1 00:00:05,750 --> 00:00:07,910 Welcome back everyone in this lecture. 2 00:00:07,910 --> 00:00:14,010 We're going to start off by talking about scope and also nested statements in general scope is the concept 3 00:00:14,010 --> 00:00:18,780 that determines where you can actually see or access a variable and if you're jumping straight into 4 00:00:18,780 --> 00:00:23,850 Python level too by now you probably actually already have an intuitive understanding of this idea of 5 00:00:23,850 --> 00:00:24,420 scope. 6 00:00:24,600 --> 00:00:29,250 But we're going to do is have a more formal discussion of this topic and show you some things that maybe 7 00:00:29,250 --> 00:00:30,540 you haven't thought about yet. 8 00:00:30,600 --> 00:00:31,350 So let's get started. 9 00:00:31,350 --> 00:00:35,280 We're going to head over to the text editor and we're just going to do a couple of experiments to show 10 00:00:35,280 --> 00:00:37,410 you this idea of scope more formally. 11 00:00:37,410 --> 00:00:40,280 All right let's begin with a quick thought experiment. 12 00:00:40,410 --> 00:00:48,630 I'm going to set a variable x equal to the string outside then I'll create a function further down called 13 00:00:48,630 --> 00:00:52,360 report and report doesn't accept any arguments. 14 00:00:52,530 --> 00:01:01,050 What it does is it sets X equal to inside and then it says return x. 15 00:01:01,070 --> 00:01:08,210 So now the question is if I were to say prints X what would you expect to see. 16 00:01:08,310 --> 00:01:12,610 Would you expect to see outside or inside. 17 00:01:13,110 --> 00:01:19,890 Well this is where the idea of scope comes in basically scope comes into play when Python is asked to 18 00:01:19,890 --> 00:01:22,690 the termine what type of variable you're referencing. 19 00:01:22,710 --> 00:01:27,560 In this case Python wants to know what X you're referencing when you call 7 here. 20 00:01:27,570 --> 00:01:29,460 So let's actually run this and see what happens. 21 00:01:30,750 --> 00:01:36,930 Notice that it says outside which basically means that it's referring to the x over here x is equal 22 00:01:36,930 --> 00:01:41,420 to outside instead of the re-assignment that's happening inside report. 23 00:01:41,430 --> 00:01:46,080 Now you may already have some intuition at that because technically you're not actually calling report 24 00:01:46,080 --> 00:01:48,650 to execute anywhere you've just define this function. 25 00:01:48,690 --> 00:01:50,230 I haven't actually called it. 26 00:01:50,460 --> 00:01:55,210 Now sometimes what's confusing for beginners is let's imagine I do call report. 27 00:01:55,410 --> 00:01:57,560 So I have Report be executed. 28 00:01:57,600 --> 00:02:01,760 In fact I say print's report. 29 00:02:01,920 --> 00:02:06,210 So I'm going to print out report which should return x in this case it's inside. 30 00:02:06,360 --> 00:02:08,160 And then going it prints X again. 31 00:02:08,160 --> 00:02:09,240 Now what do you expect. 32 00:02:09,270 --> 00:02:13,780 Do you expect to see inside outside or inside inside. 33 00:02:13,800 --> 00:02:17,430 Well let's say this and find out. 34 00:02:17,440 --> 00:02:20,250 So in this case we see inside and outside. 35 00:02:20,320 --> 00:02:25,380 So the result of running a report does give you this re-assignment x is equal to inside. 36 00:02:25,480 --> 00:02:30,260 But notice how it doesn't affect this global scope appear of exegetical to outside. 37 00:02:30,340 --> 00:02:36,020 When you say print X without any reference to this function you'll still say outside. 38 00:02:36,280 --> 00:02:39,760 So the whole reason this sort of thing is happening is because of scope. 39 00:02:39,850 --> 00:02:42,220 So let's say discuss this idea of scope. 40 00:02:42,550 --> 00:02:43,520 And in simple terms. 