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These are the user uploaded subtitles that are being translated: 1 1 00:00:05,600 --> 00:00:09,260 an important part of object-oriented programming is the concept of 2 2 00:00:09,260 --> 00:00:14,880 encapsulation so the idea here is that objects contain the data and the methods 3 3 00:00:14,880 --> 00:00:19,720 that operate on that data and don't expose the actual implementation to the 4 4 00:00:19,720 --> 00:00:24,960 outside world so it's worth noting that object oriented programming is not the 5 5 00:00:24,960 --> 00:00:30,050 only way to achieve this and languages such as modular 2 aimed to 6 6 00:00:30,050 --> 00:00:36,420 provide encapsulation by using modules rather than objects modular 2 enhance to 7 7 00:00:36,420 --> 00:00:41,080 become modular 3 which was really quite influential in the design 8 8 00:00:41,080 --> 00:00:46,670 of Java, C++ and also Python although the object approach to encapsulation was 9 9 00:00:46,670 --> 00:00:51,380 the only one adopted by Java Python allows us to encapsulate data methods 10 10 00:00:51,380 --> 00:00:56,050 using either an object orientated paradigm or a modular approach and we 11 11 00:00:56,050 --> 00:01:00,190 saw the modular approach when using the pytz module to handle time zone 12 12 00:01:00,190 --> 00:01:05,690 information earlier in this course so you don't have to know how pytz was 13 13 00:01:05,690 --> 00:01:08,080 implemented in order to use it 14 14 00:01:08,080 --> 00:01:12,180 the method that provides and the time zone information it uses are encapsulated 15 15 00:01:12,180 --> 00:01:18,730 within the pytz module in fact pytz also uses classes internally so it's possible 16 16 00:01:18,730 --> 00:01:23,590 to mix both methods quite happily now I repeatedly pointed out that everything 17 17 00:01:23,590 --> 00:01:28,560 in Python is an object so it may seem strange that a language that is really 18 18 00:01:28,560 --> 00:01:34,670 so heavily based on objects doesn't restrict us to using an object approach this is 19 19 00:01:34,670 --> 00:01:39,950 one of the important design decisions of the Python language the basic principle 20 20 00:01:39,950 --> 00:01:44,860 being that we're all responsible adults at least when programming now I mention this 21 21 00:01:44,860 --> 00:01:49,640 because many languages work by imposing restrictions on what can be done with 22 22 00:01:49,640 --> 00:01:55,320 encapsulated data Java and C++ for example restrict access to class fields 23 23 00:01:55,320 --> 00:01:59,930 by allowing them to be marked as private in Python you can indicate that a 24 24 00:01:59,930 --> 00:02:03,740 method or attribute should be considered non public but it's extremely 25 25 00:02:03,740 --> 00:02:08,460 difficult to fully enforce this once you've been introduced to the idea of 26 26 00:02:08,460 --> 00:02:12,610 encapsulation it can be tempting to spend a lot of time trying to enforce it 27 27 00:02:12,610 --> 00:02:13,470 in Python 28 28 00:02:13,470 --> 00:02:17,620 especially if he used 2 other languages such as Java that do a reasonably good 29 29 00:02:17,620 --> 00:02:22,410 job of preventing access to private data but you be more productive if you accept 30 30 00:02:22,410 --> 00:02:25,070 that Python takes a different approach 31 31 00:02:25,070 --> 00:02:28,360 neither approach is necessarily better they are just different and each has its own 32 32 00:02:28,360 --> 00:02:33,260 advantages and disadvantages so let's continue our introduction to classes in 33 33 00:02:33,260 --> 00:02:37,690 Python with another example we are going to look at the concept of a bank account and 34 34 00:02:37,690 --> 00:02:42,340 the classes are going to enable money to be deposited withdrawn and will hopefully 35 35 00:02:42,340 --> 00:02:46,660 anyway maintain the correct balance for the account so I'm going to use the same 36 36 00:02:46,660 --> 00:02:50,450 file we use previously but I'm just going to create a new Python file so we are going call this one 37 37 00:02:51,340 --> 00:03:01,130 accounts....so let's start first by creating the class 38 38 00:03:01,130 --> 00:03:08,640 so we are gonna call the class account so.... and it's going to be a simple 39 39 00:03:08,640 --> 00:03:11,990 .... 