1 00:00:00,390 --> 00:00:07,980 Previously you learned that you could get a list of Python built in modules using sys.building_module-names. 2 00:00:07,980 --> 00:00:17,190 As an example we use the time module which would freeze the script execution for a particular amount 3 00:00:17,190 --> 00:00:18,480 of seconds. 4 00:00:18,480 --> 00:00:20,310 Now let me show you something. 5 00:00:20,610 --> 00:00:29,650 I'll execute a script, onion will be printed out because it's there in the vegetables.txt file. 6 00:00:29,650 --> 00:00:33,340 However if I now delete this file, 7 00:00:37,570 --> 00:00:44,030 you'll see that the next time the script checks for the files vegetables.txt path, 8 00:00:44,140 --> 00:00:51,810 so that's a file path it's going to generate an error: "File not found error No such file or directory." 9 00:00:53,260 --> 00:00:58,260 So imagine you had this program running on a server 24/7. 10 00:00:58,390 --> 00:01:03,910 And for some reason the software that was generating these files at some point it doesn't generate the 11 00:01:03,910 --> 00:01:04,240 file. 12 00:01:04,450 --> 00:01:12,220 So your script execution will end and maybe in a minute your software will start to generate the 13 00:01:12,220 --> 00:01:19,080 files again, but your python script will not resume because it's got an error here. 14 00:01:19,090 --> 00:01:26,050 So what we want to do? We want to keep executing the python script even when the file is not in the directory 15 00:01:26,110 --> 00:01:26,650 there. 16 00:01:26,680 --> 00:01:35,950 To do that we want to make use of the OS module. Import os and you're going to notice that os is 17 00:01:35,950 --> 00:01:44,950 not among the built in Python modules. Indeed OS is a module written in Python. 18 00:01:45,060 --> 00:01:51,060 All these modules are built in which means they are written in C language inside the Python software 19 00:01:51,060 --> 00:01:57,410 implementation, OS is not one of them. To find out where OS is staying 20 00:01:57,420 --> 00:01:59,480 you can use sys.prefix. 21 00:01:59,500 --> 00:02:08,340 You're going to get a directory path there which could be different depending on what 22 00:02:08,370 --> 00:02:10,260 operating system you are using. 23 00:02:10,290 --> 00:02:12,840 So please navigate to that directory. 24 00:02:12,840 --> 00:02:20,010 I prefer to go there directly by copying it without the quotes, so copy everything without the quotes 25 00:02:21,000 --> 00:02:22,050 like that. 26 00:02:22,050 --> 00:02:30,450 Go to your command line terminal and if you are on Windows you start and paste the directory there and 27 00:02:30,450 --> 00:02:31,610 then press enter. 28 00:02:31,740 --> 00:02:36,750 If you are on Mac or Linux use open instead of start and paste the directory. 29 00:02:36,750 --> 00:02:43,620 Again press enter that will open your windows explorer or finder whatever you have to open folders. 30 00:02:44,430 --> 00:02:51,090 So that the 3.7 folder that you see in here and then go to Lib. 31 00:02:51,500 --> 00:02:59,150 Double click then find the folder Python3.7 or whatever version you have. 32 00:02:59,150 --> 00:03:01,970 So Python three point something. 33 00:03:01,970 --> 00:03:07,810 Double click it and here you'll see a number of Python files dot py. 34 00:03:07,820 --> 00:03:09,800 These are all Python files. 35 00:03:09,800 --> 00:03:16,160 These are all standard Python modules and OS is among them. 36 00:03:16,160 --> 00:03:19,310 Go to O here. 37 00:03:19,430 --> 00:03:22,790 Double Click It or open it with wherever you use 38 00:03:22,790 --> 00:03:27,300 as a text editor and you'll see that this is actually Python code. 39 00:03:27,950 --> 00:03:34,910 Make sure to close it before changing it so don't change don't modify Python standard modules. 40 00:03:35,300 --> 00:03:45,190 So I'm going to close it, and you can use deer actually to see what you have available for that module. 41 00:03:45,640 --> 00:03:52,090 So among them for example I'm going to use the path object. 42 00:03:52,270 --> 00:04:02,020 So with os.path, and path has a method called exists. The exists method gets an argument which 43 00:04:02,170 --> 00:04:09,520 should be a string and it's supposed to be the path of the file you want to check if it exists in the 44 00:04:09,520 --> 00:04:10,710 current directory. 