1 1 00:00:00,350 --> 00:00:01,970 Hi guys, and welcome back. 2 2 00:00:01,970 --> 00:00:03,800 In this video, we're going to talk about 3 3 00:00:03,800 --> 00:00:05,400 imports in Python, 4 4 00:00:05,400 --> 00:00:06,430 how to work with files, 5 5 00:00:06,430 --> 00:00:10,330 how to get code from another file into your main file. 6 6 00:00:10,330 --> 00:00:13,320 I've got two files now in my project, 7 7 00:00:13,320 --> 00:00:17,240 code.py and mymodule.py. 8 8 00:00:17,240 --> 00:00:19,890 I'm going to write some code in mymodule.py 9 9 00:00:19,890 --> 00:00:23,130 that I will then import into code.py 10 10 00:00:23,130 --> 00:00:24,513 so I can use it from there. 11 11 00:00:25,590 --> 00:00:27,070 So I've written a simple function 12 12 00:00:27,070 --> 00:00:29,600 that divides two parameters, 13 13 00:00:29,600 --> 00:00:34,600 and I'm also going to print mymodule.py colon 14 14 00:00:34,720 --> 00:00:36,257 and then, __name__. 15 15 00:00:38,027 --> 00:00:42,690 __name__ is a global variable in Python 16 16 00:00:42,690 --> 00:00:46,030 that changes depending on which file you're in. 17 17 00:00:46,030 --> 00:00:47,690 So it is a special variable 18 18 00:00:47,690 --> 00:00:49,950 because it will allow you to differentiate 19 19 00:00:49,950 --> 00:00:54,950 between the file you run and a file you import, 20 20 00:00:55,090 --> 00:00:56,560 and it also does a couple more things 21 21 00:00:56,560 --> 00:00:58,210 that we'll explain as we go along. 22 22 00:00:58,210 --> 00:01:01,530 So here in mymodule, I've defined a function, 23 23 00:01:01,530 --> 00:01:06,530 and I've also printed out the __name__ property, 24 24 00:01:06,720 --> 00:01:10,520 and I've also said that we're in mymodule.py there. 25 25 00:01:10,520 --> 00:01:14,403 So if I run this code right now, 26 26 00:01:15,950 --> 00:01:19,370 what you'll see is that you get mymodule.py, 27 27 00:01:19,370 --> 00:01:21,580 and then, you get dunder main. 28 28 00:01:21,580 --> 00:01:23,360 Instead of saying underscore underscore all the time, 29 29 00:01:23,360 --> 00:01:24,600 I'll just say dunder for now. 30 30 00:01:24,600 --> 00:01:27,490 That's the usual Python terminology. 31 31 00:01:27,490 --> 00:01:30,640 So you get mymodule.py and then dunder main. 32 32 00:01:30,640 --> 00:01:32,990 So evidently, dunder name 33 33 00:01:32,990 --> 00:01:36,500 must have equaled dunder main at the time of running. 34 34 00:01:36,500 --> 00:01:39,280 Otherwise, you wouldn't have got that printed out. 35 35 00:01:39,280 --> 00:01:42,460 So let's say that when you run a file, 36 36 00:01:42,460 --> 00:01:46,940 the dunder name variable equals dunder main 37 37 00:01:46,940 --> 00:01:48,920 because we've ran mymodule.py. 38 38 00:01:48,920 --> 00:01:51,130 That's why that value's there. 39 39 00:01:51,130 --> 00:01:53,970 If we go over to code.py now, 40 40 00:01:53,970 --> 00:01:58,610 we're going to do from mymodule import divide, 41 41 00:01:58,610 --> 00:02:02,460 and this is how you can import a specific thing 42 42 00:02:02,460 --> 00:02:04,050 from another file. 43 43 00:02:04,050 --> 00:02:05,650 So here, we're doing from 44 44 00:02:05,650 --> 00:02:08,470 and then the file that we wanna import from 45 45 00:02:08,470 --> 00:02:12,290 and then import and the thing that we wanna import. 46 46 00:02:12,290 --> 00:02:14,270 Alternatively, if you don't want to import 47 47 00:02:14,270 --> 00:02:17,270 a specific thing from the file, 48 48 00:02:17,270 --> 00:02:20,680 then you can just do import mymodule, 49 49 00:02:20,680 --> 00:02:24,380 and then, as usual, you would access the divide function 50 50 00:02:24,380 --> 00:02:27,800 inside mymodule by doing mymodule.divide. 51 51 00:02:27,800 --> 00:02:29,030 If you wanted to do that instead, 52 52 00:02:29,030 --> 00:02:30,280 that's also totally fine. 53 53 00:02:31,130 --> 00:02:32,410 There are some subtle differences 54 54 00:02:32,410 --> 00:02:33,830 between the two types of import, 55 55 00:02:33,830 --> 00:02:34,880 but they are subtle, 56 56 00:02:34,880 --> 00:02:36,490 so you won't come across them very often. 