1 1 00:00:01,970 --> 00:00:03,720 Hi guys and welcome back. 2 2 00:00:03,720 --> 00:00:07,200 In this video we're going to talk about decorators. 3 3 00:00:07,200 --> 00:00:10,200 A decorator is an advanced Python construct 4 4 00:00:10,200 --> 00:00:13,250 that is frequently confusing for beginners 5 5 00:00:13,250 --> 00:00:14,970 and with good reason. 6 6 00:00:14,970 --> 00:00:16,160 It's a bit of a weird one 7 7 00:00:16,160 --> 00:00:20,550 but we're gonna try our best to cover it in simple terms 8 8 00:00:20,550 --> 00:00:22,870 and then move over to our application 9 9 00:00:22,870 --> 00:00:24,343 so that it's a bit clearer. 10 10 00:00:25,420 --> 00:00:27,850 Imagine you've got a Python dictionary 11 11 00:00:27,850 --> 00:00:31,240 like this one that says something like, username 12 12 00:00:31,240 --> 00:00:32,993 and access level, 13 13 00:00:33,960 --> 00:00:35,540 let's say is admin. 14 14 00:00:35,540 --> 00:00:36,610 So you've got this dictionary 15 15 00:00:36,610 --> 00:00:39,620 and by the way I'm just here on a Python interpreter 16 16 00:00:39,620 --> 00:00:41,840 and so every line of code that I'm executing 17 17 00:00:41,840 --> 00:00:44,030 is just instantly running. 18 18 00:00:44,030 --> 00:00:45,520 So now the variable is defined, 19 19 00:00:45,520 --> 00:00:47,290 if you just type the variable name out, 20 20 00:00:47,290 --> 00:00:49,743 this tells you the values of it. 21 21 00:00:50,782 --> 00:00:51,653 And by the way so you know 22 22 00:00:51,653 --> 00:00:54,720 that if you've taken our complete Python course, 23 23 00:00:54,720 --> 00:00:56,340 this is gonna look very familiar. 24 24 00:00:56,340 --> 00:00:59,093 It's essentially the same example. 25 25 00:01:01,400 --> 00:01:05,130 Now let's say that you have another function, 26 26 00:01:05,130 --> 00:01:07,283 which I'm gonna call my function. 27 27 00:01:08,640 --> 00:01:12,820 And it just returns like the password for an admin panel. 28 28 00:01:12,820 --> 00:01:15,020 So the password for the admin panel 29 29 00:01:15,020 --> 00:01:18,273 is 1234, very secure password. 30 30 00:01:19,280 --> 00:01:21,840 Now whenever you call my function, 31 31 00:01:21,840 --> 00:01:25,263 it just tells you the password for the admin panel is 1234. 32 32 00:01:27,620 --> 00:01:31,373 Even if somebody comes along that isn't this user, 33 33 00:01:32,240 --> 00:01:34,070 they can still run my function 34 34 00:01:34,070 --> 00:01:36,890 and they can still get the password for the admin panel. 35 35 00:01:36,890 --> 00:01:38,320 So we wanna stop that. 36 36 00:01:38,320 --> 00:01:40,900 We wanna make sure that only users 37 37 00:01:40,900 --> 00:01:43,660 that have an access level of admin 38 38 00:01:43,660 --> 00:01:45,843 are able to run my function. 39 39 00:01:47,000 --> 00:01:48,780 So how do you do that? 40 40 00:01:48,780 --> 00:01:53,780 Well, you can say, if user access level is admin, 41 41 00:01:55,560 --> 00:01:57,193 then you can run my function, 42 42 00:01:58,250 --> 00:01:59,790 otherwise you can't. 43 43 00:01:59,790 --> 00:02:01,360 If you do this, 44 44 00:02:01,360 --> 00:02:03,190 I mean if you actually run this if statement, 45 45 00:02:03,190 --> 00:02:05,450 it tells you the password because the user 46 46 00:02:05,450 --> 00:02:08,370 does have an access level of admin. 47 47 00:02:08,370 --> 00:02:11,440 Let's try to put this inside a function, 48 48 00:02:11,440 --> 00:02:14,680 so that we can get the value out of it 49 49 00:02:14,680 --> 00:02:16,940 already with the comparison 50 50 00:02:16,940 --> 00:02:18,693 of the access level being executed. 