1 00:00:05,680 --> 00:00:07,200 Alright so I mentioned in the last video 2 00:00:07,200 --> 00:00:09,420 that we skimmed over a couple of things that I've done 3 00:00:09,420 --> 00:00:11,540 in the prior class, so let's go back and have a look 4 00:00:11,540 --> 00:00:13,640 at those in a bit more detail. 5 00:00:13,640 --> 00:00:16,610 So I'm gonna open the player.py file, so the first thing 6 00:00:16,610 --> 00:00:19,620 we did before adding the lives property was to hide 7 00:00:19,620 --> 00:00:24,450 the lives data attribute by prefixing it with an underscore. 8 00:00:24,450 --> 00:00:27,090 Now that's good practise so that the client code doesn't 9 00:00:27,090 --> 00:00:31,050 try to access the attribute directly by mistake. 10 00:00:31,050 --> 00:00:32,740 Now there's nothing to stop anyone from accessing 11 00:00:32,740 --> 00:00:35,590 the underscore lives attribute, this is Python and nothing's 12 00:00:35,590 --> 00:00:39,310 hidden, but you now have to do it deliberately. 13 00:00:39,310 --> 00:00:42,060 So getting back to the main programme, I can actually 14 00:00:42,060 --> 00:00:46,230 do something like tim._lives equals 9 print(tim) 15 00:00:52,870 --> 00:00:56,010 and then run it, and you can see that that does work, 16 00:00:56,010 --> 00:00:59,510 and obviously with the value of 9 there for lives. 17 00:00:59,510 --> 00:01:01,780 So as I've said a few times, Python doesn't enforce 18 00:01:01,780 --> 00:01:03,890 any idea of private variables. 19 00:01:03,890 --> 00:01:07,010 You're free to change anything you want, but if the creator 20 00:01:07,010 --> 00:01:10,360 of the class has hidden that data attribute by prefixing 21 00:01:10,360 --> 00:01:13,420 it's name with an underscore, then you may cause problems 22 00:01:13,420 --> 00:01:16,110 if you do go changing it. 23 00:01:16,110 --> 00:01:18,670 So I'm gonna delete line 19, now watch what happens 24 00:01:18,670 --> 00:01:20,270 when I retype it, you may have missed this the last time, 25 00:01:20,270 --> 00:01:24,430 so if I type in tim., intellij is actually dropping down 26 00:01:25,370 --> 00:01:28,230 the list of all the methods and attributes of the player 27 00:01:28,230 --> 00:01:30,890 class that we might wanna change. 28 00:01:30,890 --> 00:01:35,060 Now the list starts off as lives, level, name and score. 29 00:01:37,050 --> 00:01:39,190 After that, there are attributes and methods that 30 00:01:39,190 --> 00:01:42,120 our player class inherits from it's base class which 31 00:01:42,120 --> 00:01:43,170 is object. 32 00:01:43,170 --> 00:01:44,610 I'll come back to that a little bit later when we talk 33 00:01:44,610 --> 00:01:46,060 about inheritance. 34 00:01:46,060 --> 00:01:49,410 But notice though that the _lives doesn't appear anywhere 35 00:01:49,410 --> 00:01:53,050 in the list, so we can access it if we know it's there, 36 00:01:53,050 --> 00:01:56,930 but doing so accidentally is extremely unlikely. 37 00:01:56,930 --> 00:02:01,100 Alright so I'll finish typing the line, _lives equals 9, 38 00:02:02,640 --> 00:02:04,620 now there is another reason why we had to hide the lives 39 00:02:04,620 --> 00:02:08,000 attribute, now the result if you forget to do so can be 40 00:02:08,000 --> 00:02:11,260 extremely confusing if you're not aware of it. 41 00:02:11,260 --> 00:02:13,610 So I'm gonna pretend that I forgot, so come over here 42 00:02:13,610 --> 00:02:16,420 to the player class again. 43 00:02:16,420 --> 00:02:20,100 Alright we're gonna make the change here on line 5, 44 00:02:20,100 --> 00:02:23,300 so the easiest way is to get intelliJ to refactor it, 45 00:02:23,300 --> 00:02:27,380 so I'm gonna right-click here on the _lives on line 5, 46 00:02:27,380 --> 00:02:29,720 choose Refactor then Rename. 