1 00:00:05,360 --> 00:00:10,260 so continuing on from the last video so global keyword tells Python to 2 00:00:10,260 --> 00:00:15,140 look for a variable in the global scope which is the modules name space but the 3 00:00:15,140 --> 00:00:19,820 non local keyword tells it to look for that variable in the enclosing 4 00:00:19,820 --> 00:00:23,980 scopes and you could see what we've done differently to other videos is 5 00:00:23,980 --> 00:00:28,410 we've got an embedded function here that we didn't have at the blackjack program so 6 00:00:28,410 --> 00:00:33,540 that's why on line 7 the global keyword wasn't appropriate so again the non local 7 00:00:33,540 --> 00:00:37,830 keyword if that is telling Python to look for the variable in the enclosing 8 00:00:37,830 --> 00:00:42,090 scope so non local won't check the global name space though so a 9 00:00:42,090 --> 00:00:47,650 non local variable must exist in the enclosing scope but not be global otherwise if it 10 00:00:47,650 --> 00:00:52,540 was global of course you would use the keyword global to do that another thing to know 11 00:00:52,540 --> 00:00:56,940 about the non local variables is that they must exist in the enclosing scope 12 00:00:56,940 --> 00:01:03,070 they can't be created in other words by the non local keywords so if we change this to try and put a declaration 13 00:01:03,070 --> 00:01:08,910 in their we get an error...end of statement expected because it's looking for us just to define 14 00:01:08,910 --> 00:01:13,260 the variable to say that it's a non-local variable but it has to actually exist as 15 00:01:13,260 --> 00:01:18,790 I mentioned in the outer scope or in the enclosing scope to be more precise now 16 00:01:18,790 --> 00:01:21,860 although we didn't do this we could have created our global variables for 17 00:01:21,860 --> 00:01:24,680 blackjack in the new_game method and they would have been 18 00:01:24,680 --> 00:01:29,380 available in the main program but it's not considered good style to that and 19 00:01:29,380 --> 00:01:33,510 can lead to difficulty debugging the programs but it's there and it is actually 20 00:01:33,510 --> 00:01:38,560 allowed but the same is not true for non local variables and that's because Python 21 00:01:38,560 --> 00:01:42,340 doesn't know which enclosing scope the should have existed and has really has no 22 00:01:42,340 --> 00:01:45,860 way to know whether to create them so they must already be present in an 23 00:01:45,860 --> 00:01:51,000 enclosing scope or you get an error for that reason so at this point its a 24 00:01:51,000 --> 00:01:55,180 good time to point out that the only things that create scope in Python are 25 00:01:55,180 --> 00:02:00,520 modules functions and classes now I've already talked about variables in a module 26 00:02:00,520 --> 00:02:05,080 being defined in a while for loop and then and remaining available to the rest 27 00:02:05,080 --> 00:02:09,560 of the program but that's because they become part of the global name space now 28 00:02:09,560 --> 00:02:13,660 that can seem odd if you come from Java for example but the idea that functions 29 00:02:13,660 --> 00:02:16,630 introduced a new scope will be familiar 30 00:02:16,630 --> 00:02:20,660 know when you define a function Python creates a new name space for it and 31 00:02:20,660 --> 00:02:26,920 any variables are part of the local scope and to that end we actually printed out the local 32 00:02:26,920 --> 00:02:32,870 variables using the locals function in the blackjack game a few videos ago so non 33 00:02:32,870 --> 00:02:37,340 local tells python to look for variables in an enclosing function and 34 00:02:37,340 --> 00:02:41,000 if it doesn't find it there to check in a functioning closing that and so on 35 00:02:41,000 --> 00:02:45,210 it will not though check the global name space as I mentioned so if you remember 36 00:02:45,210 --> 00:02:50,230 that it doesn't include globals then non local is