57 57 00:02:36,490 --> 00:02:38,370 We're not gonna cover them in this course. 58 58 00:02:38,370 --> 00:02:42,000 Now that we've imported the divide function from mymodule, 59 59 00:02:42,000 --> 00:02:45,970 we can use it as if it were defined inside this file, 60 60 00:02:45,970 --> 00:02:47,350 so I will do that. 61 61 00:02:47,350 --> 00:02:49,770 Then I'm going to run this file, 62 62 00:02:49,770 --> 00:02:52,900 and you see that we get two pieces of output now. 63 63 00:02:52,900 --> 00:02:55,940 We get mymodule.py is mymodule, 64 64 00:02:55,940 --> 00:02:59,770 and you get 5.0, which is the result of dividing this here, 65 65 00:02:59,770 --> 00:03:01,290 and here is the interesting part. 66 66 00:03:01,290 --> 00:03:05,300 Notice how earlier, mymodule.py printed out dunder main, 67 67 00:03:05,300 --> 00:03:06,950 but now, it has printed mymodule. 68 68 00:03:08,140 --> 00:03:12,100 That is because only the file you run is dunder main. 69 69 00:03:12,100 --> 00:03:16,470 So if we print dunder name here and we run this again, 70 70 00:03:16,470 --> 00:03:21,440 you'll see that you get dunder main printed out inside code, 71 71 00:03:21,440 --> 00:03:24,930 but mymodule.py is still printing out mymodule. 72 72 00:03:24,930 --> 00:03:27,200 So the file you run is dunder main, 73 73 00:03:27,200 --> 00:03:28,680 and as soon as you import other files, 74 74 00:03:28,680 --> 00:03:31,320 Python will name them according to their path. 75 75 00:03:31,320 --> 00:03:34,610 So mymodule here is at the top level of our folder, 76 76 00:03:34,610 --> 00:03:36,233 so the name will be mymodule. 77 77 00:03:37,490 --> 00:03:41,040 So how does Python know where mymodule is? 78 78 00:03:41,040 --> 00:03:43,470 Does it just look in the current folder? 79 79 00:03:43,470 --> 00:03:47,000 Well, the answer is in import sys. 80 80 00:03:47,000 --> 00:03:49,310 import sys allows you to use the sys module 81 81 00:03:49,310 --> 00:03:50,660 that comes with Python, 82 82 00:03:50,660 --> 00:03:53,510 and that unlocks some certain system functionalities, 83 83 00:03:53,510 --> 00:03:56,980 and one of them is sys.path. 84 84 00:03:56,980 --> 00:04:00,420 So sys.path is the import paths 85 85 00:04:00,420 --> 00:04:04,980 where Python will look in order to find files to import. 86 86 00:04:04,980 --> 00:04:06,950 So I'm gonna press Play now, 87 87 00:04:06,950 --> 00:04:08,420 and you can see that you've got 88 88 00:04:08,420 --> 00:04:11,973 a whole bunch of different paths here inside a list. 89 89 00:04:12,810 --> 00:04:15,890 The first path that Python is going to look at 90 90 00:04:15,890 --> 00:04:17,460 is the first one in this list, 91 91 00:04:17,460 --> 00:04:21,380 which, as you can see, is this 29_imports_in_python folder 92 92 00:04:21,380 --> 00:04:22,850 that we are currently in. 93 93 00:04:22,850 --> 00:04:25,880 So whenever you try to import something called mymodule, 94 94 00:04:25,880 --> 00:04:28,820 Python is going to go into this folder first 95 95 00:04:28,820 --> 00:04:32,040 and see if something called mymodule.py is in there, 96 96 00:04:32,040 --> 00:04:34,610 or, indeed, a mymodule folder, 97 97 00:04:34,610 --> 00:04:37,580 and if it is there, it will try to use that. 98 98 00:04:37,580 --> 00:04:40,410 If it's not, then it will go into the next path 99 99 00:04:40,410 --> 00:04:42,040 and then into the next one and so on. 100 100 00:04:42,040 --> 00:04:46,170 If it reaches the end and nothing was found for that name, 101 101 00:04:46,170 --> 00:04:48,550 then you'll get an error, an import error. 102 102 00:04:48,550 --> 00:04:53,160 For example, if I try to import sysasdf, 103 103 00:04:53,160 --> 00:04:55,290 you're going to get an import error 104 104 00:04:55,290 --> 00:04:58,493 saying ModuleNotFound, No module named sysasdf. 