47 00:02:31,100 --> 00:02:32,710 And all we need to do is delete the underscore from the 48 00:02:32,710 --> 00:02:35,820 start of the line and click on Refactor, and that should 49 00:02:35,820 --> 00:02:37,970 change the code there, you can see the code was changed 50 00:02:37,970 --> 00:02:42,060 as I did that, we've got our self.lives equals 3 here, 51 00:02:42,060 --> 00:02:45,320 we've got self.lives here on line 14 and also here 52 00:02:45,320 --> 00:02:47,900 on line 17, self.lives as well. 53 00:02:48,880 --> 00:02:53,040 Alright so let's switch back to main and run this again. 54 00:02:55,530 --> 00:02:58,230 Now I'm sure that's probably not what you expected. 55 00:02:58,230 --> 00:03:00,490 Let's put an end of line at the end of the script 56 00:03:00,490 --> 00:03:02,940 so that it gets rid of that warning. 57 00:03:02,940 --> 00:03:04,960 So the problem that starts here, the reason why it's crashed 58 00:03:04,960 --> 00:03:08,710 test starts at line 3 of the player class. 59 00:03:08,710 --> 00:03:11,550 Where we set lives equal to 3. 60 00:03:11,550 --> 00:03:15,200 Now we've also got our property lives on line 19, 61 00:03:15,200 --> 00:03:19,660 so when Python sees set lives equals 3, it calls the setter 62 00:03:19,660 --> 00:03:23,250 with the value three, now inside the setter, here we're 63 00:03:23,250 --> 00:03:26,430 talking about the code here on line 12, the set of code 64 00:03:26,430 --> 00:03:31,100 which starts on line 13, the line self.lives equals lives 65 00:03:31,100 --> 00:03:34,400 is executed, so Python knows the value of lives that we 66 00:03:34,400 --> 00:03:37,540 pass to the setter is 3, so it calls the setter with 67 00:03:37,540 --> 00:03:41,090 the value 3, and inside the setter is the line self.lives 68 00:03:41,090 --> 00:03:44,910 equals lives, and you're probably getting the idea by now. 69 00:03:44,910 --> 00:03:47,180 Getting back to looking at the error down here, 70 00:03:47,180 --> 00:03:50,490 and we've got a recursion error, maximum recursion depth 71 00:03:50,490 --> 00:03:53,910 exceeded, and that's because the setter keeps calling itself 72 00:03:53,910 --> 00:03:57,580 recursively and there's no exit condition, so it just goes 73 00:03:57,580 --> 00:04:00,300 round and round until it overflows the stack and that's 74 00:04:00,300 --> 00:04:02,300 why we're getting this recursion error. 75 00:04:02,300 --> 00:04:04,980 So the important point about all this is that the data 76 00:04:04,980 --> 00:04:08,470 attribute mustn't have the same name as the property. 77 00:04:08,470 --> 00:04:11,030 Now we could have renamed the lives attribute to anything, 78 00:04:11,030 --> 00:04:13,520 as long as it wasn't the same name as we used for 79 00:04:13,520 --> 00:04:17,610 the property, getting back to here on line 19. 80 00:04:17,610 --> 00:04:20,420 So it makes sense just to put a print to just to prefix it 81 00:04:20,420 --> 00:04:24,260 with an underscore because that hides the attribute as well. 82 00:04:24,260 --> 00:04:27,520 Alright so let's go ahead now and rename the lives data 83 00:04:27,520 --> 00:04:30,710 attribute back to _lives again. 84 00:04:30,710 --> 00:04:33,770 Now we can obviously refactor again, but we can just use 85 00:04:33,770 --> 00:04:38,060 CTRL + Z on a PC, or Command + Z on a Mac to undo 86 00:04:38,060 --> 00:04:40,170 that renaming, so let's do that, making sure that we're 87 00:04:40,170 --> 00:04:44,330 in player.py, we'll do Command + Z, undo remaining variable 88 00:04:45,480 --> 00:04:46,810 _lives to lives? 