a good name for it variables that are not 37 00:02:50,230 --> 00:02:54,130 local to the current scope ok so hopefully that made a lot of sense 38 00:02:54,130 --> 00:02:59,790 earlier implied that functions can be nested inside functions and themselves 39 00:02:59,790 --> 00:03:03,970 are themselves nested in another function its quite rare to have 40 00:03:03,970 --> 00:03:07,280 more than one level of nesting in any code that did so would have to be quite 41 00:03:07,280 --> 00:03:12,550 complicated so I use a variation of the spam function in the tutorial to 42 00:03:12,550 --> 00:03:17,570 demonstrate a deeper level of nesting and bearing in mind its probably not 43 00:03:17,570 --> 00:03:20,950 something you have to do very often but the functions would be very similar and 44 00:03:20,950 --> 00:03:23,220 just returned different amounts of spam 45 00:03:23,220 --> 00:03:27,340 in this case but that will let us focus on the scope issues rather than getting 46 00:03:27,340 --> 00:03:31,300 bogged down in complicated code so we are gonna create a new Python file called 47 00:03:31,300 --> 00:03:41,170 spam.py and we are going to enter the following code in it and just do a test 48 00:03:41,170 --> 00:03:50,950 on this and have a look at these functions so... 49 00:03:50,950 --> 00:03:56,730 .... 50 00:04:01,040 --> 00:04:33,270 .... 51 00:04:34,410 --> 00:04:40,410 .... 52 00:04:41,150 --> 00:04:54,630 .....so as you can see 53 00:04:54,630 --> 00:04:59,640 here the function spam one contains another function called spam 2 and 54 00:04:59,640 --> 00:05:08,570 that's spam 2 function contains a third function called spam 3 and if we run this 55 00:05:08,570 --> 00:05:14,940 you can see that spam 1 calls the spam 2 function and then adds its result on to spam 1's text and 56 00:05:14,940 --> 00:05:18,960 spam 2 in turn adds the result of the spam 3 function on its text and 57 00:05:18,960 --> 00:05:22,730 subsequently we get a lot of spam now to get a clearer idea of what's 58 00:05:22,730 --> 00:05:26,290 happening here we can print out the locals dictionary from inside each of the 59 00:05:26,290 --> 00:05:29,690 functions I'm going to use a format string so that we can add a label 60 00:05:29,690 --> 00:05:33,150 before each dictionary to be sure where it came from so let's go ahead and do 61 00:05:33,150 --> 00:05:50,160 that i'm gonna start in spam 3.... 62 00:05:50,160 --> 00:06:09,110 .... 63 00:06:09,650 --> 00:06:20,160 .... 64 00:06:20,160 --> 00:06:26,990 so if we run this again now I've done the same thing in my haste here 65 00:06:26,990 --> 00:06:31,940 so that should have been tabbed over my bad their when I was adding the new lines so 66 00:06:31,940 --> 00:06:40,270 try that again now we can see now and you can see that 67 00:06:40,270 --> 00:06:44,130 it's showing which names are local to each function and we can see that for example 68 00:06:44,130 --> 00:06:49,840 the function spam 3 just contains the string variable z but spam 2 69 00:06:49,840 --> 00:06:57,290 contains the string variable called y you can see it here nearly off the screen but it also contains 70 00:06:57,290 --> 00:06:59,180 a function as you can see there 71 00:06:59,180 --> 00:07:05,910 called spam 3 you can see that reference their and also spam 1 has got a local variable X 72 00:07:05,910 --> 00:07:11,460 and a function called spam 2 as you can see here so within spam 1 73 00:07:11,460 --> 00:07:16,000 itself we can't refer to the variables y&z because variables in an 74 00:07:16,000 --> 00:07:21,650 inner scope aren't available to the outer scope but however inside spam 2 we can 75 00:07:21,650 --> 00:07:25,570 refer to x and inside spam 3 we can refer to both X&Y 76 00:07:26,540 --> 00:07:31,410 so an inner scope has access to the variables defined in an outer scope and we can use the 77 00:07:31,410 --> 00:07:37,910 outer variables and spam 2 spam 3 lets do that by making small change to the code so 78 00:07:37,910 --> 00:07:49,460 close it down lets have a look on what its gonna look like so if we change this now and I'll make that even.... 79 00:07:49,460 --> 00:08:08,930 .... 