51 51 00:02:20,370 --> 00:02:25,073 So this can be called something like, get password if admin. 52 52 00:02:26,728 --> 00:02:28,730 And what this will do is it will say exactly 53 53 00:02:28,730 --> 00:02:31,660 what we have there, if the user has an access level 54 54 00:02:33,940 --> 00:02:36,583 of admin then we're gonna run my function. 55 55 00:02:38,390 --> 00:02:40,340 Otherwise we're not gonna run anything. 56 56 00:02:41,220 --> 00:02:42,710 But here's the thing, 57 57 00:02:42,710 --> 00:02:46,220 what happens when you run this function? 58 58 00:02:46,220 --> 00:02:49,740 Remember, my function returns a string, 59 59 00:02:49,740 --> 00:02:52,170 it returns the string that tells you the password, 60 60 00:02:52,170 --> 00:02:53,493 it doesn't print it out. 61 61 00:02:54,360 --> 00:02:57,963 So when you run get password if admin, 62 62 00:02:58,830 --> 00:03:00,500 you get nothing. 63 63 00:03:00,500 --> 00:03:01,860 Why is that? 64 64 00:03:01,860 --> 00:03:04,740 Because this function here executed 65 65 00:03:05,740 --> 00:03:08,750 but we didn't return its value, 66 66 00:03:08,750 --> 00:03:11,620 we didn't return the value that this function returned 67 67 00:03:11,620 --> 00:03:15,070 so actually get password if admin returns none, 68 68 00:03:15,070 --> 00:03:17,713 that's the default for all Python functions. 69 69 00:03:19,870 --> 00:03:21,100 So what can you do? 70 70 00:03:21,100 --> 00:03:24,360 Well, you can say again, get password if admin, 71 71 00:03:24,360 --> 00:03:26,370 we gonna define it again but now we're gonna say, 72 72 00:03:26,370 --> 00:03:30,253 if the user access level is admin, 73 73 00:03:31,220 --> 00:03:33,063 we're gonna return my function. 74 74 00:03:35,800 --> 00:03:36,680 Okay? 75 75 00:03:36,680 --> 00:03:37,890 So let's do it again. 76 76 00:03:37,890 --> 00:03:42,890 Get password if admin, and now we get the result. 77 77 00:03:44,350 --> 00:03:45,183 Perfect. 78 78 00:03:45,183 --> 00:03:47,113 So is this a decorator? 79 79 00:03:48,080 --> 00:03:49,040 Not yet. 80 80 00:03:49,040 --> 00:03:50,250 This is not quite a decorator 81 81 00:03:50,250 --> 00:03:52,610 but it does exactly the same thing 82 82 00:03:52,610 --> 00:03:54,223 that a decorator would do. 83 83 00:03:55,222 --> 00:03:59,260 However, this still has a problem. 84 84 00:03:59,260 --> 00:04:01,760 Which is that anybody comes into this code 85 85 00:04:01,760 --> 00:04:03,510 and can say, you know, my function. 86 86 00:04:04,420 --> 00:04:06,900 And then we don't check whether their access level 87 87 00:04:06,900 --> 00:04:07,883 is admin or not. 88 88 00:04:09,200 --> 00:04:10,033 What have we done? 89 89 00:04:10,033 --> 00:04:13,520 We've actually created a new function that does the checking 90 90 00:04:13,520 --> 00:04:17,130 but we've still left the old function unchecked. 91 91 00:04:17,130 --> 00:04:20,070 Another example code. 92 92 00:04:20,070 --> 00:04:23,240 We're gonna say something like user has permission 93 93 00:04:24,130 --> 00:04:26,740 and we're gonna pass to it a function. 94 94 00:04:26,740 --> 00:04:28,453 I'm just calling it func for now. 95 95 00:04:30,730 --> 00:04:33,720 What we wanna do, is we wanna run this function 96 96 00:04:34,830 --> 00:04:38,573 if the user has the permission. 97 97 00:04:40,220 --> 00:04:45,220 So we'll say, if user access level is admin. 