89 00:04:48,560 --> 00:04:50,680 That puts the change back to what it was before we changed 90 00:04:50,680 --> 00:04:55,100 or renamed it, and if we go back to main.py, we should be 91 00:04:55,100 --> 00:04:57,350 able to run this again now. 92 00:04:58,770 --> 00:05:02,000 And you can see this time it's working as it was before. 93 00:05:02,000 --> 00:05:04,390 So watch out for that, Python really isn't happy if you 94 00:05:04,390 --> 00:05:07,240 give your property the same name as the data attribute 95 00:05:07,240 --> 00:05:08,650 backing it. 96 00:05:08,650 --> 00:05:11,990 Alright, so going back to player.py, the other thing 97 00:05:11,990 --> 00:05:14,230 that I did skim over was setting that property here 98 00:05:14,230 --> 00:05:16,230 on line 19 in player.py. 99 00:05:17,210 --> 00:05:19,520 And that's fairly straightforward, just the name 100 00:05:19,520 --> 00:05:22,530 of the property equals and a call to the properties 101 00:05:22,530 --> 00:05:26,450 class constructor to create the new property. 102 00:05:26,450 --> 00:05:29,380 Now I could hover the mouse over property and hold down 103 00:05:29,380 --> 00:05:32,940 the CTRL key or command on a Mac, you can see what 104 00:05:32,940 --> 00:05:35,180 the parameters are, but sometimes it's just easier 105 00:05:35,180 --> 00:05:39,570 to jump into the source by clicking on it which I'll do now. 106 00:05:39,570 --> 00:05:43,120 So the first parameter here is self, and that's because 107 00:05:43,120 --> 00:05:45,310 we're using this inside a class definition, so we can skip 108 00:05:45,310 --> 00:05:49,320 that, the next we specify up the three methods. 109 00:05:49,320 --> 00:05:51,680 And as they say in the doc string below the underscore 110 00:05:51,680 --> 00:05:55,110 underscore init method definition, if get is the name 111 00:05:55,110 --> 00:05:58,640 down here, function that's used for getting the attribute 112 00:05:58,640 --> 00:06:01,820 value, if set is the name of a function that will set 113 00:06:01,820 --> 00:06:05,290 the value and if del identifies a function that can 114 00:06:05,290 --> 00:06:08,300 be used to delete the attribute. 115 00:06:08,300 --> 00:06:11,240 And the final parameter, doc over here, that's a doc string 116 00:06:11,240 --> 00:06:14,130 you can use to provide a description of the property. 117 00:06:14,130 --> 00:06:17,640 Now we don't have all four parameters, so if I need to 118 00:06:17,640 --> 00:06:20,070 supply at the parameters to get and set 119 00:06:20,070 --> 00:06:21,820 our lives properties. 120 00:06:23,230 --> 00:06:25,500 You can see that on line 19. 121 00:06:25,500 --> 00:06:27,930 Now a delete a method doesn't really make sense when dealing 122 00:06:27,930 --> 00:06:30,790 with numbers, but could well be useful if the property 123 00:06:30,790 --> 00:06:33,550 is a list and you wanted a quick way to clear out 124 00:06:33,550 --> 00:06:36,240 all the items in the list. 125 00:06:36,240 --> 00:06:38,920 One thing to watch out for though, looking at line 19 again, 126 00:06:38,920 --> 00:06:42,460 is that you provide the names of the methods. 127 00:06:42,460 --> 00:06:44,740 Now if you place opening and closing parentheses after 128 00:06:44,740 --> 00:06:46,410 the names like this, 129 00:06:51,280 --> 00:06:53,550 then you're actually calling the method and the property's 130 00:06:53,550 --> 00:06:56,230 getter will be set to whatever the result of calling 131 00:06:56,230 --> 00:07:01,070 get_lives is, almost certainly not anything useful. 132 00:07:01,070 --> 00:07:03,080 I'll leave you to run the programme and see what happens 133 00:07:03,080 --> 00:07:05,640 if you do with the parentheses instead of just passing 134 00:07:05,640 --> 00:07:07,380 the method names. 