80 00:08:08,930 --> 00:08:12,280 and run this to make sure it works so you could see that is working ok 81 00:08:13,000 --> 00:08:18,330 and because we're not assigning to X&Y in the inner functions the names don't shadow 82 00:08:18,330 --> 00:08:22,330 the variable in the outer scope and as a result the values are then available for 83 00:08:22,330 --> 00:08:27,420 use now I'm gonna demonstrate scope and it's not necessary sensible thing 84 00:08:27,420 --> 00:08:30,950 to do here but it will demonstrate what we're trying to get across to you and 85 00:08:30,950 --> 00:08:35,659 the reason is... 86 00:08:35,659 --> 00:08:37,600 lines 16 and 17 87 00:08:37,600 --> 00:08:42,409 cam be combined into a single expression and when I change them lets have a look 88 00:08:42,409 --> 00:08:47,500 and see what happens so this here I'm gonna make... 89 00:08:49,800 --> 00:08:55,410 now the mini challenge here is before actually running this program can you work at 90 00:08:55,410 --> 00:08:59,450 what's going to happen and why its going to happen because I only done a simple thing 91 00:08:59,450 --> 00:09:04,940 as you can see I've merged lines 16 and 17 to make it one line but I want you to see if you can 92 00:09:04,940 --> 00:09:10,540 figure out what's going to happen and why so pause the video now and have a think about it and when 93 00:09:10,540 --> 00:09:13,540 your ready to continue come back and we'll go through it 94 00:09:17,860 --> 00:09:22,070 ok so hopefully you have a look at it you might be actually curious if you didn't 95 00:09:22,070 --> 00:09:27,640 figure it out so let's talk about what actually happens now the change I made was 96 00:09:27,640 --> 00:09:31,320 quite simple and it might appear at face value to be a simple optimization 97 00:09:31,320 --> 00:09:36,570 but in actual fact that simple optimization breaks the code and without 98 00:09:36,570 --> 00:09:40,890 starting the spam 2 function it wasn't obvious that the code was gonna break 99 00:09:40,890 --> 00:09:45,780 so lets run it first to confirm that there is a problem and you can see we get 100 00:09:45,780 --> 00:09:50,190 these weird errors so three variable X is referenced before assignment in closing 101 00:09:50,190 --> 00:09:51,130 scope 102 00:09:51,130 --> 00:09:54,540 we got errors here on line 2016 and 11 103 00:09:54,540 --> 00:10:00,400 so what actually happened is that spam 2 the function attempts to use the 104 00:10:00,400 --> 00:10:06,340 value of x but spent spam 2 is called during the initial assignment to X so x doesn't 105 00:10:06,340 --> 00:10:12,730 yet have a value as a result we get the error free variable X 106 00:10:12,730 --> 00:10:16,070 this down here on the bottom free variable x referenced before 107 00:10:16,070 --> 00:10:20,100 assignment in closing scope so theirs actually 3 lessons to learn from the 108 00:10:20,100 --> 00:10:26,960 simple change I'm gonna move the it their so we can look at the source code at the same time while we are talking about it so 109 00:10:26,960 --> 00:10:31,530 the three things are wherever possible try to write functions so that they only 110 00:10:31,530 --> 00:10:37,300 use local variables and parameters only access global and non-local variables 111 00:10:37,300 --> 00:10:42,170 when it's absolutely necessary if you can see an unexpected code such as our 112 00:10:42,170 --> 00:10:50,250 use of 2 lines to assign x and even also to assign y on line 11 and 12 think carefully before 113 00:10:50,250 --> 00:10:53,590 changing them because it could be that the programmer who wrote them was having a 114 00:10:53,590 --> 00:10:58,830 bad day but also their could be a very valid reason why they used 2 lines 115 00:10:58,830 --> 00:11:03,310 when one would appear to do so