41 00:02:43,520 --> 00:02:47,160 Basically the idea of scope can be described in three general rules. 42 00:02:47,260 --> 00:02:51,840 One is that name assignments will create or change local names by default. 43 00:02:52,560 --> 00:02:57,180 Two is that name references search at most for scopes. 44 00:02:57,180 --> 00:03:00,000 So let's talk about these for scopes. 45 00:03:00,000 --> 00:03:03,300 They're known as the LGB l e G-B rule. 46 00:03:05,040 --> 00:03:15,080 And that stands for local inclosing global and built in. 47 00:03:15,810 --> 00:03:21,300 So what's going to happen is Python is going to search in this order of scopes to try to figure out 48 00:03:21,390 --> 00:03:23,580 what you're actually referencing. 49 00:03:23,580 --> 00:03:27,630 So if it can't find something locally they'll start looking in the inclosing function. 50 00:03:27,630 --> 00:03:32,370 If it can't find the reference and inclosing function will start looking globally at the module level. 51 00:03:32,370 --> 00:03:37,980 Basically your entire Python script if it still can't find it then will start looking at builtin keywords 52 00:03:37,980 --> 00:03:42,100 with Python such as the Eliane length function. 53 00:03:42,110 --> 00:03:45,460 Now in order to actually build the understanding of each of these scopes. 54 00:03:45,530 --> 00:03:47,330 Actually work through some examples of them. 55 00:03:47,360 --> 00:03:49,270 I'm going to clear out this code right here. 56 00:03:50,110 --> 00:03:52,260 And we'll come back to it later on. 57 00:03:52,360 --> 00:03:56,950 For now let me show you an example of local and that's actually basically what we just did inside of 58 00:03:56,950 --> 00:03:58,090 that function. 59 00:03:58,150 --> 00:04:02,740 So I'm going to say report X equal to local. 60 00:04:02,740 --> 00:04:04,520 So this is a local assignment 61 00:04:07,850 --> 00:04:14,150 and then if I say print x x is going to be searched for first locally. 62 00:04:14,150 --> 00:04:16,190 So right here at the local scope 63 00:04:19,020 --> 00:04:23,210 melts take a look at this next scope which is called inclosing. 64 00:04:23,260 --> 00:04:26,090 So this is an example of an inclosing function local. 65 00:04:26,110 --> 00:04:28,750 So this first one this is a local assignment. 66 00:04:28,750 --> 00:04:30,720 Now I want to check out inclosing. 67 00:04:30,910 --> 00:04:34,630 So this is going to occur when you have a function inside of another function. 68 00:04:34,630 --> 00:04:36,610 We have actually seen an example of this so far. 69 00:04:36,610 --> 00:04:38,230 So let's try it out. 70 00:04:38,320 --> 00:04:41,960 I'm going to say x is equal to a string. 71 00:04:41,980 --> 00:04:49,730 This is global level and it's global level because it has no indentation and it's not within another 72 00:04:49,730 --> 00:04:54,630 function and to show this I'm going to comets out our entire previous function. 73 00:04:54,650 --> 00:04:58,000 So now this is the only life code inside of this python script. 74 00:04:58,160 --> 00:05:11,650 So I'm saying X is a global level parameter or variable then I will say the F inclosing colon and I'll 75 00:05:11,650 --> 00:05:13,110 say x is equal to. 76 00:05:13,270 --> 00:05:19,000 And here I'm going to assign it inclosing level. 77 00:05:19,080 --> 00:05:24,000 Then inside of my inclosing function ongoings create another function. 78 00:05:24,590 --> 00:05:27,920 So notice here how this function is actually inside the enclosing function. 79 00:05:27,920 --> 00:05:38,850 Take a look at my indentation carefully and then I'm going to print x and then I'm going to call inside. 80 00:05:38,850 --> 00:05:42,600 So again take very careful attention of the actual indentation. 81 00:05:42,960 --> 00:05:47,390 All of this code right here is within this inclosing function. 