40 40 00:03:15,200 --> 00:03:27,650 .... 41 41 00:03:27,650 --> 00:03:34,370 ....and incidentally the convention is to start class names with a capital letter and to use 42 42 00:03:34,370 --> 00:03:38,790 camel case being that each time you got a different word you capitalized 43 43 00:03:38,790 --> 00:03:44,710 just that word so if we're calling this the account number class we would type something like that 44 44 00:03:44,710 --> 00:03:46,430 capitalize each 45 45 00:03:46,430 --> 00:03:51,090 word but basically start with a capital letter so that is the convention in Python so 46 46 00:03:51,090 --> 00:03:55,730 ok will continue on with our init method so we want to passed name and balance to 47 47 00:03:55,730 --> 00:04:04,270 it and we are going to assign the attributes.... 48 48 00:04:04,270 --> 00:04:16,990 ..... 49 49 00:04:16,990 --> 00:04:22,370 .... 50 50 00:04:22,370 --> 00:04:30,230 ...and we are going to just check the amount that is passed 51 51 00:04:30,230 --> 00:04:39,960 to make sure that its greater than 0...and only add it to our balance if its greater than 0 and we will add the amount to what 52 52 00:04:39,960 --> 00:04:49,330 the current balance was like so....next let's do a withdraw method.... 53 53 00:04:49,330 --> 00:04:56,510 ... 54 54 00:04:56,510 --> 00:05:23,400 .... 55 55 00:05:23,400 --> 00:05:31,970 .....ok so there's our class and as I just mentioned the convention is to 56 56 00:05:31,970 --> 00:05:37,320 start class names with a capital letter and and to use camel case with that said you'll find 57 57 00:05:37,320 --> 00:05:41,880 many the built-in classes in Python do use lowercase nevertheless IntelliJ will 58 58 00:05:41,880 --> 00:05:45,390 give you a warning if you use lowercase or separate paths of the class name with 59 59 00:05:45,390 --> 00:05:58,100 underscores...another warning as you can see should only use case convention it's not 60 60 00:05:58,100 --> 00:06:05,220 happy if we use underscores now I've included a dot string on line 2 at the start of the class and it's a good idea to do 61 61 00:06:05,220 --> 00:06:09,620 this with all your classes although it can appear to be a bit redundant with simple classes like 62 62 00:06:09,620 --> 00:06:14,400 this one which he can really figure out the functionality just by glancing it's 63 63 00:06:14,400 --> 00:06:17,410 a good habit to get into and once your class structures get more complicated 64 64 00:06:18,230 --> 00:06:21,910 you will be glad that you did do this and so will anyone else who uses your classes 65 65 00:06:21,910 --> 00:06:27,160 and I'll be talking more about doc strings later when I show you what they used for in 66 66 00:06:27,160 --> 00:06:31,790 more detail now the init method as you recall is used to initialize the 67 67 00:06:31,790 --> 00:06:36,540 class and it's extremely rare to create a class without providing an init method 68 68 00:06:36,540 --> 00:06:41,530 now in an earlier video I used the term constructor to described the init method but 69 69 00:06:41,530 --> 00:06:46,210 really that's not strictly true in the new style Python classes that will be 70 70 00:06:46,210 --> 00:06:51,700 using for most of this section the Class creation process involves two steps so 71 71 00:06:51,700 --> 00:06:56,380 the first method to be called when a class instance is created is new and this 72 72 00:06:56,380 --> 00:07:01,450 takes care of the actual creation the init method then customizes the instance 73 73 00:07:01,450 --> 00:07:07,100 performing tasks such as giving values to the data attributes for example so 74 74 00:07:07,100 --> 00:07:11,890 technically the class constructor is the new method which is actually calling 75 75 00:07:11,890 --> 00:07:16,050 underscore underscore new underscore underscore as you probably expect 76 76 00:07:16,050 --> 00:07:20,580 generally speaking you don't need to define new except in special cases 77 77 00:07:21,160 --> 00:07:25,200 when subclassing certain types of classes so I won't be discussing you 78 78 00:07:25,200 --> 00:07:29,610 much in this section as a result now old style classes