Would you like to inspect the original subtitles? These are the user uploaded subtitles that are being translated: 1 00:00:00,319 --> 00:00:06,270 Hey welcome to this new lecture! This is the most important lecture you have had 3 00:00:06,270 --> 00:00:12,240 so far in this course, so please try to make the most of it and this is actually 5 00:00:12,240 --> 00:00:15,780 a new video that I am adding to the course. I didn't have this lecture a few 7 00:00:15,780 --> 00:00:22,320 times ago so I had this quite a while after I published the course and until 9 00:00:22,320 --> 00:00:26,550 now I had explained this concept, so the errors little by little in other 11 00:00:26,550 --> 00:00:32,610 lectures but I felt that was not enough and I was seeing that students were 13 00:00:32,610 --> 00:00:37,140 still having difficulties in understanding and dealing with errors in 15 00:00:37,140 --> 00:00:44,070 Python so I decided to make a dedicated lecture for errors in Python. So here we 17 00:00:44,070 --> 00:00:48,629 are and I said this is the most important lecture because you know we 19 00:00:48,629 --> 00:00:54,510 we learnt about functions and strings and numbers until now in the previous 21 00:00:54,510 --> 00:01:00,480 videos and these are individual concepts that you know you'll learn, now or later you'll 23 00:01:00,480 --> 00:01:05,760 learn functions and strings and etc, but if you don't know how to read an error 25 00:01:05,760 --> 00:01:11,220 how to to understand an error and deal with it if you don't know that you will 27 00:01:11,220 --> 00:01:15,930 have trouble dealing with everything every other object like functions and 29 00:01:15,930 --> 00:01:22,560 streams and numbers etc. So understanding and error and knowing how to deal with 31 00:01:22,560 --> 00:01:28,350 it, how to fix it is very important and even the most experienced programmers 33 00:01:28,350 --> 00:01:34,140 they make errors in their programs. The difference is that they know how to read 35 00:01:34,140 --> 00:01:39,420 these errors. How to understand and fix them, but everyone makes errors. So if 37 00:01:39,420 --> 00:01:44,460 an error shows in your program don't panic just for all the instructions that 39 00:01:44,460 --> 00:01:48,840 I'll give you in this video on how to fix the error so that's what this 41 00:01:48,840 --> 00:01:54,930 lecture is about. So what is an error anyway? An error in a program is a bug 43 00:01:54,930 --> 00:02:00,509 that causes the program to function incorrectly. Now in Python we have 45 00:02:00,509 --> 00:02:06,229 basically two types of errors. We have syntax errors and we have exceptions. 47 00:02:06,229 --> 00:02:10,950 Let's first focus on syntax errors and let's forget about exceptions. We'll go 49 00:02:10,950 --> 00:02:16,840 to exceptions after we explain syntax errors. I have this code here and 51 00:02:16,840 --> 00:02:23,019 actually this is a Python file. This is the icon showing on a Mac so currently 53 00:02:23,019 --> 00:02:27,909 I am on a Mac computer but that doesn't make the slightest difference. No matter 55 00:02:27,909 --> 00:02:33,280 where you are on Linux Windows or Mac everything is the same. So I have the 57 00:02:33,280 --> 00:02:37,750 terminal here and let me go ahead and execute this program. This has a few 59 00:02:37,750 --> 00:02:44,890 errors so let me show them. On Mac you can call Python 3 with the python3 61 00:02:44,890 --> 00:02:49,870 command, on Windows you can just call python or whatever command you are 63 00:02:49,870 --> 00:02:56,049 using. You can also use any editor that you like and so errors.py is the 65 00:02:56,049 --> 00:03:07,290 name I gave to the script, execute. So this is now an example of a syntax error 67 00:03:07,290 --> 00:03:14,230 and this is the entire body of the error message and this is very important, so 69 00:03:14,230 --> 00:03:19,450 whenever you get an error the first advice I give you is don't panic just 71 00:03:19,450 --> 00:03:23,730 focus on the error. Read it line by line just like you're reading a poem or 73 00:03:23,730 --> 00:03:32,739 whatever so the first line of the error points you to the name of the file that 75 00:03:32,739 --> 00:03:37,840 has the error. In this case is errors.py. Then you have a comma and after 77 00:03:37,840 --> 00:03:40,620 the comma you have the line where the error occurred. 