1 00:00:05,100 --> 00:00:06,980 So in the last few videos, 2 00:00:06,980 --> 00:00:09,350 I've digressed to look at exceptions 3 00:00:09,350 --> 00:00:11,340 and exception handling in Python, 4 00:00:11,340 --> 00:00:13,910 but now it's time to get back to databases. 5 00:00:13,910 --> 00:00:15,510 Now what we're going to do is have a look at 6 00:00:15,510 --> 00:00:18,980 rolling back transactions in situations 7 00:00:18,980 --> 00:00:22,520 where it's important that all transactions are successful. 8 00:00:22,520 --> 00:00:23,420 Now if you recall, 9 00:00:23,420 --> 00:00:25,640 we discussed one example of that earlier 10 00:00:25,640 --> 00:00:28,640 and that was transferring money between bank accounts. 11 00:00:28,640 --> 00:00:29,730 In that scenario, 12 00:00:29,730 --> 00:00:32,049 we really don't wanna end up in a situation 13 00:00:32,049 --> 00:00:33,700 where one account is debited 14 00:00:33,700 --> 00:00:36,240 without the corresponding amount also being stored 15 00:00:36,240 --> 00:00:37,810 in the other account. 16 00:00:37,810 --> 00:00:41,390 So what we're going to do is use a simple account class, 17 00:00:41,390 --> 00:00:44,900 basically very similar to what we did in an earlier video, 18 00:00:44,900 --> 00:00:47,870 to see how to ensure that both transactions occur 19 00:00:47,870 --> 00:00:49,370 or neither of them do. 20 00:00:49,370 --> 00:00:50,230 Now in the process, 21 00:00:50,230 --> 00:00:52,000 we're also gonna look at a neat way 22 00:00:52,000 --> 00:00:55,360 to do the contents of our database from IntelliJ. 23 00:00:55,360 --> 00:00:56,820 So what I've done is I've created 24 00:00:56,820 --> 00:01:00,900 a new IntelliJ Python project calling it rolling back 25 00:01:00,900 --> 00:01:05,220 and I've created a new Python file called rollback.py 26 00:01:05,220 --> 00:01:08,320 and we're just gonna quickly type in the account class 27 00:01:08,320 --> 00:01:10,340 that we're gonna be working with. 28 00:01:10,340 --> 00:01:14,500 So it's gonna be class Account(object): 29 00:01:16,110 --> 00:01:20,190 and we're gonna start by defining the init method 30 00:01:21,400 --> 00:01:24,810 and that's going to have also a name: str 31 00:01:26,210 --> 00:01:28,460 and also an opening balance 32 00:01:30,660 --> 00:01:33,820 which is a float the default value 0.0 33 00:01:35,640 --> 00:01:38,570 and we're going to assign the parameter 34 00:01:38,570 --> 00:01:41,400 so self.name = name, 35 00:01:41,400 --> 00:01:44,400 self._balance = opening_balance 36 00:01:50,560 --> 00:01:53,120 and then we'll just print something to the console 37 00:01:53,120 --> 00:01:57,280 Account created for two curly braces dot and a space 38 00:01:59,490 --> 00:02:01,720 and it's gonna be .format 39 00:02:01,720 --> 00:02:03,720 and then we do self.name 40 00:02:06,350 --> 00:02:09,009 and end equals two single quotes 41 00:02:10,150 --> 00:02:11,980 then self.show_balance 42 00:02:13,940 --> 00:02:17,500 which we obviously need to write. 43 00:02:17,500 --> 00:02:19,700 All right, next we're going to start off with our 44 00:02:19,700 --> 00:02:23,770 or start with the deposit method so def deposit 45 00:02:23,770 --> 00:02:25,150 self is automatic 46 00:02:25,150 --> 00:02:28,450 and then amount which is gonna be a float. 47 00:02:28,450 --> 00:02:31,070 We can also add an annotation here for the return type 48 00:02:31,070 --> 00:02:33,450 which we've talked about 49 00:02:33,450 --> 00:02:35,300 to be a float. 