were the only type 79 79 00:07:29,610 --> 00:07:34,750 of class available in Python 2 up until 2.2 when the new style classes are 80 80 00:07:34,750 --> 00:07:40,050 introduced so in Python 2.2 onwards you could use both types of 81 81 00:07:40,050 --> 00:07:44,700 classes I will discuss the Python 2 classes later on 82 82 00:07:44,700 --> 00:07:48,980 so that your aware of the differences and can recognize them but in Python 3 83 83 00:07:48,980 --> 00:07:52,760 there are only the new style classes and as a results I'm going to be 84 84 00:07:52,760 --> 00:07:57,290 concentrating on those as we move forward so getting back to the account 85 85 00:07:57,290 --> 00:08:01,730 class the init method takes two parameters in addition to self and these 86 86 00:08:01,730 --> 00:08:07,300 are used to set the values of the data attributes name and balance now notice that all 87 87 00:08:07,300 --> 00:08:10,480 the methods takes self as their 1st parameter and you saw IntelliJ 88 88 00:08:10,480 --> 00:08:14,270 automatically adding that for us and whenever we want to refer to the data 89 89 00:08:14,270 --> 00:08:18,350 attributes we have to use the form self. and whatever the name of the attribute is 90 90 00:08:18,350 --> 00:08:23,980 again as you see me type in this class in Python there is no shorthand and self 91 91 00:08:23,980 --> 00:08:28,640 must be specified if your used to java you can omit the word this before 92 92 00:08:28,640 --> 00:08:32,980 a field if their references is not ambiguous but in Python you can't do that so you need 93 93 00:08:32,980 --> 00:08:37,330 to get into the habit of doing that each time I just wanna remember i just fixed that 94 94 00:08:37,330 --> 00:08:44,050 those warnings and add the extra space to keep IntelliJ happy now the positive withdrawal methods 95 95 00:08:44,050 --> 00:08:48,370 obviously a pretty simple and they adjust the balance by the amount passed to the parameter 96 96 00:08:48,370 --> 00:08:53,160 they also check that the amounts are positive although as withdrawal its just a negative 97 97 00:08:53,160 --> 00:08:57,410 deposit and a deposit is just a negative withdrawal we could have used a single 98 98 00:08:57,410 --> 00:09:01,890 method to handle both cases now there are extra steps that are really sure of 99 99 00:09:01,890 --> 00:09:05,010 implemented in the withdrawal method to make sure there is enough money in the 100 100 00:09:05,010 --> 00:09:09,550 account though so it makes sense to keep the two transaction type separate the 101 101 00:09:09,550 --> 00:09:14,330 show balance method again very simple it just prints the balance data attribute so now the 102 102 00:09:14,330 --> 00:09:17,860 that the classes been defined we can create accounts with people and start 103 103 00:09:17,860 --> 00:09:22,160 looking out for their money so I'll start by creating account for myself 104 104 00:09:22,160 --> 00:09:27,400 of course with a zero balance and then deposit withdraw some cash let's go ahead and do 105 105 00:09:27,400 --> 00:09:29,120 that 106 106 00:09:29,120 --> 00:10:00,400 .... 107 107 00:10:00,400 --> 00:10:09,710 .....and lets 108 108 00:10:09,710 --> 00:10:15,620 run this to make sure its workings account created for Tim the original balance was 109 109 00:10:15,620 --> 00:10:21,080 0 which would expect balances is a thousand when we deposited 1000 and we've 110 110 00:10:21,080 --> 00:10:26,220 withdrawn 500 and shows a balance of 500 so that's working fine 111 111 00:10:27,300 --> 00:10:30,610 so this is a very simple example as I said and theirs plenty of room for improvement 112 112 00:10:31,760 --> 00:10:35,190 now rather than having to call the show balance method after each transaction 113 113 00:10:35,190 --> 00:10:39,330 will probably be helpful if the balance if the deposit and withdrawal methods 114 114 00:10:39,330 --> 00:10:45,210 did this for us automatically I will also reluctantly fix the problem that lets me take as much 115 115 00:10:45,210 --> 00:10:49,520 money as I want regardless of the actual balance so for both methods I'm just gonna 116 116 00:10:49,520 --> 00:10:53,350 call the show balance method after adjusting the balance preventing 