1 00:00:05,720 --> 00:00:09,720 so as I mentioned at the end of the previous video theirs a fundamental problem 2 00:00:09,720 --> 00:00:15,220 with the code for the account class that we've created and although fixing it is 3 00:00:15,220 --> 00:00:20,280 easy there's more than one way to do so so I'm gonna cover that later but so I 4 00:00:20,280 --> 00:00:23,390 will let you see if you can spot the problem until we get to that part 5 00:00:23,390 --> 00:00:28,140 of this course so now that we have a program that is using the class to maintain an 6 00:00:28,140 --> 00:00:32,350 account balance it's easy to see hopefully easy to see how the class 7 00:00:32,350 --> 00:00:37,660 encapsulates the balance attributes and the methods to manipulate it without any client of 8 00:00:37,660 --> 00:00:42,060 the class having to worry about the details now a client is anything 9 00:00:42,060 --> 00:00:46,950 that uses a particular class or this class so clients don't have to worry about the 10 00:00:46,950 --> 00:00:50,960 implementation details they just need to call the deposit method to put more money 11 00:00:50,960 --> 00:00:56,180 in and the withdrawal method to take money out so one advantage provided 12 00:00:56,180 --> 00:01:00,500 by encapsulation is that we can change the way that the methods are implemented 13 00:01:00,500 --> 00:01:05,269 as long as we don't change their signature in a way that is it that is 14 00:01:05,269 --> 00:01:09,400 incompatible with the current signature so if we do that any client code will 15 00:01:09,400 --> 00:01:14,190 continue to work now if you remember back from the lectures on functions that a 16 00:01:14,190 --> 00:01:18,920 signature is the definition of the name and parameters of a function as well as 17 00:01:18,920 --> 00:01:25,220 any return value and applies to methods as well so to demonstrate that what I'll do is I'll keep the bank 18 00:01:25,220 --> 00:01:29,330 orders as happy by including a transaction log to track the details of all deposits 19 00:01:29,330 --> 00:01:34,960 and withdrawals now will start by just tracking deposits because the same code 20 00:01:34,960 --> 00:01:38,760 will ultimately be used for withdrawals and you know rather than duplicating the 21 00:01:38,760 --> 00:01:42,360 code I'll use that as an opportunity to introduce class methods 22 00:01:43,040 --> 00:01:47,330 and first I'll add a list to the classes data attributes and modify the deposit 23 00:01:47,330 --> 00:01:51,470 method to add the transaction details to the list the details to be log will be 24 00:01:51,470 --> 00:01:55,180 using the date and time of the transaction also the amount so we'll need 25 00:01:55,180 --> 00:01:59,580 to import the date time module and remember from the videos in section eleven that 26 00:01:59,580 --> 00:02:04,990 to we really should install local time so we'll also used to pytz module so 27 00:02:04,990 --> 00:02:10,400 that we can log our location and where UTC time so lets go ahead and start the changes and 28 00:02:10,400 --> 00:02:13,320 the first we need to do is import those 2 29 00:02:13,320 --> 00:02:20,780 modules so....and remember if you having 30 00:02:20,780 --> 00:02:25,350 problems with pytz getting that to work we did install that in a previous sections so go 31 00:02:25,350 --> 00:02:26,400 back and check that out 32 00:02:26,400 --> 00:02:31,420 as its not installed usually by default with the Python installation alright so what we're 33 00:02:31,420 --> 00:02:36,190 going to do is we need to add the list to the class attributes so that we can store 34 00:02:36,190 --> 00:02:37,850 the log and we'll do that in our 35 00:02:37,850 --> 00:02:43,770 init method so we'll come down here to the init and.... 36 00:02:43,770 --> 00:02:51,710 ....and what we need to do is in 37 00:02:51,710 --> 00:02:56,150 the deposit method we need to add the call that is going to append the amount 38 00:02:56,150 --> 00:03:01,720 of the deposit with the UTC time so we'll do that by typing... 39 00:03:01,720 --> 00:03:06,710 ... 40 00:03:09,140 --> 00:03:23,020 .... 