50 00:02:35,300 --> 00:02:38,960 Then the code is going to be if amount > 0.0 51 00:02:41,210 --> 00:02:45,380 then we're going to do self._balance += amount 52 00:02:49,360 --> 00:02:51,260 and then we're gonna print a message as well 53 00:02:51,260 --> 00:02:55,420 two curly braces deposited.format(amount) 54 00:03:00,710 --> 00:03:01,580 and then in either case 55 00:03:01,580 --> 00:03:04,250 we're gonna return self._balance 56 00:03:05,410 --> 00:03:07,180 and that's our deposit method. 57 00:03:07,180 --> 00:03:08,760 Now the withdraw method, 58 00:03:08,760 --> 00:03:09,760 def withdraw 59 00:03:11,840 --> 00:03:13,040 it's gonna have a self obviously 60 00:03:13,040 --> 00:03:16,660 and then it's going to be amount: float 61 00:03:16,660 --> 00:03:19,140 and another annotation for this for the return type 62 00:03:19,140 --> 00:03:21,470 which is going to be a float 63 00:03:22,460 --> 00:03:24,180 and then the code is going to be 64 00:03:24,180 --> 00:03:27,340 if 0 < amount <= to self._balance 65 00:03:32,070 --> 00:03:35,740 and we wanna do self._balance -= amount 66 00:03:38,560 --> 00:03:40,510 then print something to the console 67 00:03:40,510 --> 00:03:44,680 so print two curly braces withdrawn.format(amount) 68 00:03:51,640 --> 00:03:52,720 return amount 69 00:03:54,590 --> 00:03:55,420 else: 70 00:03:56,520 --> 00:03:59,520 we're going to print a message basically saying 71 00:03:59,520 --> 00:04:01,130 the amount must be greater than zero 72 00:04:01,130 --> 00:04:02,630 no more than your account balance 73 00:04:02,630 --> 00:04:05,880 so the amount must be greater than zero 74 00:04:08,180 --> 00:04:11,260 and no more than your account balance 75 00:04:14,600 --> 00:04:15,430 return 0.0 76 00:04:17,440 --> 00:04:20,870 and lastly we want to define the show balance method 77 00:04:20,870 --> 00:04:24,350 that we're calling upon line seven up here 78 00:04:24,350 --> 00:04:27,520 so we need to define that show_balance 79 00:04:29,800 --> 00:04:30,950 and the code for that is going to be 80 00:04:30,950 --> 00:04:32,950 print Balance on account 81 00:04:37,320 --> 00:04:39,550 curly braces, left to right curly braces, 82 00:04:39,550 --> 00:04:42,720 is and another set of curly braces left to right 83 00:04:42,720 --> 00:04:46,890 .format(self.name, self._balance) 84 00:04:52,040 --> 00:04:54,590 okay so there's our account class. 85 00:04:54,590 --> 00:04:55,430 As I mentioned, 86 00:04:55,430 --> 00:04:58,870 I've added annotations for the return values, 87 00:04:58,870 --> 00:05:00,270 but I also added annotations, 88 00:05:00,270 --> 00:05:02,190 you probably noticed as I was typing it, 89 00:05:02,190 --> 00:05:04,670 for the parameters as well. 90 00:05:04,670 --> 00:05:07,090 And where this differs, 91 00:05:07,090 --> 00:05:10,100 this class differs from our earlier attempt 92 00:05:10,100 --> 00:05:13,240 is both the deposit and withdraw methods 93 00:05:13,240 --> 00:05:15,620 now also return a value but other than that, 94 00:05:15,620 --> 00:05:17,220 it's very basic. 95 00:05:17,220 --> 00:05:20,110 It's pretty much the same code that we've seen previously. 96 00:05:20,110 --> 00:05:21,920 Now just to test this out, 97 00:05:21,920 --> 00:05:23,780 let's write a bit of code to do just that. 98 00:05:23,780 --> 00:05:27,860 So if __name__ is equal to and in quotes __main__ 99 00:05:35,890 --> 00:05:40,060 now we're gonna put john = Account("John") 100 00:05:41,820 --> 00:05:45,160 and john.deposit(10.10) 101 00:05:47,500 --> 00:05:50,250 and john.deposit another 10 cents 102 00:05:53,540 --> 00:05:55,960 john.deposit another 10 cents 103 00:05:58,690 --> 00:06:00,860 john.deposit 30 more cents 104 00:06:03,150 --> 00:06:07,030 and then john.withdraw(0) 105 00:06:07,030 --> 00:06:08,860 and john.show_balance. 