79 00:03:40,620 --> 00:03:47,169 So it's line 3 and you can see that line 3 is this one here but you also have for 81 00:03:47,169 --> 00:03:53,379 your convenience Python prints out the line in the terminal so int 9. Here is 83 00:03:53,379 --> 00:03:59,199 where the error is and just after that you have the type of the error so it's a 85 00:03:59,199 --> 00:04:03,849 syntax error, you also have other types of errors such as name error, type error, 87 00:04:03,849 --> 00:04:07,780 but those are exceptions and I'll explain later why we call them 89 00:04:07,780 --> 00:04:12,519 exceptions, so this is one type of error a syntax error and the others are 91 00:04:12,519 --> 00:04:18,900 exceptions. So this is the type and then you have the description after the colon. 93 00:04:18,900 --> 00:04:24,990 Sometimes the description is more specific more detailed. This time is just 95 00:04:24,990 --> 00:04:29,870 invalid syntax. So you have to figure out where 97 00:04:29,870 --> 00:04:36,259 you did some error where you missed some syntax of your program. 99 00:04:36,259 --> 00:04:45,080 And you also have this arrow here pointing upwards and the arrow points you even at the 101 00:04:45,080 --> 00:04:51,770 token that the error is occurring or at the end of the token, so a token is you 103 00:04:51,770 --> 00:04:58,190 know, this is the token in this case 9. So it can be a number, it can be string, 105 00:04:58,190 --> 00:05:04,400 anything, so at this point this is pointing at this, at the token but 107 00:05:04,400 --> 00:05:11,780 it could also be like that. 999 save the script, execute. In this 109 00:05:11,780 --> 00:05:16,220 case you see that it's pointing you at the end of the token. So here's, around 111 00:05:16,220 --> 00:05:21,729 here you have an error now and I know that int is actually a function and 113 00:05:21,729 --> 00:05:29,780 functions important they need to have brackets so you need to pass 999 inside 115 00:05:29,780 --> 00:05:42,650 brackets like that and you leave that. Save and execute again. All right, you've 117 00:05:42,650 --> 00:05:49,729 got another error but don't panic again. This is my advice always, instead 119 00:05:49,729 --> 00:05:54,979 read the error it says file errors.py the file line 5, oh line 5 this time. 121 00:05:54,979 --> 00:06:02,210 So it's not line 3 anymore which means that line 3 was fixed so what Python is 123 00:06:02,210 --> 00:06:06,949 doing is it's going through all the lines one by one from top to bottom, it 125 00:06:06,949 --> 00:06:11,330 checks the first line it says: "Oh it's fine, no error there". Goes to the next 127 00:06:11,330 --> 00:06:16,130 line no error, goes through the third line, no error this time, fourth line is 129 00:06:16,130 --> 00:06:21,860 also fine, and fifth line has an error so again it's a syntax error and this one 131 00:06:21,860 --> 00:06:28,630 is more specifically described so missing parentheses in call to print. 133 00:06:28,630 --> 00:06:33,560 Note that you get this error only if you are on Python 3. If you are on Python 2 135 00:06:33,560 --> 00:06:40,810 this won't be an error because in Python 2 print was a statement and was not 137 00:06:40,810 --> 00:06:46,210 function so with statements like return or print in Python - you didn't 139 00:06:46,210 --> 00:06:49,150 have to pass brackets. This syntax would be correct. 141 00:06:49,150 --> 00:06:56,190 So again this arrow here is pointing you and the token so we just add 143 00:06:56,190 --> 00:07:04,810 brackets there, ctrl s to save the script and execute again. This time we don't get 145 00:07:04,810 --> 00:07:09,280 any error so the script executed fine and print it out the output here. 147 00:07:09,280 --> 00:07:15,400 1 2 3 so we had 3 print functions printing out output. This doesn't return 149 00:07:15,400 --> 00:07:21,340 any output because we're not printing anything so this just gets the function 151 00:07:21,340 --> 00:07:26,410 input and produces some output but doesn't print anything. If you want to 153 00:07:26,410 --> 00:07:30,550 print it, you want to pass the print statement function there. There could 155 00:07:30,550 --> 00:07:36,940 also be other types of syntax errors and you know such as you want to define a 157 00:07:36,940 --> 00:07:44,800 least 1, 2, 3, but instead of closing it with a square brackets, you use 159 00:07:44,800 --> 00:07:50,260 a round bracket instead like that. Save the script and execute and you see that 161 00:07:50,260 --> 00:07:58,030 you got a syntax error, invalid syntax at line 5 which is this one in here. Again 163 00:07:58,030 --> 00:08:04,780 the arrow points you at the token so you need to figure out how to fix this round 165 00:08:04,780 --> 00:08:11,020 bracket here and you know that. You need to close it with a square bracket so 167 00:08:11,020 --> 00:08:15,760 that should fix the issue. So these are Ao that's about syntax errors. They are 169 00:08:15,760 --> 00:08:22,000 very easy to fix. In the next lecture I will explain exceptions so see you there! 10246