41 00:03:23,020 --> 00:03:30,520 ....and attribute called transaction list has been created in 42 00:03:30,520 --> 00:03:35,820 the init method and that sent to an empty list initially and in the deposit method 43 00:03:35,820 --> 00:03:41,270 we then appended the transaction details to the list which is the date and the 44 00:03:41,270 --> 00:03:46,020 amount in this case now I'll come back to how the dates is calculated in a 45 00:03:46,020 --> 00:03:51,220 minute because their have been a quite a few video since the ones on dates and times in Python 46 00:03:52,200 --> 00:03:56,240 and I have chose to store the details as a tuple containing the date time and the amount 47 00:03:56,240 --> 00:04:00,110 so I'll add the method to print out the transaction log after the show balance 48 00:04:00,110 --> 00:04:06,870 method so let's go ahead and do that so... 49 00:04:06,870 --> 00:04:22,450 ... 50 00:04:22,450 --> 00:04:24,180 .... 51 00:04:24,180 --> 00:04:38,820 ... 52 00:04:38,820 --> 00:05:18,810 .... 53 00:05:20,070 --> 00:05:30,860 .....and that should be it so before I make any changes to the main 54 00:05:30,860 --> 00:05:36,010 program down here lets just run it to make sure that it still works even though I've 55 00:05:36,010 --> 00:05:40,330 made changes to the implementation of course of the deposit method by adding the call 56 00:05:40,330 --> 00:05:48,500 to add the transaction log so if I just run that so it still works but the 57 00:05:48,500 --> 00:05:53,190 class now has got additional functionality the ability to display a 58 00:05:53,190 --> 00:05:57,370 transaction list so any existing code doesn't break as a result of the changes 59 00:05:57,370 --> 00:06:01,400 we made to the deposit method bit if we want to we can also print out the transaction 60 00:06:01,400 --> 00:06:07,690 list by adding that to our main so we can come down here after the withdrawal.... 61 00:06:07,690 --> 00:06:11,610 ...and if we run that 62 00:06:12,670 --> 00:06:21,690 ....you can see $1000 deposited on the date of the transaction in UTC time and in the local time which is 63 00:06:21,690 --> 00:06:27,070 works out to 8:32 in the morning Australian time so before I move on you might be wondering 64 00:06:27,070 --> 00:06:33,310 about the for loop in line 30 or on line 30 where the code iterates through the 65 00:06:33,310 --> 00:06:38,300 transaction list so we created the transaction list to contain tuples and in 66 00:06:38,300 --> 00:06:42,480 the videos on tuples I showed you that you can assign a value to several variables in a 67 00:06:42,480 --> 00:06:46,140 single assignment statement by using a tuple and that's all it's really 68 00:06:46,140 --> 00:06:50,240 happening in the for loop instead of just assigning a value to a single variable for 69 00:06:50,240 --> 00:06:51,800 each value in the list 70 00:06:51,800 --> 00:06:56,910 Python unpacks the tuple and assigns the value to both data and amount at the same 71 00:06:56,910 --> 00:06:57,390 time 72 00:06:57,390 --> 00:07:02,020 ok so our class now logs deposits and I could do the same thing in the 73 00:07:02,020 --> 00:07:05,790 withdrawal method to log withdrawals as well and to do that that would involve 74 00:07:05,790 --> 00:07:11,390 duplicating the code that creates a date and time with time zone information so its 75 00:07:11,390 --> 00:07:15,820 probably better to create a function to do that instead now I could do that by 76 00:07:15,820 --> 00:07:19,660 adding another method to the class but the method wouldn't be acting on any 77 00:07:19,660 --> 00:07:23,060 data attributes and there's no point including it in the namespace of every 78 00:07:23,060 --> 00:07:27,750 single instance when a function in the would do the job just fine so 79 00:07:27,750 --> 00:07:32,690 it's a good place to introduce static methods which is the same term used in 80 00:07:32,690 --> 00:07:37,790 Java and C++ so what i'll do is i'll start by creating it as a method which I 81 00:07:37,790 --> 00:07:43,690 put right at the start of the class so we are going 82 00:07:43,690 --> 00:07:50,830 put it above the init so.... 83 00:07:50,830 --> 00:08:02,170 .... 