105 105 00:04:59,340 --> 00:05:01,460 These paths aren't defined in any specific order, 106 106 00:05:01,460 --> 00:05:05,450 so the first path is always the path of the file you ran. 107 107 00:05:05,450 --> 00:05:06,823 So here, we've ran code.py, 108 108 00:05:07,720 --> 00:05:11,230 so the first path is the path to code.py 109 109 00:05:11,230 --> 00:05:13,010 except, of course, the file name. 110 110 00:05:13,010 --> 00:05:14,810 So whenever you run a Python file, 111 111 00:05:14,810 --> 00:05:17,840 you will be able to import other Python files or folders 112 112 00:05:17,840 --> 00:05:21,410 inside the same folder that you ran. 113 113 00:05:21,410 --> 00:05:24,290 The second path, if defined, 114 114 00:05:24,290 --> 00:05:27,060 is an environment variable called PYTHONPATH. 115 115 00:05:27,060 --> 00:05:29,540 At the moment, we don't have a PYTHONPATH defined, 116 116 00:05:29,540 --> 00:05:31,510 so that's not there, 117 117 00:05:31,510 --> 00:05:33,920 and then, you've got all your other paths 118 118 00:05:33,920 --> 00:05:36,783 for your imports that are defined at a Python level. 119 119 00:05:37,760 --> 00:05:41,110 In UNIX systems, you can do export PYTHONPATH, 120 120 00:05:41,110 --> 00:05:43,230 and then, give it a path. 121 121 00:05:43,230 --> 00:05:46,263 For example, you can do /Users. 122 122 00:05:47,670 --> 00:05:49,780 Then, if you run this code again, 123 123 00:05:49,780 --> 00:05:53,660 you'll see that the second path now in sys.path 124 124 00:05:53,660 --> 00:05:56,220 is the /Users path. 125 125 00:05:56,220 --> 00:05:58,640 Again, the first path will always be 126 126 00:05:58,640 --> 00:06:01,610 the one that you currently ran your file in, 127 127 00:06:01,610 --> 00:06:05,250 and the second one will be your PYTHONPATH if it is defined, 128 128 00:06:05,250 --> 00:06:07,040 and finally, the rest of your paths. 129 129 00:06:07,040 --> 00:06:08,560 In Windows, instead of export, 130 130 00:06:08,560 --> 00:06:10,580 you'll have to use the keyword set 131 131 00:06:10,580 --> 00:06:13,270 to set an environment variable. 132 132 00:06:13,270 --> 00:06:15,930 Let's go back to importing mymodule, 133 133 00:06:15,930 --> 00:06:17,270 and now, from mymodule, 134 134 00:06:17,270 --> 00:06:20,170 we're going to import something in a folder. 135 135 00:06:20,170 --> 00:06:22,780 So here, in my project, I'm going to create a new folder. 136 136 00:06:22,780 --> 00:06:25,190 I'm gonna call it libs for libraries, 137 137 00:06:25,190 --> 00:06:26,230 and inside that folder, 138 138 00:06:26,230 --> 00:06:29,063 we're going to create a library called mylib.py. 139 139 00:06:30,023 --> 00:06:33,040 mylib.py is going to contain just mylib.py 140 140 00:06:33,040 --> 00:06:35,290 and then dunder name, 141 141 00:06:35,290 --> 00:06:38,450 just so you can see how the dunder name operates. 142 142 00:06:38,450 --> 00:06:42,123 Here, in mymodule, we're going to import libs.mylib, 143 143 00:06:43,940 --> 00:06:46,460 and then here, we're gonna import mymodule. 144 144 00:06:46,460 --> 00:06:49,493 So the imports go code imports mymodule, 145 145 00:06:49,493 --> 00:06:51,720 mymodule imports libs.mylib, 146 146 00:06:51,720 --> 00:06:53,850 and then, mylib prints out the name. 147 147 00:06:53,850 --> 00:06:56,040 Let's run this and see what happens. 148 148 00:06:56,040 --> 00:06:59,700 So as you can see, we got mymodule.py imported, 149 149 00:06:59,700 --> 00:07:02,600 and the name at the time was mymodule, 150 150 00:07:02,600 --> 00:07:05,270 and then, that imported mylib.py, 151 151 00:07:05,270 --> 00:07:07,900 and the name was libs.mylib. 152 152 00:07:07,900 --> 00:07:11,590 So again, it is the path that you use for importing 153 153 00:07:11,590 --> 00:07:12,753 that is relevant. 154 154 00:07:13,810 --> 00:07:17,220 If you're using Python 2, then this will not work 155 155 00:07:17,220 --> 00:07:19,080 because in some Python versions, 156 156 00:07:19,080 --> 00:07:23,120 you have to also create a dunder init file 157 157 00:07:23,120 --> 00:07:25,630 inside any folder that you wanna import from. 158 158 00:07:25,630 --> 00:07:27,790 So here in the libs folder, 159 159 00:07:27,790 --> 00:07:31,700 I have created a __init__ file. 160 160 00:07:31,700 --> 00:07:33,180 I would recommend that you get accustomed 161 161 00:07:33,180 --> 00:07:34,280 to creating those files 162 162 00:07:34,280 --> 00:07:36,360 because they are sometimes required. 