106 00:06:11,720 --> 00:06:13,320 So before I run this programme, 107 00:06:13,320 --> 00:06:16,790 we're gonna be depositing a total of $10 108 00:06:16,790 --> 00:06:21,490 and that actually should have been a withdrawal. 109 00:06:21,490 --> 00:06:24,060 So we're depositing a total of $10.30, 110 00:06:24,060 --> 00:06:26,070 $10.10 plus another 20 111 00:06:26,070 --> 00:06:28,830 then we're withdrawing 30 cents or pennies 112 00:06:28,830 --> 00:06:30,980 or whatever you like to call it. 113 00:06:30,980 --> 00:06:32,480 So if we run this, 114 00:06:34,510 --> 00:06:38,010 and I'm just putting this down the bottom, 115 00:06:39,590 --> 00:06:40,430 well what's going on here? 116 00:06:40,430 --> 00:06:42,800 So we got the account must be greater than zero 117 00:06:42,800 --> 00:06:43,880 no more than your account balance 118 00:06:43,880 --> 00:06:46,130 so it seemingly has worked, 119 00:06:46,980 --> 00:06:51,650 but at the moment the balance on account John is $9.999999 120 00:06:51,650 --> 00:06:53,720 so obviously that really should have been, 121 00:06:53,720 --> 00:06:57,550 the balance remaining should have been $10. 122 00:06:57,550 --> 00:06:59,900 So we're using floating point numbers here 123 00:06:59,900 --> 00:07:02,690 and in just about every computer language, 124 00:07:02,690 --> 00:07:05,970 these floating point numbers are subject to rounding errors 125 00:07:05,970 --> 00:07:08,860 when converting between binary and decimal. 126 00:07:08,860 --> 00:07:10,540 Now integer values can be converted 127 00:07:10,540 --> 00:07:13,560 between any two number systems with complete accuracy, 128 00:07:13,560 --> 00:07:15,880 but as soon as you introduce fractions, 129 00:07:15,880 --> 00:07:18,170 you'll often get rounding errors. 130 00:07:18,170 --> 00:07:20,250 Now you're probably familiar with pi 131 00:07:20,250 --> 00:07:21,890 and the fact that it can't be represented 132 00:07:21,890 --> 00:07:23,830 exactly in decimal. 133 00:07:23,830 --> 00:07:26,780 Now other fractions such as 1/3 134 00:07:26,780 --> 00:07:29,800 also can't be represented accurately in decimal 135 00:07:29,800 --> 00:07:32,800 so it comes out as 0.3333 recurring. 136 00:07:33,700 --> 00:07:38,330 Now when 1/10 or 0.1 in decimal is represented in binary, 137 00:07:38,330 --> 00:07:41,470 that also has the same issue with recurring, 138 00:07:41,470 --> 00:07:43,020 that also recurs. 139 00:07:43,020 --> 00:07:43,860 So in other words, 140 00:07:43,860 --> 00:07:46,340 we lose some accuracy every time we use 0.1 141 00:07:46,340 --> 00:07:49,000 in our example on screen 142 00:07:49,000 --> 00:07:53,300 and 0.3 also recurs in binary as would 0.4, 143 00:07:53,300 --> 00:07:58,140 0.5 is 1/2 which can be represented exactly in binary. 144 00:07:58,140 --> 00:08:01,600 Now Richard Pryor's character in the film Superman 3 145 00:08:01,600 --> 00:08:04,550 takes advantage of this to make a small fortune 146 00:08:04,550 --> 00:08:07,000 by creaming up the small fractions of a cent 147 00:08:07,000 --> 00:08:09,770 for millions of transactions but in the real world, 148 00:08:09,770 --> 00:08:13,940 our answer here of 9.99 is obviously not satisfactory. 