84 00:08:02,170 --> 00:08:04,990 ... 85 00:08:04,990 --> 00:08:14,670 .....so two things firstly you notice I've use a underscore at the start when defining 86 00:08:14,670 --> 00:08:19,970 the method and also we've got a warning so IntelliJ has detected that the 87 00:08:19,970 --> 00:08:22,730 self parameter isn't use and suggest that the method 88 00:08:22,730 --> 00:08:29,110 can be implemented as a static method instead...so detecting 89 00:08:29,110 --> 00:08:34,210 a method which may safely be made static so you can just repeat what I said it's detected that the 90 00:08:34,210 --> 00:08:38,570 self parameter isn't being used in the method that was created and its 91 00:08:38,570 --> 00:08:43,179 suggesting that this method can be implemented as static method instead a 92 00:08:43,179 --> 00:08:47,190 static method is shared by all instances of the class in the same way that the 93 00:08:47,190 --> 00:08:51,490 power_source class attribute of our kettle class was shared 94 00:08:51,490 --> 00:08:56,560 by all instances so making the method static is pretty simple we just remove the 95 00:08:56,560 --> 00:09:01,150 self parameter and put an annotation before the method definition so if I 96 00:09:01,150 --> 00:09:09,490 removed self after we delete that parameter we also have to put annotation so above the method we put... 97 00:09:09,490 --> 00:09:14,790 ...and notice how the warnings disappeared 98 00:09:14,790 --> 00:09:20,060 and if we hold the Ctrl key down or command on a Mac and hover over the annotation 99 00:09:20,060 --> 00:09:25,980 you can see that it's coming up and showing its a class and as I said everything in 100 00:09:25,980 --> 00:09:31,460 Python is an object so if we now click while holding down control or command on this 101 00:09:31,460 --> 00:09:38,040 annotation you see definitions showing up here in the built ins module that useful 102 00:09:38,040 --> 00:09:41,430 to remember again that everything is an object but unless your experience in object 103 00:09:41,430 --> 00:09:44,320 oriented programming I wouldn't worry too much about this implementation 104 00:09:44,320 --> 00:09:51,520 details just yet so the current time method uses the date time class or the date time 105 00:09:51,520 --> 00:09:56,470 module to get the current date and time in UTC format and then uses the pytz 106 00:09:56,470 --> 00:10:00,980 module to localize it so that it stores time zone information for the current location 107 00:10:00,980 --> 00:10:08,310 and then on line 10 it returns the localize UTC date time so we looked at pytz and 108 00:10:08,310 --> 00:10:12,570 time zones in section eleven and here I'm just making sure that I store UTC time 109 00:10:12,570 --> 00:10:16,320 but have the ability to also display the times in the local time that the 110 00:10:16,320 --> 00:10:21,210 transaction happen and I've also named the current underscore time method 111 00:10:21,210 --> 00:10:25,250 with the leading underscore to indicate to anyone using this class that this 112 00:10:25,250 --> 00:10:29,720 method isn't really part of the normal usage of the class now this conventional 113 00:10:29,720 --> 00:10:34,190 the convention is that names starting with an underscore are non-public 114 00:10:34,190 --> 00:10:38,700 even though if you remember theirs nothing in Python that enforces this so the account 115 00:10:38,700 --> 00:10:43,450 class is concerned with managing bank accounts not with dates and times so 116 00:10:43,450 --> 00:10:46,480 although clients can call the current underscore time method if they want to 117 00:10:46,480 --> 00:10:51,040 the underscore makes it clear that this method isn't intended to be used outside 118 00:10:51,040 --> 00:10:55,420 of the class now if someone who does use the current time method from outside our 119 00:10:55,420 --> 00:11:00,160 account class they run the risk that we may remove it in later version or change 120 00:11:00,160 --> 00:11:03,340 it to remove to return something