135 00:07:07,380 --> 00:07:10,200 Now IntelliJ warns us that this is a strange argument, 136 00:07:10,200 --> 00:07:12,810 and that a callable is expected. 137 00:07:12,810 --> 00:07:15,370 Which is slightly ironic considering it tries to add 138 00:07:15,370 --> 00:07:18,910 the parentheses for us when we type the method names. 139 00:07:18,910 --> 00:07:21,890 And just to show you that, you probably saw this before 140 00:07:21,890 --> 00:07:26,070 but if I type _set_lives, it's automatically adding 141 00:07:26,070 --> 00:07:29,030 the parentheses, so it's sort of missing the point here, 142 00:07:29,030 --> 00:07:31,970 I will delete these two parentheses anyway. 143 00:07:31,970 --> 00:07:35,650 Now if you don't specify a method to use for the setter, 144 00:07:35,650 --> 00:07:38,410 then the property is gonna be read only which is sometimes 145 00:07:38,410 --> 00:07:42,590 useful, now if you specify setter but no getter, 146 00:07:42,590 --> 00:07:44,730 then you can change the value of the property but you can't 147 00:07:44,730 --> 00:07:47,330 display it, and that's really the less useful, 148 00:07:47,330 --> 00:07:49,850 possibly maybe for a password attribute. 149 00:07:49,850 --> 00:07:52,080 If you can think of a use for a write only property, 150 00:07:52,080 --> 00:07:54,150 then that's how you'd actually do it. 151 00:07:54,150 --> 00:07:56,870 The other thing I want to mention is that STI method 152 00:07:56,870 --> 00:07:59,100 and the strange replacement fields that I've used on line 153 00:07:59,100 --> 00:08:02,620 21 and 22, now I've been using replacement fields in 154 00:08:02,620 --> 00:08:07,530 the .format method a lot, but so far we've either used 155 00:08:07,530 --> 00:08:10,420 just opening and closing braces, or put a number inside them 156 00:08:10,420 --> 00:08:14,470 to indicate which of the values should go where. 157 00:08:14,470 --> 00:08:18,110 But when dealing with classes, you can also use this form. 158 00:08:18,110 --> 00:08:21,260 Now because the value supplied to format is an object, 159 00:08:21,260 --> 00:08:24,270 in this case self, the current instance of our player class, 160 00:08:24,270 --> 00:08:26,920 we only need to specify it once. 161 00:08:26,920 --> 00:08:29,400 We can then access it's data attributes and properties 162 00:08:29,400 --> 00:08:32,049 inside the replacement fields. 163 00:08:32,049 --> 00:08:34,870 So I think you'd agree that the way we've got it there 164 00:08:34,870 --> 00:08:37,809 is a lot more readable than another scenario which I'll 165 00:08:37,809 --> 00:08:40,559 just copy and paste this code in. 166 00:08:43,120 --> 00:08:44,530 Than that method there, and I'll just comment one of these 167 00:08:44,530 --> 00:08:46,620 out to keep things happy. 168 00:08:49,230 --> 00:08:52,580 So I think the first method, 0.name, 0.lives, 0.level, 169 00:08:52,580 --> 00:08:54,520 is a lot more readable than that second version which 170 00:08:54,520 --> 00:08:57,620 we've been using so far in the course. 171 00:08:57,620 --> 00:08:59,930 I'll just put it back to where it was. 172 00:08:59,930 --> 00:09:02,900 Alright so I think at this point, it's time for challenge. 173 00:09:02,900 --> 00:09:06,580 So here's the challenge, so the challenge is to modify 174 00:09:06,580 --> 00:09:10,790 the player class so that the player score is increased 175 00:09:10,790 --> 00:09:15,300 to 1000 every time their level increases by one. 176 00:09:15,300 --> 00:09:17,050 So if they jump up two levels, they'll get a bonus 177 00:09:17,050 --> 00:09:19,400 of 2000 added to their score. 