149 00:08:14,780 --> 00:08:17,990 So what can we actually do about this to solve the problem? 150 00:08:17,990 --> 00:08:20,930 Well, one solution is to use the decimal class 151 00:08:20,930 --> 00:08:22,590 that comes with Python. 152 00:08:22,590 --> 00:08:24,640 Now I'm gonna not cover that right now. 153 00:08:24,640 --> 00:08:25,950 If there's sufficient demand, 154 00:08:25,950 --> 00:08:28,960 then it could be a good topic for a future video. 155 00:08:28,960 --> 00:08:32,110 But what I will do is show an equivalent version 156 00:08:32,110 --> 00:08:34,539 of this programme on the screen for a few seconds 157 00:08:34,539 --> 00:08:37,240 just so you can get an idea for how it's used. 158 00:08:37,240 --> 00:08:40,900 So I'm gonna create a new Python file called rollback2 159 00:08:40,900 --> 00:08:43,460 and paste the code in there just so you've got it 160 00:08:43,460 --> 00:08:44,640 and you can also refer to that 161 00:08:44,640 --> 00:08:48,750 by downloading the source code for this video. 162 00:08:48,750 --> 00:08:52,090 So I'm gonna create a new file rollback2 163 00:08:55,660 --> 00:08:59,020 and I'll just grab that and paste that in. 164 00:08:59,020 --> 00:09:02,390 Okay so there's the code there. 165 00:09:02,390 --> 00:09:05,540 So essentially the main programme still uses float values, 166 00:09:05,540 --> 00:09:09,010 but the accounts class converts them to decimal objects 167 00:09:09,010 --> 00:09:11,250 and you can see that here on line nine, 168 00:09:11,250 --> 00:09:14,420 line 14 and also on line 21 down here. 169 00:09:15,670 --> 00:09:18,420 Now you can work entirely in decimal values, 170 00:09:18,420 --> 00:09:20,890 but the class is a bit cumbersome to use 171 00:09:20,890 --> 00:09:22,940 so personally I like this approach of allowing 172 00:09:22,940 --> 00:09:25,300 clients of the class to work in floats 173 00:09:25,300 --> 00:09:28,610 and then converting them for storage and arithmetic. 174 00:09:28,610 --> 00:09:30,130 All right so I will close that file now 175 00:09:30,130 --> 00:09:31,550 but again as I mentioned, 176 00:09:31,550 --> 00:09:33,180 it's in the source code for this video 177 00:09:33,180 --> 00:09:35,100 so you can refer to that at your leisure. 178 00:09:35,100 --> 00:09:38,300 Let's go back to rollback.py that we were working on. 179 00:09:38,300 --> 00:09:40,180 So the other thing we can do 180 00:09:40,180 --> 00:09:42,930 is work entirely with integer values. 181 00:09:42,930 --> 00:09:43,790 Now as I've said, 182 00:09:43,790 --> 00:09:46,780 integers can be stored exactly in binary 183 00:09:46,780 --> 00:09:48,590 so we won't get any rounding errors 184 00:09:48,590 --> 00:09:51,910 if we perform all our arithmetic on ints. 185 00:09:51,910 --> 00:09:53,900 So when we need to display values, 186 00:09:53,900 --> 00:09:57,820 we divide by 100 so that the programme shows dollars and cents 187 00:09:57,820 --> 00:10:00,180 or pounds and pennies or whatever currency units 188 00:10:00,180 --> 00:10:01,940 that you're actually dealing with. 189 00:10:01,940 --> 00:10:03,880 And if the programme receives input, 190 00:10:03,880 --> 00:10:07,500 we need to multiply by 100 to convert to an integer. 191 00:10:07,500 --> 00:10:09,990 So basically we're working in cents or pennies 192 00:10:09,990 --> 00:10:12,390 rather than dollars or pounds. 193 00:10:12,390 --> 00:10:13,260 So what we need to do now 194 00:10:13,260 --> 00:10:16,300 is make some changes to our source code 195 00:10:16,300 --> 00:10:18,800 to replace that with integers. 