no matter how trivial a change do you think you've 116 00:11:03,310 --> 00:11:08,110 made test the program thoroughly to make sure you haven't broken anything that's 117 00:11:08,110 --> 00:11:11,690 often simple changes like that that will actually break code and major changes 118 00:11:11,690 --> 00:11:15,450 will probably get through testing anyway but very simple modifications can get 119 00:11:15,450 --> 00:11:20,100 overlooked and this will be a case of the change that I've made on the 16 and 17 to 120 00:11:20,100 --> 00:11:22,830 merge them into one line would be something with that would often 121 00:11:22,830 --> 00:11:29,000 get overlooked and actually there's a fourth lesson here as well and that is that if your write unexpected 122 00:11:29,000 --> 00:11:34,250 code such as our use of 2 lines we assigning X&Y make sure you include a 123 00:11:34,250 --> 00:11:38,780 comment to explain why so the next person who works on your code doesn't do 124 00:11:38,780 --> 00:11:45,820 what you've just done so what we do is undo the change put it back to how it was 125 00:11:45,820 --> 00:11:54,150 before and we come back and say.... 126 00:11:55,070 --> 00:12:04,070 .....and on lines 11 and 12 we would 127 00:12:04,070 --> 00:12:09,480 say something like.... 128 00:12:13,110 --> 00:12:20,920 ...you will actually be surprised how often that next person is 129 00:12:20,920 --> 00:12:25,080 you so coming back to the code perhaps have looked at the program you have written for a 130 00:12:25,080 --> 00:12:29,990 number of days weeks months or even years so the person that you are writing 131 00:12:29,990 --> 00:12:36,870 this comments for could ultimately be used and I just put in an extra space here should have bee watching the 132 00:12:36,870 --> 00:12:43,250 warnings to keep the compiler happy so yeah the person could actually be you 133 00:12:43,250 --> 00:12:47,490 that you're trying to help out here and you can quickly really forget why you did something 134 00:12:47,490 --> 00:12:51,320 in a certain way so using comments like this to explain anything you do that's 135 00:12:51,320 --> 00:12:55,450 not obvious is very very highly recommended now before I finish this 136 00:12:55,450 --> 00:12:58,820 section out theirs two more things to look at the first is very simple 137 00:12:58,820 --> 00:13:03,150 their at the module level the local scope is the same as the global scope so we can 138 00:13:03,150 --> 00:13:08,710 see that by printing both out at the end of the program so lets add these couples of lines of output 139 00:13:08,710 --> 00:13:19,250 to just check that so...and if you run that 140 00:13:26,840 --> 00:13:30,870 and if we scroll across you should find that the basically the two outputs from locals to globals 141 00:13:30,870 --> 00:13:34,240 is giving us the identical output here you can see this looking pretty well 142 00:13:34,240 --> 00:13:39,300 identical right through to the end everything is basically identical that's 143 00:13:39,300 --> 00:13:43,670 for those last two lines of output which correspond to the output from line 22 to 23 144 00:13:45,290 --> 00:13:50,170 so that confirms that in the module scope locals and globals are the same 145 00:13:50,170 --> 00:13:55,040 second thing though is a bit odd and you might be wondering about it if 146 00:13:55,040 --> 00:13:58,010 you paid careful attention to the program's output since I added the non 147 00:13:58,010 --> 00:14:07,180 local variables so lets have a look at that again so spam 3 original had a single variable called z but 148 00:14:07,180 --> 00:14:14,220 now you can see up here it also has y as local and spam 2 you see that has 149 00:14:14,220 --> 00:14:19,610 now got an x its got the x as local as well now this is very odd it seems to go 150 00:14:19,610 --> 00:14:23,730 against everything I have just discussed about non local variables but in fact 151 00:14:23,730 --> 00:14:27,970 the explanation