --> 00:02:21,850 so the terms data attribute and method are used to distinguish between the two 29 29 00:02:21,850 --> 00:02:27,260 types of attribute now the other aspect that I want to mention is that classes also 30 30 00:02:27,260 --> 00:02:31,490 have attributes and once again the Smalltalk term instance variable is 31 31 00:02:31,490 --> 00:02:36,660 useful because it contains the word instance now the data attributes in the 32 32 00:02:36,660 --> 00:02:42,400 kettle example as such as make and price have both been attributes of the instances and each 33 33 00:02:42,400 --> 00:02:47,240 instance has its own values for them so its also possible for the class to have 34 34 00:02:47,240 --> 00:02:51,400 attributes which is shared by all the instances so to stick with the analogy 35 35 00:02:51,400 --> 00:02:55,830 of a house plan each house that we built from the plan will have different attributes such 36 36 00:02:55,830 --> 00:03:00,500 as its address however all the houses will share some attributes such as the type of 37 37 00:03:00,500 --> 00:03:05,070 house now all house are built from a plan for a four-bedroom lodge would be four 38 38 00:03:05,070 --> 00:03:10,100 bedroom lodges so our kettle class is modelling electric kettles so we can 39 39 00:03:10,100 --> 00:03:13,630 introduce a class attribute called power source that all instances will share 40 40 00:03:14,830 --> 00:03:18,290 so lets go ahead and do that so we are gonna go back up to the top of the definition for the kettle 41 41 00:03:18,290 --> 00:03:31,870 class and on line 32 I'm going to add....so 42 42 00:03:31,870 --> 00:03:35,180 that's a class attribute that we've added called power source 43 43 00:03:36,540 --> 00:03:43,160 so now we've done that we can demonstrate that all instances share a single version of this class attribute so we are gonna add the following 44 44 00:03:43,160 --> 00:03:51,440 at the end of the file but first I'm going to comment out line 47 to remove that error so we 45 45 00:03:51,440 --> 00:04:01,220 can try doing... 46 46 00:04:02,030 --> 00:04:04,140 ... 47 47 00:04:04,140 --> 00:04:14,060 .....now if you run that...you could see that all three have got 48 48 00:04:14,060 --> 00:04:19,610 electricity as the power source so the two instances kenwood and hamilton plus the class 49 49 00:04:19,610 --> 00:04:27,200 itself kettle have got this power source class attribute so think of that if you know 50 50 00:04:27,200 --> 00:04:33,130 Java or C++ this is very similar to static fields in those languages similar 51 51 00:04:33,130 --> 00:04:39,270 but not exactly the same but although its a useful comparison if you are used to 52 52 00:04:39,270 --> 00:04:43,640 those other languages don't take this to literally it is useful to examine the 53 53 00:04:43,640 --> 00:04:47,310 namespaces of the three objects to verify that the two instances are 54 54 00:04:47,310 --> 00:04:52,200 sharing the same attribute which only exists in the class and we can access the 55 55 00:04:52,200 --> 00:05:02,540 namespace via the dic attribute so lets go ahead and do that so... 56 56 00:05:02,540 --> 00:05:08,750 .... 57 57 00:05:08,750 --> 00:05:21,410 ....and 58 58 00:05:21,410 --> 00:05:30,180 I'm going to change or run it first move this as a bit more space I'm gonna move to bottom we can see 59 59 00:05:30,180 --> 00:05:34,500 its a little bit better and looking at that output down the bottom of the screen the 60 60 00:05:34,500 --> 00:05:38,680 kettle class namespace contains quite a lot of items and we can see power 61 61 00:05:38,680 --> 00:05:42,630 source showing their and we can see some of the other functions we 62 62 00:05:42,630 --> 00:05:46,230 can see an init function and we should be able to see our switch on 63 63 00:05:46,230 --> 00:05:50,540 method if we scroll over and their is the switch on 64 64 00:05:50,540 --> 00:05:59,040 kettle.switch_on so that's their and the 2 instances below that you can see that 65 65 00:05:59,040 --> 00:06:05,710 they've got far less in their namespace and in fact theirs only the instance variables make price and on 66 66 00:06:05,710 --> 00:06:10,740 and you see Kenwood has got the extra power attribute that's the second from 67 67 00:06:10,740 --> 00:06:13,630 bottom one that we added earlier in this code but other than that 68 68 00:06:13,630 --> 00:06:17,790 their the same so what happens is that when we try to access the power_source 69 69 00:06:17,790 --> 00:06:23,010 attribute for the instances Python checks to see if the power source exists in the 70 70 00:06:23,010 --> 00:06:28,080 instance name space if it doesn't which is the case here it then checks the class 71 71 00:06:28,080 --> 00:06:32,290 for the instance and finds power source in the kettle class and that's why the 72 72 00:06:32,290 --> 00:06:36,390 reason it printed out because basically it got it from the class attribute so if 73 73 00:06:36,390 --> 00:06:43,690 you close it down if you go ahead and make a change with the power source if we do something like this 74 74 00:06:43,690 --> 00:06:54,480 so.... 75 75 00:06:55,420 --> 00:06:58,420 .... 