196 00:10:19,730 --> 00:10:22,180 We're gonna start up here on line three. 197 00:10:22,180 --> 00:10:25,440 We're going to put int = 0 198 00:10:25,440 --> 00:10:28,230 no decimal anymore so just int = 0 199 00:10:28,230 --> 00:10:32,260 and for the deposit method, amount is going to be an int. 200 00:10:32,260 --> 00:10:35,290 We're also going to still return a float though. 201 00:10:35,290 --> 00:10:36,590 And for printing out, 202 00:10:36,590 --> 00:10:39,010 we're gonna use some formatting for printing out now. 203 00:10:39,010 --> 00:10:41,480 So within the curly braces, 204 00:10:41,480 --> 00:10:44,730 we're gonna put :.2f two decimal points 205 00:10:46,560 --> 00:10:49,210 and we're gonna put amount format is going to be 206 00:10:49,210 --> 00:10:51,210 amount / 100. 207 00:10:53,450 --> 00:10:56,620 For the withdrawal method, we need to convert that. 208 00:10:56,620 --> 00:10:58,180 The input's gonna be an int. 209 00:10:58,180 --> 00:11:00,310 We're still gonna return a float. 210 00:11:00,310 --> 00:11:01,600 In terms of formatting, 211 00:11:01,600 --> 00:11:04,120 exactly the same as we've done for the deposit method, 212 00:11:04,120 --> 00:11:06,450 :.2f within the curly braces 213 00:11:07,820 --> 00:11:11,980 and formatting it's gonna be amount / 100 214 00:11:13,090 --> 00:11:17,640 and then I'm also gonna return amount / 100 215 00:11:17,640 --> 00:11:21,800 and of course we also need to return self._balance / 100 216 00:11:23,890 --> 00:11:26,740 to convert that back to a float as well. 217 00:11:26,740 --> 00:11:30,270 And show balance, we need to make a change to that, 218 00:11:30,270 --> 00:11:32,750 so the second set of curly braces in the print line 219 00:11:32,750 --> 00:11:36,510 is gonna have the same formatting :.2f 220 00:11:36,510 --> 00:11:38,920 and the balance we're gonna do the same thing there 221 00:11:38,920 --> 00:11:41,420 self._balance / 100 222 00:11:42,790 --> 00:11:44,620 and in terms of the deposit function, 223 00:11:44,620 --> 00:11:47,710 we're not going to use decimal points anymore 224 00:11:47,710 --> 00:11:49,720 so we're using whole numbers, integers, 225 00:11:49,720 --> 00:11:51,630 so it's gonna be 10.10, 226 00:11:52,940 --> 00:11:55,270 this is gonna be 10, 10, 30. 227 00:11:57,090 --> 00:11:59,990 So basically I changed all the float parameters to int 228 00:11:59,990 --> 00:12:02,060 and divided the values by 100 229 00:12:02,060 --> 00:12:03,890 before returning or printing them. 230 00:12:03,890 --> 00:12:05,860 And you saw me specify 231 00:12:05,860 --> 00:12:09,440 the format of :.2f in the replacement fields 232 00:12:09,440 --> 00:12:10,600 so that the amounts printing 233 00:12:10,600 --> 00:12:13,410 is floating them with two decimal places. 234 00:12:13,410 --> 00:12:15,240 So if we run this now, 235 00:12:17,070 --> 00:12:19,520 you could see we're now getting the correct formatting 236 00:12:19,520 --> 00:12:20,640 for all our numbers 237 00:12:20,640 --> 00:12:23,300 and also the balance returned now 238 00:12:23,300 --> 00:12:26,380 is as we would expect it to be 10.00. 239 00:12:27,340 --> 00:12:29,520 All right, so I'm gonna finish the video here now. 240 00:12:29,520 --> 00:12:30,450 In the next video, 241 00:12:30,450 --> 00:12:33,570 let's start storing these account details in a database. 242 00:12:33,570 --> 00:12:35,730 See you in the next video.