Original subtitles

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Hey welcome to this new lecture! This is

the most important lecture you have had

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so far in this course, so please try to

make the most of it and this is actually

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a new video that I am adding to the

course. I didn't have this lecture a few

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times ago so I had this quite a while

after I published the course and until

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now I had explained this concept, so the

errors little by little in other

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lectures but I felt that was not enough

and I was seeing that students were

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still having difficulties in

understanding and dealing with errors in

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Python so I decided to make a dedicated

lecture for errors in Python. So here we

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are and I said this is the most

important lecture because you know we

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we learnt about functions and strings

and numbers until now in the previous

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videos and these are individual concepts

that you know you'll learn, now or later you'll

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learn functions and strings and etc, but

if you don't know how to read an error

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how to to understand an error and deal

with it if you don't know that you will

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have trouble dealing with everything

every other object like functions and

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streams and numbers etc. So understanding

and error and knowing how to deal with

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it, how to fix it is very important and

even the most experienced programmers

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they make errors in their programs. The

difference is that they know how to read

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these errors. How to understand and

fix them, but everyone makes errors. So if

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an error shows in your program don't

panic just for all the instructions that

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I'll give you in this video on how to

fix the error so that's what this

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lecture is about. So what is an error

anyway? An error in a program is a bug

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that causes the program to function

incorrectly. Now in Python we have

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basically two types of errors. We have

syntax errors and we have exceptions.

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Let's first focus on syntax errors and

let's forget about exceptions. We'll go

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to exceptions after we explain

syntax errors. I have this code here and

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actually this is a Python file. This is

the icon showing on a Mac so currently

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I am on a Mac computer but that doesn't

make the slightest difference. No matter

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where you are on Linux Windows or Mac

everything is the same. So I have the

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terminal here and let me go ahead and

execute this program. This has a few

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errors so let me show them. On Mac you

can call Python 3 with the python3

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command, on Windows you can just call

python or whatever command you are

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using. You can also use any editor that

you like and so errors.py is the

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name I gave to the script, execute.

So this is now an example of a syntax error

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and this is the entire body of the error

message and this is very important, so

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whenever you get an error the first

advice I give you is don't panic just

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focus on the error. Read it line by

line just like you're reading a poem or

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whatever so the first line of the error

points you to the name of the file that

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has the error. In this case is errors.py.

Then you have a comma and after

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the comma you have the line where the

error occurred.

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So it's line 3 and you can see that line

3 is this one here but you also have for

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your convenience Python prints out the

line in the terminal so int 9. Here is

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where the error is and just after that

you have the type of the error so it's a

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syntax error, you also have other types

of errors such as name error, type error,

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but those are exceptions and I'll

explain later why we call them

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exceptions, so this is one type of error

a syntax error and the others are

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exceptions. So this is the type and then

you have the description after the colon.

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Sometimes the description is more

specific more detailed. This time is just

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invalid syntax.

So you have to figure out where

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you did some error where you

missed some syntax of your program.

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And you also have this arrow here pointing

upwards and the arrow points you even at the

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token that the error is occurring or at

the end of the token, so a token is you

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know, this is the token in this case 9.

So it can be a number, it can be string,

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anything, so at this point this is

pointing at this, at the token but

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it could also be like that.

999 save the script, execute. In this

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case you see that it's pointing you at

the end of the token. So here's, around

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here you have an error now and I know

that int is actually a function and

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functions important they need to have

brackets so you need to pass 999 inside

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brackets like that and you leave that.

Save and execute again. All right, you've

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got another error but don't panic again.

This is my advice always, instead

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read the error it says file errors.py

the file line 5, oh line 5 this time.

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So it's not line 3 anymore which means

that line 3 was fixed so what Python is

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doing is it's going through all the

lines one by one from top to bottom, it

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checks the first line it says: "Oh it's

fine, no error there". Goes to the next

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line no error, goes through the third

line, no error this time, fourth line is

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also fine, and fifth line has an error so

again it's a syntax error and this one

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is more specifically described so

missing parentheses in call to print.

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Note that you get this error only if you

are on Python 3. If you are on Python 2

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this won't be an error because in Python

2 print was a statement and was not

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function so with statements like

return or print in Python - you didn't

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have to pass brackets. This syntax would

be correct.

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So again this arrow here is pointing you

and the token so we just add

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brackets there, ctrl s to save the script

and execute again. This time we don't get

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any error so the script executed fine

and print it out the output here.

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1 2 3 so we had 3 print functions

printing out output. This doesn't return

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any output because we're not printing

anything so this just gets the function

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input and produces some output but

doesn't print anything. If you want to

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print it, you want to pass the print

statement function there. There could

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also be other types of syntax errors and

you know such as you want to define a

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least 1, 2, 3, but instead of

closing it with a square brackets, you use

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a round bracket instead like that. Save

the script and execute and you see that

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you got a syntax error, invalid syntax at

line 5 which is this one in here. Again

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the arrow points you at the token so you

need to figure out how to fix this round

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bracket here and you know that. You need

to close it with a square bracket so

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that should fix the issue. So these are

Ao that's about syntax errors. They are

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very easy to fix. In the next lecture I

will explain exceptions so see you there!