76 76 00:06:59,860 --> 00:07:07,670 so if we run this again you can see what happens there is that atomic we are switching to atomic power and the three 77 77 00:07:07,670 --> 00:07:12,570 printouts are now all the same again as well and only updated the class attribute 78 78 00:07:12,570 --> 00:07:18,200 but it has changed the other two instances one automatically which is more or less sort of proof that the 79 79 00:07:18,200 --> 00:07:23,130 instances for Hamilton and kenwood are looking at the class attribute at that 80 80 00:07:23,130 --> 00:07:27,450 point and this is probably not surprising if Python is your first 81 81 00:07:27,450 --> 00:07:30,700 experience of object oriented programming and will probably make 82 82 00:07:30,700 --> 00:07:34,220 perfect sense after our discussion of global variables and functions that we've talk 83 83 00:07:34,220 --> 00:07:41,080 about previously so as long as we only access the class data attribute via the class rather than via 84 84 00:07:41,080 --> 00:07:47,740 instance then when we want to assign a new value to it this works as expected so all 3 objects are still sharing their 85 85 00:07:47,740 --> 00:07:52,050 common power source in this scenario and show the same value in this case atomic 86 86 00:07:52,050 --> 00:07:55,950 but that's not true if you try and change this in different ways so if we 87 87 00:07:55,950 --> 00:08:06,240 come down to say the kenwood instance below their will do.... 88 88 00:08:06,240 --> 00:08:12,010 ... 89 89 00:08:13,520 --> 00:08:22,360 ...so if we run that now...you can see in the bottom here we've got atomic for the class attribute we change kenwood to gas and that is showing 90 90 00:08:22,360 --> 00:08:25,040 as gas and Hamilton still showing atomic 91 91 00:08:25,040 --> 00:08:30,080 and the class attribute is still showing as atomic so this is probably not that 92 92 00:08:30,080 --> 00:08:34,780 surprising if Python is your first experience of object-oriented programming and would 93 93 00:08:34,780 --> 00:08:38,260 probably make perfect sense after our discussion in previous videos about 94 94 00:08:38,260 --> 00:08:42,440 global variables and functions remember that as soon as we try to assign a value 95 95 00:08:42,440 --> 00:08:44,270 to a global variable 96 96 00:08:44,270 --> 00:08:48,980 Python created a new local variable that shattered the global one so a 97 97 00:08:48,980 --> 00:08:53,470 similar thing is actually happening in this scenario when we added the code to 98 98 00:08:53,470 --> 00:09:01,280 switch the Kenwood power source to gas the code on line 53 so although kenwood 99 99 00:09:01,280 --> 00:09:05,190 is now running on gas hamilton's is still running on atomic power which is one of 100 100 00:09:05,190 --> 00:09:10,250 the ways that class attributes differ from Java static fields and examining 101 101 00:09:10,250 --> 00:09:18,280 the source if we run this again and look at the namespaces notice now that Kenwood second to 102 102 00:09:18,280 --> 00:09:22,500 bottom one has now got its own data attribute called power_source that 103 103 00:09:22,500 --> 00:09:26,530 shadows the class attribute so this is definitely something to watch out for as 104 104 00:09:26,530 --> 00:09:31,090 it's quite easy to make the mistake of assigning a new value to a class attribute 105 105 00:09:31,090 --> 00:09:35,000 via an instance variable and Java will give you a warning if you try to access 106 106 00:09:35,000 --> 00:09:39,230 as a static field via an instance variable but you won't get any such warnings in 107 107 00:09:39,230 --> 00:09:45,550 Python so that's the basic of classes and their is a quite a bit of jargon and new concepts that we've thrown in 108 108 00:09:45,550 --> 00:09:49,410 there and it may not make complete sense at the moment but don't worry because I'm 109 109 00:09:49,410 --> 00:09:52,620 gonna be using a lot of examples in the next set of videos that will consolidate 110 110 00:09:52,620 --> 00:09:57,090 what's been covered so far a simple kettle class doesn't really do very much 111 111 00:09:57,090 --> 00:10:02,490 but as we create more class and using them to encapsulate data and functionality it'll 112 112 00:10:02,490 --> 00:10:06,220 start making a lot more sense now also we've introduced other object-oriented 113 113 00:10:06,220 --> 00:10:10,100 concepts such as inheritance and composition so that we can build more 114 114 00:10:10,100 --> 00:10:14,890 complicated class structures and the other thing I do is also introduce other 115 115 00:10:14,890 --> 00:10:19,790 object-oriented concepts such as inheritance and composition so that we 116 116 00:10:19,790 --> 00:10:23,580 can build more complicated class structures so we'll start off slow in 117 117 00:10:23,580 --> 00:10:27,020 the next few videos will look at some more examples of using classes see you 118 118 00:10:27,020 --> 00:10:27,440 in the next video 14394