163 163 00:07:36,360 --> 00:07:38,010 After doing all those imports, 164 164 00:07:38,010 --> 00:07:40,760 you can actually check sys.modules 165 165 00:07:40,760 --> 00:07:42,200 to see what's imported. 166 166 00:07:42,200 --> 00:07:47,200 So I will also import sys from here and press Play, 167 167 00:07:47,260 --> 00:07:49,330 and then, you can see that there's a bunch of things 168 168 00:07:49,330 --> 00:07:52,330 that have been imported at the time. 169 169 00:07:52,330 --> 00:07:57,180 So you have all of these built-in modules, 170 170 00:07:57,180 --> 00:08:00,860 including the sys module that has a bunch of other things, 171 171 00:08:00,860 --> 00:08:02,860 _frozen_importlib, and you know, 172 172 00:08:02,860 --> 00:08:04,170 there's loads of different ones 173 173 00:08:04,170 --> 00:08:06,730 that already come with Python you can use, 174 174 00:08:06,730 --> 00:08:09,400 and then, at the very end, you've got your own. 175 175 00:08:09,400 --> 00:08:12,121 So you have libs.mylib, 176 176 00:08:12,121 --> 00:08:14,670 libs, and mymodule, 177 177 00:08:14,670 --> 00:08:17,110 and these are all imported by the imports 178 178 00:08:17,110 --> 00:08:18,500 that we've written. 179 179 00:08:18,500 --> 00:08:21,050 So whenever you type another import 180 180 00:08:21,050 --> 00:08:22,530 somewhere else in your programme, 181 181 00:08:22,530 --> 00:08:24,690 Python is going to check these modules 182 182 00:08:24,690 --> 00:08:26,810 and see if that import is there, 183 183 00:08:26,810 --> 00:08:29,540 and if it is, then it's going to use that 184 184 00:08:29,540 --> 00:08:31,620 instead of trying to import again. 185 185 00:08:31,620 --> 00:08:32,980 Why does that matter? 186 186 00:08:32,980 --> 00:08:35,590 Because have you seen, when we import a file 187 187 00:08:35,590 --> 00:08:37,080 such as import mymodule, 188 188 00:08:37,080 --> 00:08:40,610 it runs through the file in order to be able to determine 189 189 00:08:40,610 --> 00:08:43,800 what is inside the file that we can use. 190 190 00:08:43,800 --> 00:08:46,710 So if you were to import the same thing many times 191 191 00:08:46,710 --> 00:08:49,460 and every time you ran that same thing, 192 192 00:08:49,460 --> 00:08:51,690 you might find that your code is running a lot of times 193 193 00:08:51,690 --> 00:08:53,880 over and over without achieving anything. 194 194 00:08:53,880 --> 00:08:55,860 That's why Python keeps track of the things 195 195 00:08:55,860 --> 00:08:57,330 that have already been imported, 196 196 00:08:57,330 --> 00:08:59,510 and if they have, you won't run through them again. 197 197 00:08:59,510 --> 00:09:01,750 You'll just be able to use the one that's already ran. 198 198 00:09:01,750 --> 00:09:03,250 Hopefully that makes sense. 199 199 00:09:03,250 --> 00:09:05,500 To recap, importing runs through a file 200 200 00:09:05,500 --> 00:09:08,210 and allows you to access the properties defined inside it. 201 201 00:09:08,210 --> 00:09:11,350 The dunder name variable is equal to dunder main 202 202 00:09:11,350 --> 00:09:13,180 in the file you run 203 203 00:09:13,180 --> 00:09:16,350 and is equal to the import path on other files, 204 204 00:09:16,350 --> 00:09:19,040 and you can import from files or folders 205 205 00:09:19,040 --> 00:09:22,840 as long as they are in your sys paths. 206 206 00:09:22,840 --> 00:09:24,620 Importing from a folder usually requires 207 207 00:09:24,620 --> 00:09:27,520 a dunder init.py file to be defined, 208 208 00:09:27,520 --> 00:09:30,040 especially in older Python versions. 209 209 00:09:30,040 --> 00:09:31,350 Thanks for joining me in this video. 210 210 00:09:31,350 --> 00:09:32,290 Hope it's been useful. 211 211 00:09:32,290 --> 00:09:33,740 I'll see you in the next one.