117 117 00:10:53,350 --> 00:10:57,710 unlimited withdrawals easily done just to confirm the way the withdrawal method is 118 118 00:10:57,710 --> 00:11:04,510 written at the moment I could quite happily withdraw 50,000 and it would 119 119 00:11:04,510 --> 00:11:09,100 work all be with a $49,000 overdrawn balance but the point is it would work and we need 120 120 00:11:09,100 --> 00:11:13,210 to restrict that so that when we are withdrawing money we can only withdraw the 121 121 00:11:13,210 --> 00:11:18,760 amount to the maximum amount we've got currently in the account so lets go ahead and do that so 122 122 00:11:18,760 --> 00:11:24,800 we are going to add to the deposit method so..... 123 123 00:11:24,800 --> 00:11:30,780 ... 124 124 00:11:30,780 --> 00:11:45,310 .... 125 125 00:11:46,260 --> 00:11:49,630 .... 126 126 00:11:51,210 --> 00:12:10,490 .... 127 127 00:12:10,490 --> 00:12:16,740 ....so we'll leave the initial show balance call on line 27 128 128 00:12:16,740 --> 00:12:22,450 but will comment out the other one because they can be called automatically 129 129 00:12:22,450 --> 00:12:27,400 when we call those methods and lets put a valid withdrawal if we went back into 500 130 130 00:12:27,400 --> 00:12:36,570 then we will put a invalid one in.....so lets run this one 131 131 00:12:36,570 --> 00:12:44,020 you can see balance is 0 account was created for 132 132 00:12:44,020 --> 00:12:45,860 Tim balance is 0 133 133 00:12:45,860 --> 00:12:52,700 balance is 1000 it is calling the show balance method now as we are calling the deposit 134 134 00:12:52,700 --> 00:12:56,070 and withdrawal methods balance is deducted to 500 and we got a message 135 135 00:12:56,070 --> 00:13:02,320 here when we called line 34 when line 34 was executed the amount must be 136 136 00:13:02,320 --> 00:13:06,640 greater than 0 no more than your account balance and it also reprinted the 137 137 00:13:06,640 --> 00:13:09,880 balance you can see the balance is still the same as it was when that method 138 138 00:13:09,880 --> 00:13:13,830 was called so this seems to be working fine and it does show that you can 139 139 00:13:13,830 --> 00:13:18,070 call methods from other methods as I mentioned earlier that you must prefix 140 140 00:13:18,070 --> 00:13:22,350 all attributes with self and that applies to method calls in this class and you 141 141 00:13:22,350 --> 00:13:27,050 probably saw when I was typing one of the show balance methods that I forgot to put the self their and 142 142 00:13:27,050 --> 00:13:32,790 IntelliJ was flagging as an error so this may catch Java programmers out for 143 143 00:13:32,790 --> 00:13:36,650 a while and it shouldn't be a huge problem because as you saw IntelliJ will let you 144 144 00:13:36,650 --> 00:13:41,910 know as soon as you tried to use attribute or a method call within a method without using 145 145 00:13:41,910 --> 00:13:49,870 self again just to confirm what I'm talking back we can delete that we then get an error un-resolve 146 146 00:13:49,870 --> 00:13:56,490 reference so we have to use self and likewise for the attributes we get an error 147 147 00:13:56,490 --> 00:13:59,840 unless you specifically do it so It will probably catch you out if your coming from another 148 148 00:13:59,840 --> 00:14:02,710 language such as Java but after you 149 149 00:14:02,710 --> 00:14:07,240 done a bit of Python coding it becomes pretty straightforward and 150 150 00:14:07,240 --> 00:14:12,120 again IntelliJ is helping you by flagging the error so you need to fix the 151 151 00:14:12,120 --> 00:14:15,930 errors to run the program so sooner or later you'll figure it out anyway but 152 152 00:14:15,930 --> 00:14:23,270 with that said there is a fundamental problem with the code for this account class and 153 153 00:14:23,270 --> 00:14:26,500 although fixing it is easy their more than one way to do so 154 154 00:14:26,500 --> 00:14:30,620 I'm gonna cover that later but for now I'm gonna see if you can spot the problem 155 155 00:14:30,620 --> 00:14:34,070 until we get to that section where our I will show you how to get around it 156 156 00:14:34,070 --> 00:14:36,480 so we'll start working on that in the next video 18372