completely different in the future 121 00:11:03,340 --> 00:11:06,490 starting the name again with an underscore is the Python way of 122 00:11:06,490 --> 00:11:10,060 informing them of this they used the method at their own risk 123 00:11:10,760 --> 00:11:16,450 ok so now we've implemented this static method we can go back to our deposit 124 00:11:16,450 --> 00:11:21,120 method and used that static method instead of performing the conversion in 125 00:11:21,120 --> 00:11:26,160 places as we currently got on line 22 so come down and make a change to that so let's 126 00:11:26,160 --> 00:11:27,210 make the change here 127 00:11:27,210 --> 00:11:32,880 delete all this code now in the transaction_list.append 128 00:11:32,880 --> 00:11:38,510 then brackets that will only be a matter of now adding an extra set of brackets and 129 00:11:38,510 --> 00:11:41,510 .... 130 00:11:42,850 --> 00:11:49,540 ...so that's all we need to do now so its 131 00:11:49,540 --> 00:11:54,950 now calling the static method that we defined on line 8 returning or adding the 132 00:11:54,950 --> 00:12:01,410 returned date and then also as it was doing before appending the amount and we 133 00:12:01,410 --> 00:12:07,490 can go back to our withdrawal method and in the code that's confirmed that 134 00:12:07,490 --> 00:12:13,860 we're able to do the withdraw after we deducted the amount we put.... 135 00:12:13,860 --> 00:12:30,450 .... 136 00:12:30,450 --> 00:12:35,280 ...so if we run this to confirm its running you can see now we've got our code working 137 00:12:35,280 --> 00:12:38,830 quite nicely we got our deposit and withdrawal by showing nicely in it's 138 00:12:38,830 --> 00:12:42,860 formatted as it should be and obviously we could do a bit of work if we wanted to 139 00:12:42,860 --> 00:12:45,270 do to fix these dates up but we will leave them for now 140 00:12:45,270 --> 00:12:49,220 so the point here is that despite the additional functionality that the class 141 00:12:49,220 --> 00:12:53,410 now has the program still runs fine without any modification to the main 142 00:12:53,410 --> 00:12:58,290 part in other words we didn't modify anything here in the code from line 143 00:12:58,290 --> 00:13:02,950 46 onwards that's still as it was before and its still working despite us making 144 00:13:02,950 --> 00:13:08,300 these changes in the various class methods and looking at lines 145 00:13:08,300 --> 00:13:13,860 22 and 28 you notice that when we are calling our static method were 146 00:13:13,860 --> 00:13:18,540 using the class name then the name of the method so we're not using self in other 147 00:13:18,540 --> 00:13:23,890 words again that's because the current time method is a static or class method now you 148 00:13:23,890 --> 00:13:28,480 can call class method on an instance as I said some in other words I can change that and 149 00:13:28,480 --> 00:13:37,170 just make this say here....and that's will work just as well as but theirs no advantage to 150 00:13:37,170 --> 00:13:41,580 doing so and in actual fact performance slightly suffers because 151 00:13:41,580 --> 00:13:45,660 Python will first attempt to find the method in the instances namespace and 152 00:13:45,660 --> 00:13:50,790 when it fails it then checks the class name space so if you know you got a static methods best to leave it 153 00:13:50,790 --> 00:13:54,730 there just a to say that little bit of time and to make it clear also in the 154 00:13:54,730 --> 00:13:59,430 code that it is a static method so some ways needed to tell the difference 155 00:13:59,430 --> 00:14:02,980 between deposits and withdrawals and negating the amount is a quick way to do 156 00:14:02,980 --> 00:14:08,590 this you could as an alternative add a transaction type item to the tuple if you wanted a tuple 157 00:14:08,590 --> 00:14:12,470 if you wanted to do that so I'm going to end the video here and then the next video we're 158 00:14:12,470 --> 00:14:16,550 going to talk a little bit more about the transactions and start a mini 159 00:14:16,550 --> 00:14:18,050 challenge so see you in the next video