82 00:05:47,400 --> 00:05:50,750 So what's happening here is I'm defining inclosing function. 83 00:05:50,880 --> 00:05:53,970 I'm setting this variable x equal to the inclosing level. 84 00:05:54,300 --> 00:06:00,840 Then I say this other function inside is going to print out X and then I'm actually going to call the 85 00:06:00,840 --> 00:06:02,490 inside function to execute. 86 00:06:02,490 --> 00:06:10,800 Which means when I call the inclosing function to x Q It says X defines inside and then executes inside. 87 00:06:10,800 --> 00:06:13,470 So now here X I have this global level. 88 00:06:13,470 --> 00:06:17,280 So now the question arises if that is further down in my script. 89 00:06:17,280 --> 00:06:18,780 Let me get a little bit of space here. 90 00:06:21,470 --> 00:06:26,320 Maybe you can actually see the indentation very clearly but if further down in my script script hour 91 00:06:26,360 --> 00:06:29,840 to execute inclosing What do I expect to see. 92 00:06:29,940 --> 00:06:35,320 Do I expect to see this is a global level or inclosing level. 93 00:06:35,630 --> 00:06:38,100 Well let's think of the L EJB rule. 94 00:06:38,540 --> 00:06:42,680 So in this case we're going to follow L E G B. 95 00:06:42,680 --> 00:06:48,410 So we first look for something local roomer something local looks like this. 96 00:06:48,650 --> 00:06:53,740 When the uncomment this so we can review this is a local assignment. 97 00:06:53,750 --> 00:06:55,240 So X just looks locally. 98 00:06:55,250 --> 00:06:59,070 See if find something at the same indentation level and there it is local. 99 00:06:59,120 --> 00:07:01,380 So that's what Python wants to look for first. 100 00:07:02,400 --> 00:07:04,590 So here we're saying Prince X and pythons. 101 00:07:04,610 --> 00:07:04,990 OK. 102 00:07:05,010 --> 00:07:10,860 Let me see if I can find something where x is defined locally and it looks for the entire inside function 103 00:07:11,130 --> 00:07:12,130 and it can't find it. 104 00:07:12,240 --> 00:07:12,750 It says. 105 00:07:12,750 --> 00:07:13,100 All right. 106 00:07:13,110 --> 00:07:14,410 So L is done. 107 00:07:14,550 --> 00:07:16,970 Now it's time for me to search inclosing. 108 00:07:17,040 --> 00:07:19,570 So the inclosing function is this entire function. 109 00:07:20,070 --> 00:07:23,790 And then it can find right here x is equal to inclosing level. 110 00:07:23,790 --> 00:07:24,900 So that's what it goes with. 111 00:07:24,900 --> 00:07:26,000 It stops right there. 112 00:07:26,010 --> 00:07:30,240 It no longer needs to jump up further and start looking for a global variable. 113 00:07:30,720 --> 00:07:34,770 So if I were to save this and run it to make sure everything else commented out. 114 00:07:35,630 --> 00:07:40,850 Let's run the script and there we see inclosing level as expected. 115 00:07:40,850 --> 00:07:42,590 So this is no longer local. 116 00:07:42,590 --> 00:07:44,150 Instead it's inclosing. 117 00:07:44,480 --> 00:07:45,100 OK. 118 00:07:45,320 --> 00:07:47,660 Now let's talk about the global level. 119 00:07:47,900 --> 00:07:52,400 So I'm going to comment out this assignment here. 120 00:07:53,730 --> 00:07:55,440 I'm going to save this again. 121 00:07:55,650 --> 00:08:00,310 And remember the order that pi is going to look for it's going to say let me try to find something locally. 122 00:08:00,420 --> 00:08:02,320 In this case it won't be able to find it. 123 00:08:02,340 --> 00:08:06,380 Let me try to find something within the inclosing function actually commented out. 124 00:08:06,390 --> 00:08:07,740 So I don't find it there. 125 00:08:07,740 --> 00:08:12,810 So the next step is hey let me start looking for something at the global level basically with zero and 126 00:08:12,810 --> 00:08:13,480 then Station. 