178 00:09:19,400 --> 00:09:22,090 If the player drops back a level, they'll lose 1000 179 00:09:22,090 --> 00:09:24,850 for each level they drop back. 180 00:09:24,850 --> 00:09:27,160 They can't go below Level One, so your solution should 181 00:09:27,160 --> 00:09:29,230 prevent that from happening. 182 00:09:29,230 --> 00:09:31,590 So the aim of this challenge is to practise properties, 183 00:09:31,590 --> 00:09:34,730 so although it may make more sense to add methods 184 00:09:34,730 --> 00:09:37,240 to increase and decrease the level, please don't do it 185 00:09:37,240 --> 00:09:39,650 that way, use a property. 186 00:09:39,650 --> 00:09:42,080 Alright, so that's the challenge, see how you go with that, 187 00:09:42,080 --> 00:09:46,240 pause the video now and I'll see you when you get back. 188 00:09:48,270 --> 00:09:50,840 Alright, so because this video is about getters and setters, 189 00:09:50,840 --> 00:09:54,770 the solution I was looking for is to create a property 190 00:09:54,770 --> 00:09:57,660 for the level and then create the getter and a setter 191 00:09:57,660 --> 00:10:01,370 to change the score as the level changes. 192 00:10:01,370 --> 00:10:04,680 Now we're going to create a property called level, 193 00:10:04,680 --> 00:10:06,910 so the first thing is to hide our level attribute 194 00:10:06,910 --> 00:10:09,590 by renaming it to _level. 195 00:10:09,590 --> 00:10:14,360 So I'm gonna right click the level on line 6 and refactor it 196 00:10:14,360 --> 00:10:17,610 Rename, and we can rename it to _level. 197 00:10:20,230 --> 00:10:24,730 Next we need to create a getter and set our methods. 198 00:10:24,730 --> 00:10:26,720 So we're gonna add them under the getters and setters 199 00:10:26,720 --> 00:10:28,220 methods for lives. 200 00:10:29,780 --> 00:10:33,940 So def _get_level (self), and we've got a return, 201 00:10:38,880 --> 00:10:43,050 self._level, so the getter's obviously trivial there, 202 00:10:44,170 --> 00:10:48,170 we're just returning the value of the _level data attribute. 203 00:10:48,170 --> 00:10:50,690 Now the setter on the other hand has to check that the new 204 00:10:50,690 --> 00:10:55,120 level was greater than 0, then calculate the bonus that will 205 00:10:55,120 --> 00:10:59,900 be added to the score, so let's go ahead and write that. 206 00:10:59,900 --> 00:11:04,070 So def _set_level, adding self, and we also need to pass 207 00:11:06,360 --> 00:11:08,810 the parameter level. 208 00:11:08,810 --> 00:11:12,370 So I'm gonna start off by putting if level is greater than 0 209 00:11:12,370 --> 00:11:16,120 then put delta equals level take self._level. 210 00:11:20,880 --> 00:11:25,050 Then we wanna put self.score plus equals delta multiplied 211 00:11:26,890 --> 00:11:31,050 by 1000, by 1000, and self.level, self._level I should say, 212 00:11:34,620 --> 00:11:39,490 equals level, otherwise we're gonna put an else there, 213 00:11:39,490 --> 00:11:43,660 print("Level can't be less than 1"). 214 00:11:49,050 --> 00:11:51,370 So if the new level is greater than 0, so 1 or more, 215 00:11:51,370 --> 00:11:55,510 we subtract the current level from it to find out how much 216 00:11:55,510 --> 00:11:57,090 it's increased by. 217 00:11:57,090 --> 00:11:59,830 Now if the level's gone down, delta will be negative 218 00:11:59,830 --> 00:12:02,390 and will be subtracting the bonus from the score, 219 00:12:02,390 --> 00:12:06,230 otherwise the bonus is added to the score. 220 00:12:06,230 --> 00:12:09,070 We're then updating the score and set the underscore level 221 00:12:09,070 --> 00:12:11,790 attribute to the new value. 