45 00:04:13,450 --> 00:04:21,580 So let's write the directory path to the file, and I get false because the file was not in the current 46 00:04:21,580 --> 00:04:22,350 directory. 47 00:04:22,390 --> 00:04:28,150 If I go there and create that file inside the files folder 48 00:04:32,170 --> 00:04:39,760 so now I have a files/vegetables.txt file path. If I execute that again I'm going to get 49 00:04:39,760 --> 00:04:47,740 true and we can make use of that fact in our Python program. 50 00:04:47,740 --> 00:04:55,990 So what we want to do is before opening the file in read mode we want to check if the file exists 51 00:04:56,290 --> 00:05:03,760 because if we don't do that and the file is not there this line here is going to generate that file 52 00:05:03,790 --> 00:05:04,740 not found error. 53 00:05:05,230 --> 00:05:17,180 So before doing that we want to check if os.path.exists files/vegetables.txt 54 00:05:17,300 --> 00:05:26,840 and since this is a conditional we want to use a colon there and after that wherever you want to 55 00:05:26,840 --> 00:05:29,320 do it has to be indented. 56 00:05:29,660 --> 00:05:41,450 So with it's going to be under if. This is also part of the with block so indent that as well. 57 00:05:41,560 --> 00:05:48,410 And then else goes inside the while loop again. What you want to do under else? 58 00:05:48,520 --> 00:05:58,650 Well maybe we want to print out "File does not exist" and then be careful here. 59 00:05:58,690 --> 00:06:02,440 Time sleep has to be outside of the if else block. 60 00:06:02,500 --> 00:06:11,350 Because you want to execute this independently of the conditional block which means if the file 61 00:06:11,350 --> 00:06:12,850 exists, 62 00:06:12,850 --> 00:06:16,360 this is going to be executed. if the file does not exist 63 00:06:16,360 --> 00:06:24,100 this is going to be executed, and after either of these is executed, always we are going to execute 64 00:06:24,100 --> 00:06:25,540 time.sleep. 65 00:06:25,540 --> 00:06:30,220 So let me save the script and execute it. 66 00:06:30,220 --> 00:06:33,050 And we got an error, os it's not defined. 67 00:06:33,050 --> 00:06:38,750 I am glad that this showed up because modules have to be imported. 68 00:06:39,820 --> 00:06:46,420 Otherwise you're going to get that name error because os before being used in here it was not 69 00:06:46,420 --> 00:06:53,790 imported, and it's not a built in function or methods such as print which you don't need to import. 70 00:06:53,800 --> 00:07:01,390 So its name is not available immediately in the namespace. Save the script again, execute, and we get an 71 00:07:01,390 --> 00:07:05,650 empty string because vegetables is currently empty. 72 00:07:05,650 --> 00:07:13,560 Now if I go and delete some vegetables like that, wait a while 73 00:07:17,570 --> 00:07:19,870 and we get this printed out. 74 00:07:20,360 --> 00:07:21,770 If you create the file again. 75 00:07:25,340 --> 00:07:33,830 Write something there, save the file, don't forget, pretending that the software it's generating this file. 76 00:07:36,110 --> 00:07:39,210 So you get garlic there. 77 00:07:39,230 --> 00:07:47,510 Now I can interrupt that script execution with control C and those are both standard Python modules 78 00:07:47,840 --> 00:07:49,390 and they are exactly the same. 79 00:07:49,400 --> 00:07:54,780 So you can use dir with them. 80 00:07:55,710 --> 00:07:59,030 So all the Python rules apply to this modules. 81 00:07:59,040 --> 00:08:05,280 The only difference is that time is written in C language, and it's built in inside the python software 82 00:08:05,580 --> 00:08:13,320 and OS comes as a .py script and it came here when you installed Python so it was basically copied 83 00:08:13,740 --> 00:08:20,580 in one of your computer directories and you can import those, all those modules standard modules 84 00:08:21,000 --> 00:08:22,230 in Python. 85 00:08:22,230 --> 00:08:23,580 Now, of course 86 00:08:23,610 --> 00:08:26,940 not everything comes with your Python installation. 87 00:08:27,030 --> 00:08:29,520 Some things you have to install. 88 00:08:29,610 --> 00:08:31,350 So I'll show you that in the next video.