Runtime Errors |Section 2|Celestial Warrior

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Hi again and welcome to this new lecture!

In the previous lecture we talked about

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errors in general and I focused on

syntax errors which is one of the two

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types of errors that you have in Python.

So you have syntax errors and you have

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exceptions. Now we're going to talk about

exceptions in Python, so every other type

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of error that is not a syntax error is

an exception. And often all errors,

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syntax errors and exceptions they are

referred to as errors so you're going to

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hear this all over the place. Now I have

a new script here that I created for

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this lecture.

Try to guess what I'm going to get as

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output when I execute this script, so I

have a equal to 1, n equals to characters 2.

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Print int 2.5 and print a plus b. So

guess what I'm going to get for a moment.

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Okay, now let me run the script Python 3.

Errors.py. File errors.py line 4

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which is the last line, this one. We get

a syntax error, invalid syntax. Now this

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can be very confusing for you now

because you know you're looking at the

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print a plus b expression, but you don't

see any error there. So for a beginner of

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this can be quite frustrating but try to

look back on the left of arrow here, so

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think of the arrow that the arrow is

pointing at the token and token has not

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been written correctly in the script, so

in this case you know the problem

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here is that this line here it has an

open bracket, round bracket

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and than it has an int function, and then the int

function has its own brackets that is

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wrapping inside its input, so these are

the brackets of the int function, but the

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print function doesn't have a closing

bracket. What you did instead, what I did

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actually, what I did is that instead of

putting a bracket there I actually wrote

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another print function so Python was

expecting a closing round

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bracket, but I wrote I typed in a print

function. That's why this is saying that

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this print function is not in the

correct position. That's the idea.

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So always when you see this arrow first

look at this line event look before that

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also. Sorry about this! So always when you

see this arrow here, first look at this

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line here but also don't forget that the

problem might also be before this

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line.

Okay, so that was about the syntax error.

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I'll explain why I talked about this

syntax error here let me execute this

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again. So the syntax error was fixed now,

but we still have another error. So the

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reason I included a syntax error in my

code was to show you that Python

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actually first checks for syntax errors

so basically it parses the code, it looks

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for syntax errors, it doesn't execute the

code yet. So when I executed Python 3

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errors that's why i in here the code was

not executed but the interpreter just

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checks for syntax errors, it doesn't

check for exceptions just yet. So first

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you need to fix the syntax errors and

that's what we did here. We added

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these brackets and we fixed the syntax errors.

Now Python is throwing out an exception and

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let's, here is where I executed the code

and see the next line we have number two

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there which is coming from the output of

this line here, so from line three we

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got the output correctly. So basically

again Python executes the script from top

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to bottom

if it doesn't find

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syntax errors. If it finds syntax errors

it doesn't execute anything. So 2 is

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printed out and then you get this trace

back of the error which starts there and

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ends in here that's a block of the error.

Sometimes the errors may have more than

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one type of error. So in this case

we have only one type of error which is

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a type error, but you may have multiple

blocks here. However the most important

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error that you must focus on is the lost

line of the error, so the last block of

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of the error. In this case we have only

one block so we're focused on that.

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And line 4 which is this one in here. This

is the line. Again the line is printing out

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just like in the case of syntax error.

And you have a type of the error here

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which is a type error. So what's a type

error? A type error means that there is

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something wrong with one of your object

types in your script, and you have the

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description here. Unsupported operand

type for plus so this is trying to say

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that the plus operator has an

unsupported type so the plus operator in

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other words doesn't support one of the

types that you have given to it, so it's

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either the type of variable a or the

type of variable b. So it says int and

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string, so it doesn't specifically say

whether int or string is an object but

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it says that you cannot use a plus

operator with an integer and a string.

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And that's logically wrong

because you cannot add a number to some

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text. That's what Python does not

understand so it throws a type error.

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So exceptions are like logical errors

and you need to use your logic now to

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fix the error. And you use your logic by

carefully inspecting the error and

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that's what we did.

So what we want to do, we want to fix

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this. Now it's up to me whether I

meant to concatenate these two objects

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or do a mathematical addition operation.

So let's say if I was intending to

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do an addition between these two numbers

then I would have to convert b to a float

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or an integer.

Save that, execute, and yeah in this case you

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don't get any error. You get 2 printed

out from the second, from the third line

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and 3.0 from the last line that we have

just fixed. However if my intention was

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to actually print out the concatenation

between these two strings, instead of

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converting b to a float, I would have to

convert a to a string.

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In this case I would get the text

12. 12, so that would be

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a string object, not a number. Even though

here it shows as a number it's just how

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the terminal prints it out. So again these

are errors that occur in runtime, so when

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the script executes. Syntax errors are

parsing errors so the interpreter tries

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to understand whether the script is

syntactically correct, whether

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you have followed the Python

syntax rules. So you need a bracket,

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a closing bracket for an opening bracket.

You need a closing quote after the opening

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quote and so on. Now there are also

other types of exceptions, not only type

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error. You know you may have a name error.

Let's say here instead of printing out

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that, we print out c.

Save, execute and here is the trace back.

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Again 2 was printed out from the third line

and the trace back says line 4 at

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print c, name error name c is not

defined. So again this is not a syntax

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error because you haven't made any

mistakes with the syntax. You know you have

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this variable name which is correct,

you have brackets opening, round brackets

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and closing round bracket, so everything

is syntactically correct both this c

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object Python doesn't know this. You

haven't defined this c variable so

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Python doesn't know what to print out.

You know python is able to print out a

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because it knows that a refers to the

1 to integer 1 and so it prints out

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integer 1, but c doesn't have anything

so you get this name error and whenever

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you get a name error you know that this

name here has not been defined by you so

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to fix this you may want to define c

like that and execute and you don't get

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any error. You may also have other

types of errors such as c divided by 0.

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See what you get. Zero division

error division by 0. That's the

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description of this error so division by

0 is not mathematically possible or

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meaningful and since python is based on

mathematics it throws an error in

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and you need to fix that. You need to

remove that expression that divides by 0.

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Yeah, that's about errors in Python.

I hope you understood a great deal of this

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lecture and the previous lecture as well.

So it's relatively easy to fix errors,

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however sometimes

there are errors that you might have

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difficulties understanding and fixing

them so later for example we're going to

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work on libraries and sometimes

libraries they have different kinds of

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errors, however nothing to worry about

because there are other things that you

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can do

for an arrow that you don't understand.

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And I'll talk about that in the next

lecture, so I'll see you there!

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