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[No audio]

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Hi again and welcome to this new

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lecture. In the previous lecture, we

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talked about errors in general, and I

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focused on syntax errors, which is one

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of the two types of errors you have in

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Python. So you have syntax errors, and

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you have exceptions. Now we're going to

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talk about exceptions in Python. So

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every other type of error that is not a

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syntax error as an exceptions, and often

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all errors, syntax errors and

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exceptions, they are referred to as

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errors. So you're going to hear this all

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over the place. Now I have a new script

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here, that I created for this lecture.

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Try to guess what I'm going to get as

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output when I execute the script. So I

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have a equals to 1, b equals to

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character 2, print int 2.5, and print a + b.

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So guess what I'm going to get

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for a moment. Again, now let me run the

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script, python3 errors.py.

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[No audio]

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File errors.py line 4, which is the last line.

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[No audio]

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This one, we get a syntax error,

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invalid syntax.

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[No audio]

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Now, this can be very

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confusing for you now, because you know,

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you're looking at the prints a + b

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expression, but you don't see any error

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there. Also, for a beginner, this can be

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quite frustrating. But try to look back

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on the left of arrow here. So think of

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the arrow that the arrow is pointing at

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the token, and the token has not been

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written correctly in the script. So in

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this case, you know, the problem here is

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that this line here, it has an open

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bracket, round bracket, and then it has

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int function, and then the int function

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has its own brackets that is wrapping

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inside its inputs. So these are the

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brackets of the int function, but the print

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function doesn't have a closing

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brackets. What you did instead, what I

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did actually, what I did is that

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instead of putting a bracket there, I

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actually wrote another print

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function. So Python was expecting a

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closing round bracket, but I

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wrote, I typed in a print function.

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That's why this is saying that this

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print function is not in the correct

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position, that's the idea. As always,

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when you see this arrow, first look at

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this line, but then look before that,

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also. Sorry about this.

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So always, when

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you see this arrow here, a first look at

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this line here, but also don't forget

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that the problem might also be before

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this line. Okay, so that was about the

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syntax error. I'll explain why I talked

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about the syntax error here. Let us

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execute this again. So the syntax error

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was fixed now, but we still have another

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error. So the reason I included a syntax

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error in my code was to show you that

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Python actually first checks for syntax

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errors. So basically, it parses the

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code, it looks for syntax errors, it

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doesn't execute the code yet. So when I

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executed python3 errors.py in

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here, the code was not executed, but the

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interpreter just checks for syntax

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errors. It doesn't check for exceptions

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just yet. So first, you need to fix the

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syntax errors, and that's what we did

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here, we added this bracket, and we fixed

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the syntax errors. Now Python is

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throwing out an exception, and let's

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here is where I executed the code, and

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see the next line we have number 2

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there, which is coming from the output

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of this line here. So from line 3,

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we got the output correctly. So

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basically, again, Python execute the

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script from top to bottom

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if it doesn't find syntax errors, if it

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finds syntax errors, it doesn't execute

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anything. So 2 is printed out, and

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then you get this Traceback of the error

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[No audio]

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which starts there and ends in here, that's

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the block of the error.

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[No audio]

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Sometimes, the errors may have more than one type

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of error. So in this case, we have only

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one type of error, which is the type

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error, but you may have multiple blocks

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here. However, the most important error

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that you must focus on is the last

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line of the error. So the last block of

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of the error. In this case, we have only

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one block. So we're focus on in that.

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In the line 4, which is this one in here.

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This is the line, again the line is

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printing out, just like in the case of

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syntax error, and here you have a type of

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the error here, which is a TypeError.

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So what's a TypeError? A TypeError

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means that there is something wrong with

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one of your object types in your script.

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And you have the description here,

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unsupported operand type for '+'. So this

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is trying to say that the + operator

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has an unsupported type. So the +

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operator, in other words, doesn't

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support one of the types that you have

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given to it. So it either a type of

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variable a, or the type of variable b. So

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it says int and string. So it doesn't

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specifically say that whether int or

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string is object. But it says that you

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cannot use a + operator with an

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integer and a string, and that's

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logically wrong, because you

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cannot add a number to some text. That's

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what Python doesn't understand, and it

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throws a TypeError. So exceptions are

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like logical errors, and you need to use

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your logic now to fix the error, and use

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your logic by carefully inspecting the

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error, and that's what we did. So

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what we want to do, we want to fix this. Now

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it's up to me whether I meant to

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concatenate this two objects, or do a

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mathematical addition operation. So now

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let's say if I was intending to do an

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addition between these two numbers, then

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I would have to convert b to float or an integer.

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Save that execute, and yeah

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in this case, you don't get any error, you

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get 2 print it out from the second, from

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the third line, and 3.0 from the last

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line that we have just fixed. However,

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if my intention was to actually

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[No audio]

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print out the concatenation between these two

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strings, instead of converting b to a

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float without have to convert a to a string.

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In this case, I'll get the text

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1 and 2, 1 and 2. So that would

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be a string object, not a number. Even

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though here, it shows as a number. It's

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just how the terminal prints it out. So

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again, these are errors that occur in

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runtime so while the script executes.

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Syntax errors are parsing errors, so the

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interpreter tries to understand whether

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the script is syntactically correct,

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whether we are full, whether you have

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followed the Python syntax rules. So you

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need brackets, a closing bracket for

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opening bracket, you need a closing

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quote, after the opening quote, and so on.

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[No audio]

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Now, there are also other types of

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exceptions, not only TypeError, you

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know, you may have a name error. Let's

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say here, instead of printing out all that,

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we print out c,

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[No audio]

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save, execute. And here is a

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TraceBack, again 2 was printed out from

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the third line, and the TraceBack says line

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4 at print c, NameError, name c is not defined.

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[No audio]

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So again, this is not a

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syntax error, because you haven't made

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any mistakes with the syntax. You know,

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you have this variable name, which is

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correct. You have brackets opening

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round brackets and closing round

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brackets. So everything is syntactically

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correct, both this c object Python

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doesn't know this, you haven't defined

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this c variable. So Python doesn't know

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what to print out. You know Python is able to

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print out a because it knows that a

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refers to the 1, to integer 1, so it

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prints out integer 1, but c doesn't

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have anything. So you get this NameError,

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and whenever you get a NameError,

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you know that's this name here has

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not been defined by you. So to fix this,

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you may want to defined c, like that and

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execute and you don't get any error. You

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may also have all types of errors such

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as c divided by 0, see what you get.

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[No audio]

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A ZeroDivisionError, division by zero.

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That's the description of this error. So

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division by zero is not mathematically

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possible or meaningful, and since Python

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is based on mathematics, then it throws

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an error. In the end, you need to fix

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that you need to remove that expression

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that divides by 0, and yeah, that's

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about errors in Python.

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[No audio]

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I hope you understood a great deal of this lecture

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and the previous lecture as well. So

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it's relatively easy to fix errors.

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However, sometimes,

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there are errors that you might have

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difficulties understanding and fixing

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them. So later, for example, we're

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going to work on libraries, and

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sometimes the libraries they have

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different kinds of errors. However,

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nothing to worry about, because there

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are other things that you can do

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for an error that you don't

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understand, and I'll talk about that in

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the next lecture. So I will see you there, see you.