222 00:12:11,790 --> 00:12:13,520 The last thing we have to do which I haven't done yet 223 00:12:13,520 --> 00:12:16,500 is to define the property and tell it to use our two methods 224 00:12:16,500 --> 00:12:19,030 as the getter and setter. 225 00:12:19,030 --> 00:12:21,670 So let's go ahead and do that. 226 00:12:21,670 --> 00:12:23,700 I'm gonna add this on line 31. 227 00:12:23,700 --> 00:12:27,860 So level is equal to property _get_level and _set_level, 228 00:12:32,200 --> 00:12:35,390 and it's almost like IntelliJ's got it in for us here, 229 00:12:35,390 --> 00:12:37,830 because it's always trying to add a call to a method 230 00:12:37,830 --> 00:12:41,300 when it offers it's suggestion as it did there. 231 00:12:41,300 --> 00:12:43,230 So that's why I mentioned it earlier and I'm mentioning 232 00:12:43,230 --> 00:12:45,890 it again now, we very definitely don't want the opening 233 00:12:45,890 --> 00:12:49,770 and closing parentheses after the method names. 234 00:12:49,770 --> 00:12:51,880 As that will result in the methods being called 235 00:12:51,880 --> 00:12:54,740 and their return value used for getter and setter, 236 00:12:54,740 --> 00:12:57,470 and it would be a very unusual getter that returned 237 00:12:57,470 --> 00:13:00,440 a method, and even if it did, it's not likely to be one 238 00:13:00,440 --> 00:13:03,450 that we'd want to use for our getter. 239 00:13:03,450 --> 00:13:05,920 Now a setter doesn't specify a value to return so it would 240 00:13:05,920 --> 00:13:09,770 return none, and the property setter would stay set to none. 241 00:13:09,770 --> 00:13:13,110 So in both cases not something that we want to happen. 242 00:13:13,110 --> 00:13:15,300 So just be very careful not to include the parentheses 243 00:13:15,300 --> 00:13:17,940 when setting up your properties. 244 00:13:17,940 --> 00:13:20,320 Alright so at this point now, I need to put a colon there 245 00:13:20,320 --> 00:13:21,230 on line 27. 246 00:13:22,870 --> 00:13:25,310 So at this point we can go back to our main.py and we 247 00:13:25,310 --> 00:13:27,620 can actually test this out. 248 00:13:27,620 --> 00:13:30,140 Alright so we're going to below the first definition 249 00:13:30,140 --> 00:13:32,140 of setting the lives to 9. 250 00:13:32,140 --> 00:13:35,990 Let's come down here and we're going to put tim.level 251 00:13:35,990 --> 00:13:40,160 equals 2, print tim, then on the next line, tim.level 252 00:13:42,760 --> 00:13:44,840 plus equals 5, print tim. 253 00:13:47,130 --> 00:13:49,870 Now hopefully we get a score ere, so that's just you see 254 00:13:49,870 --> 00:13:51,670 what it's doing. 255 00:13:51,670 --> 00:13:53,810 And we can see that we've got the results here, 256 00:13:53,810 --> 00:13:56,870 a score of 1000 then 6000 when we increase the level by 5 257 00:13:56,870 --> 00:13:59,050 more which is correct. 258 00:13:59,050 --> 00:14:01,170 Now decreasing the level should also work because 259 00:14:01,170 --> 00:14:05,910 I have delta, let's confirm that, tim.level equals 3, 260 00:14:05,910 --> 00:14:06,740 print tim. 261 00:14:10,880 --> 00:14:13,510 The level's going down by a total of 4, so our score 262 00:14:13,510 --> 00:14:17,000 is going down by 4000 in this case. 263 00:14:17,000 --> 00:14:19,720 So if your solution produces the same results, 264 00:14:19,720 --> 00:14:21,650 even if your code is different tomorrow, 265 00:14:21,650 --> 00:14:24,490 then congratulations, challenge passed. 266 00:14:24,490 --> 00:14:26,060 Alright, now there's one more thing that I need to talk 267 00:14:26,060 --> 00:14:28,930 about for these properties, but let's look at doing that 268 00:14:28,930 --> 00:14:30,430 in the next video.