behavior of non local variables is perfectly correct so 152 00:14:27,970 --> 00:14:32,400 there's nothing to worry about what we are seeing is a result of the way the locals 153 00:14:32,400 --> 00:14:37,490 function works and some optimization of the part of Python and that's because whether 154 00:14:37,490 --> 00:14:42,090 you use a non-local variable in the way we've done python adds it to the local 155 00:14:42,090 --> 00:14:45,360 name space so that it doesn't have to go hunting through all the enclosing 156 00:14:45,360 --> 00:14:49,760 scopes every time you use it so it still referring to the non local variable 157 00:14:49,760 --> 00:14:55,810 its just a reference it in locals to make everything run faster and it might be worth your while 158 00:14:55,810 --> 00:15:03,770 having a look at the documentation for the locals function as well I'm just gonna bring that up on the screen 159 00:15:03,770 --> 00:15:06,770 theirs the link on the screen as you can see up there so we go to that 160 00:15:10,270 --> 00:15:18,470 theirs locals so a free variable is a variable that's used in a scope where it has not been 161 00:15:18,470 --> 00:15:24,160 defined which is exactly the same thing as a non-local variable when working 162 00:15:24,160 --> 00:15:27,420 with the scope of variables it is helpful to remember the analogy 163 00:15:27,420 --> 00:15:33,670 LEGB and that stands for local enclosing global built-ins and list the order to 164 00:15:33,670 --> 00:15:40,530 which Python searches for names so again its local enclosing global and built-ins so 165 00:15:40,530 --> 00:15:43,910 closing refers to non-local or free variables that I've discussed in this 166 00:15:43,910 --> 00:15:48,920 lecture now globals we have also seen and we have looked at how to use it and assigned to 167 00:15:48,920 --> 00:15:54,820 global variables of course in our functions built in refers to the name that are built-in to Python 168 00:15:54,820 --> 00:15:58,360 and that we've been using throughout the course we've got into the builtins.py module 169 00:15:58,360 --> 00:16:01,960 quite a few times now to have a look at the various functions and built-ins are 170 00:16:01,960 --> 00:16:06,030 the final place that Python will possible look to when trying to resolve a 171 00:16:06,030 --> 00:16:10,800 name now if you remember and understand that LEGB in order and again that is local enclosing 172 00:16:10,800 --> 00:16:15,230 global built-ins if remember that for resolving names and take note of the 173 00:16:15,230 --> 00:16:19,170 warnings that IntelliJ gives you should be quite comfortable with the way Python 174 00:16:19,170 --> 00:16:23,740 scoping works and then should also be able to use non-local and global variables 175 00:16:23,740 --> 00:16:24,650 when you need to 176 00:16:24,650 --> 00:16:31,210 ok finally that brings this section to an end it's been a very very long one we've 177 00:16:31,210 --> 00:16:34,430 looked at a few modules from the Python standard library and also the 178 00:16:34,430 --> 00:16:38,720 documentation so that other modules should be much easier to use when a program 179 00:16:38,720 --> 00:16:42,770 needs the functionality they provide and of course we also had a look at GUI 180 00:16:42,770 --> 00:16:47,880 programming called TKinter we did a lot with that but we are not done with that yet and theirs few more 181 00:16:47,880 --> 00:16:51,510 things we need to come before you can use it to its full potential and then 182 00:16:51,510 --> 00:16:54,930 covered functions showing you how they are or what they are I should say and how to write your own 183 00:16:54,930 --> 00:16:59,990 and in the process we learn about scope in Python now classes are also 184 00:16:59,990 --> 00:17:01,440 created a new namespace 185 00:17:01,440 --> 00:17:05,160 and we are gonna be covering object oriented programming in classes in the next section 186 00:17:05,160 --> 00:17:06,349 so I'll see you their