127 00:08:13,560 --> 00:08:15,870 And there we do see this global level variable. 128 00:08:16,080 --> 00:08:18,730 So I'm going to save that run it. 129 00:08:18,860 --> 00:08:19,970 And now I get back. 130 00:08:19,970 --> 00:08:27,380 This is global level so we can clearly see here how Python is taking the steps of local inclosing global 131 00:08:27,440 --> 00:08:28,610 and built in. 132 00:08:28,700 --> 00:08:31,660 And in fact let's go ahead and show them all right now. 133 00:08:33,050 --> 00:08:35,900 So let me make sure that's been answered correctly. 134 00:08:37,870 --> 00:08:46,090 I'm going to say local here so let's run this again and whips I make sure my indentation is correct. 135 00:08:46,120 --> 00:08:47,490 There we go run this again. 136 00:08:47,750 --> 00:08:53,670 And now I see local so I can see there's a local level and if I were to comment that I'll save it and 137 00:08:53,670 --> 00:08:54,820 run it again. 138 00:08:54,920 --> 00:09:00,620 Then I see the inclosing level up here if I'm to comment that I'll save it and run it again. 139 00:09:00,720 --> 00:09:02,670 Then I see the global level. 140 00:09:02,670 --> 00:09:06,180 So hopefully this is a really clear picture of how Python is going. 141 00:09:06,180 --> 00:09:08,480 Local inclosing and global. 142 00:09:08,670 --> 00:09:12,440 Now let's say you can't find something in global. 143 00:09:12,500 --> 00:09:15,250 It's going to then look for built in namespace. 144 00:09:15,380 --> 00:09:18,920 And in this case these are the type of functions and keywords that you're actually already familiar 145 00:09:18,920 --> 00:09:27,140 with things like length or some books or some or max those kinds of things. 146 00:09:27,140 --> 00:09:31,280 So if it can't find it as a built in variable then it's just going to give you an error and say hey 147 00:09:31,340 --> 00:09:33,060 X isn't defined. 148 00:09:33,140 --> 00:09:36,430 So if we run this I'll say hey sorry name x is not the find. 149 00:09:36,440 --> 00:09:39,230 Well maybe it won't be so apologetic but you get the idea. 150 00:09:40,300 --> 00:09:44,680 The last thing I want to cover here is just local variables versus global variables. 151 00:09:44,740 --> 00:09:51,910 We already understand scope in general how we go local inclosing global and builtin basically the order 152 00:09:51,910 --> 00:09:55,220 that Python is going to search for variable definitions in. 153 00:09:55,570 --> 00:09:59,720 But let's go back to the original example of the report. 154 00:10:00,070 --> 00:10:03,760 So remember I'm going to delete everything here. 155 00:10:03,760 --> 00:10:16,820 Remember we had something that said X outside and then we say for Port x is equal to inside and we return 156 00:10:16,970 --> 00:10:17,470 X 157 00:10:20,860 --> 00:10:25,710 and then I say go ahead and print out report for me. 158 00:10:26,230 --> 00:10:32,260 When our toes run this notice I get back inside because right now we understand that it's going to look 159 00:10:32,350 --> 00:10:34,720 locally for that scope first. 160 00:10:34,720 --> 00:10:41,200 Now there may be times where you actually do want to change this particular value that is global. 161 00:10:41,200 --> 00:10:43,210 So the question is Well how do I do that. 162 00:10:43,300 --> 00:10:48,580 I actually reach out and tell Python Hey when I'm doing this x is equal to inside I actually want to 163 00:10:48,580 --> 00:10:51,820 change this global variable as well in order to do that. 164 00:10:51,840 --> 00:10:53,870 Use the global keyword. 165 00:10:53,890 --> 00:10:58,950 Keep in mind you should be very careful with this and most Python users don't use the global keyword 166 00:10:58,960 --> 00:10:59,380 often. 