98 98 00:04:52,520 --> 00:04:55,540 Now we wanna run the function 99 99 00:04:55,540 --> 00:04:59,190 so we will say return func, okay? 100 100 00:04:59,190 --> 00:05:02,600 Pretty much exactly the same as we had up here. 101 101 00:05:02,600 --> 00:05:04,400 But if we say, 102 102 00:05:04,400 --> 00:05:09,283 my function equal user has permission with my function, 103 103 00:05:10,300 --> 00:05:14,080 what happens now is that my function 104 104 00:05:14,920 --> 00:05:18,950 which used to be this unsecured function up here, 105 105 00:05:18,950 --> 00:05:23,430 my function has now become the result of running 106 106 00:05:23,430 --> 00:05:27,223 user has permission with my function passed into it. 107 107 00:05:28,200 --> 00:05:30,300 So if we go just up here, 108 108 00:05:30,300 --> 00:05:32,620 we can see what happens when you do that. 109 109 00:05:32,620 --> 00:05:35,300 My function becomes func, 110 110 00:05:35,300 --> 00:05:38,300 and then we check if the user's access level is admin 111 111 00:05:38,300 --> 00:05:41,730 then return the value of running func. 112 112 00:05:41,730 --> 00:05:43,620 When you run my function you get, 113 113 00:05:43,620 --> 00:05:46,090 password for admin panel is 1234. 114 114 00:05:46,090 --> 00:05:49,410 So user has permission will return, 115 115 00:05:49,410 --> 00:05:51,590 password for admin panel is 1234 116 116 00:05:51,590 --> 00:05:54,690 because of the time that we run this line, 117 117 00:05:54,690 --> 00:05:58,390 the user had an admin access level. 118 118 00:05:58,390 --> 00:06:02,120 So now, my function is no longer a function, 119 119 00:06:02,120 --> 00:06:07,000 now my function is the result of running the check 120 120 00:06:07,000 --> 00:06:08,910 and returning this function. 121 121 00:06:08,910 --> 00:06:11,670 This is not what we wanted to do. 122 122 00:06:11,670 --> 00:06:13,863 So I'm gonna clear it a bit, 123 123 00:06:14,950 --> 00:06:16,800 and then we're gonna copy this again. 124 124 00:06:19,000 --> 00:06:21,347 We're gonna once again, get my function 125 125 00:06:21,347 --> 00:06:23,640 and we're gonna make it into a function once again, 126 126 00:06:23,640 --> 00:06:26,980 in order for us to get to our final example. 127 127 00:06:26,980 --> 00:06:30,990 So now, my function gives us the password 128 128 00:06:30,990 --> 00:06:33,490 for the admin panel is 1234 but it is a function 129 129 00:06:33,490 --> 00:06:34,673 we do have to run it. 130 130 00:06:35,740 --> 00:06:37,670 What we're gonna do now is we're gonna define 131 131 00:06:37,670 --> 00:06:40,607 another user has permission function 132 132 00:06:40,607 --> 00:06:43,740 and it's also gonna take in func, 133 133 00:06:43,740 --> 00:06:46,800 a function as a parameter. 134 134 00:06:46,800 --> 00:06:49,420 But inside of this function, 135 135 00:06:49,420 --> 00:06:51,513 we're going to define another function. 136 136 00:06:54,110 --> 00:06:56,540 Inside this function, we're gonna say, 137 137 00:06:56,540 --> 00:07:01,540 if user access level is admin, 138 138 00:07:01,630 --> 00:07:03,753 then we're gonna return func. 139 139 00:07:06,340 --> 00:07:08,650 So this user has permission function 140 140 00:07:08,650 --> 00:07:10,910 actually defined a function inside of it 141 141 00:07:10,910 --> 00:07:13,090 that does the checking and runs 142 142 00:07:13,090 --> 00:07:15,623 whatever function was originally passed to it. 143 143 00:07:17,210 --> 00:07:22,080 What should user has permission do with that function? 144 144 00:07:22,080 --> 00:07:24,680 It should return it. 145 145 00:07:24,680 --> 00:07:25,830 And here's why. 146 146 00:07:25,830 --> 00:07:30,830 Now, if we run my function, it's completely unsecured 147 147 00:07:31,240 --> 00:07:35,387 but we should now say, my function is equal to user 148 148 00:07:36,420 --> 00:07:41,093 has permission with my function passed into it. 149 149 00:07:41,960 --> 00:07:44,200 So what this going to do when we run it, 150 150 00:07:44,200 --> 00:07:47,020 is it's gonna evaluate 151 151 00:07:47,020 --> 00:07:49,743 the result of running user has permission. 152 152 00:07:50,890 --> 00:07:53,870 And it's gonna replace my function 153 153 00:07:53,870 --> 00:07:56,610 with whatever that returns. 154 154 00:07:56,610 --> 00:08:01,080 So, once again, my function is gonna be passed in as func, 155 155 00:08:01,080 --> 00:08:04,300 we're gonna define another function and inside that one, 156 156 00:08:04,300 --> 00:08:06,750 we're gonna check the access level to see if it's admin 157 157 00:08:06,750 --> 00:08:09,810 and if it is, we're going to run the original function 158 158 00:08:09,810 --> 00:08:11,203 which is my function. 159 159 00:08:12,450 --> 00:08:15,680 Then we return this inner function 160 160 00:08:15,680 --> 00:08:19,560 and that is what my function becomes. 161 161 00:08:19,560 --> 00:08:23,323 So my function becomes this secure func. 162 162 00:08:25,240 --> 00:08:27,370 So if we run my function now, 163 163 00:08:27,370 --> 00:08:30,730 we get the password for the admin panel is 1234 164 164 00:08:30,730 --> 00:08:34,010 but if we define another user with say, 165 165 00:08:34,010 --> 00:08:36,403 username bob and access level, 166 166 00:08:37,670 --> 00:08:38,503 guest. 167 167 00:08:39,770 --> 00:08:43,410 And we run my function again, now you get nothing. 168 168 00:08:43,410 --> 00:08:45,960 Because the user at the time of running, 169 169 00:08:45,960 --> 00:08:48,303 did not have the appropriate access level. 170 170 00:08:49,330 --> 00:08:50,720 What we've just defined here, 171 171 00:08:50,720 --> 00:08:53,373 this is a decorator in Python. 172 172 00:08:54,300 --> 00:08:57,770 A decorator is a function that returns another function 173 173 00:08:58,650 --> 00:09:02,750 and the inner function runs the function that was passed 174 174 00:09:02,750 --> 00:09:04,743 to the decorator as an argument. 175 175 00:09:06,070 --> 00:09:07,280 And the reason for that, 176 176 00:09:07,280 --> 00:09:09,570 is because it allows us to extend 177 177 00:09:09,570 --> 00:09:11,620 the original function with extra things. 178 178 00:09:11,620 --> 00:09:14,010 In this case we are doing if the user's access level 179 179 00:09:14,010 --> 00:09:16,023 as admin will run this function. 180 180 00:09:17,750 --> 00:09:19,070 So that's what a decorator is for, 181 181 00:09:19,070 --> 00:09:21,170 a decorator is for extending the functionality 182 182 00:09:21,170 --> 00:09:22,350 of an existing function 183 183 00:09:23,370 --> 00:09:25,643 while leaving the original, unchanged. 184 184 00:09:28,490 --> 00:09:31,660 And I know that this is a bit of a strange way 185 185 00:09:31,660 --> 00:09:35,200 of looking at decorators but we're gonna now move over 186 186 00:09:35,200 --> 00:09:37,850 to our web application and you're gonna see 187 187 00:09:37,850 --> 00:09:40,430 how this applies in a real app 188 188 00:09:40,430 --> 00:09:42,260 as opposed to just example code. 189 189 00:09:42,260 --> 00:09:45,043 So please bear with me and see you on the next video.