167 00:10:59,380 --> 00:11:03,270 In fact it should be avoided because of how powerful it is. 168 00:11:03,340 --> 00:11:08,470 Basically you could really easily reassign everything within a function within a script and kind of 169 00:11:08,470 --> 00:11:12,390 screw yourself up so that's where you want to do this with great caution. 170 00:11:12,400 --> 00:11:14,710 I will show you how it works. 171 00:11:14,710 --> 00:11:16,860 So the way this works is the following manner. 172 00:11:17,940 --> 00:11:24,750 I'm going to say x is equal to outside and then within my report I'm going to say Global X and this 173 00:11:24,750 --> 00:11:35,550 basically tells Python Hey grab the global level x variable and then once it's done that then it's going 174 00:11:35,550 --> 00:11:39,510 to reassign the global x. 175 00:11:39,540 --> 00:11:41,970 So this is no longer just a local assignment. 176 00:11:41,970 --> 00:11:45,200 It's actually telling Python Hey keyword global. 177 00:11:45,270 --> 00:11:50,310 That means this X is going to reference this outside Global X and then when you say x is equal to the 178 00:11:50,310 --> 00:11:52,580 inside you're going to return x. 179 00:11:52,620 --> 00:11:57,000 So when we actually run this we won't see anything different occur. 180 00:11:57,030 --> 00:12:06,150 We will still see inside what will be different is if we were to ask for Python to Prince X. 181 00:12:06,190 --> 00:12:10,000 After calling that report function knows what happens. 182 00:12:10,000 --> 00:12:12,760 Now I get inside inside. 183 00:12:12,850 --> 00:12:19,030 If I were to remove that global call here so I'm still having x is equal to outside. 184 00:12:19,300 --> 00:12:21,720 And now I'm just re-assigning locally. 185 00:12:22,210 --> 00:12:29,860 When I save and run this that I see inside outside it's only when I call that global keyword that Python 186 00:12:29,890 --> 00:12:33,630 basically now has permission to go into the global namespace. 187 00:12:33,670 --> 00:12:37,300 Grab the x variable and do the re-assignment globally. 188 00:12:37,300 --> 00:12:44,150 Until then you won't actually be able to affect the scope of this x variable and this function. 189 00:12:44,480 --> 00:12:47,340 OK so that's the very basics of scopes. 190 00:12:47,350 --> 00:12:52,300 Again the global keyword in general if you're starting to learn at this level of Python you should just 191 00:12:52,300 --> 00:12:54,070 pretty much avoid it entirely. 192 00:12:54,070 --> 00:12:55,620 Instead you should pass it in. 193 00:12:55,630 --> 00:12:57,630 If you ever want to work with it. 194 00:12:57,780 --> 00:13:03,600 So what I mean by that is if for some reason you do want to work with this outside a variable where 195 00:13:03,610 --> 00:13:10,330 you can do is you can just pass it in report X and then say x is equal to inside return x. 196 00:13:10,540 --> 00:13:14,780 And so you can say X here and then play around with it that way. 197 00:13:14,830 --> 00:13:18,060 So here you'll notice we get inside and then we still get outside. 198 00:13:18,060 --> 00:13:24,550 So this is a much safer way of doing any sort of reassignments while still having access to the initial 199 00:13:24,820 --> 00:13:25,680 global. 200 00:13:25,690 --> 00:13:35,760 So for example I could still say print x here and so I should see outside inside outside and there it 201 00:13:35,760 --> 00:13:38,410 is outside inside outside. 202 00:13:38,790 --> 00:13:42,380 OK if you have any questions feel free to post the Kewney forums. 203 00:13:42,420 --> 00:13:48,240 Hopefully you start to really get a fundamental foundation to your already intuitive understanding of 204 00:13:48,240 --> 00:13:48,680 scope. 205 00:13:48,810 --> 00:13:54,000 Just from your lectures on functions in the next lecture we'll start talking about object oriented programming. 206 00:13:54,060 --> 00:13:54,770 We'll see if.