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ok so let's start with a challenge so

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the challenges is for you to create a

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new python file and write a short

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program that asks the user to type 2

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integer numbers then prints out the

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first number / the second now the

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program shouldn't crash no matter what

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the user types in although you shouldn't

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have to worry about control keys and

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just as a hint here if you have to do

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the same thing more than once that

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sounds like a good use for a function so

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that's you challenge see how you go

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pause the video and I'll see you when

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you get back ok so let's go through my

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solution to this challenge and as

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always yours might be different

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the important thing here is that you've

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got a solution that works

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not that you've got identical code to me

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so create a new file for this

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i'm going to right click over here click

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on new select python file will call this

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one...okay we're

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gonna be asking for a number more than

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once so i put the code to get the number

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from the user into a function so...

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...

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...

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so the function will keep asking for a

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number until something valid is entered

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to put everything into an infinite loop

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using while true on line 2 so that loop

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just keeps looping so we'll have to use

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break or return statement to get out of

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it obviously using the return statement

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on line five next comes the try except

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blocks so we've seen how to get numbers

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from the keyboard a few times we now

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use the input function then convert the

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users text into an int now that's

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the first place where an exception can

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be thrown when python attempts to

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convert the user's input into an int value

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now obviously the conversion

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to an int will only work if the users

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has actually entered numbers and

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nothing else

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so we need to wrap the conversion in a

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try block so we can trap that invalid

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input so how did I know what exception

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that we should be handling well one

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approach to this would be to read

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through the list of exceptions in the

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documentation link that i showed you in

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the previous video that's certainly one

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way to find out

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now I'm a great believer in using

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documentation and I've been providing

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links to the documentation throughout

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this course you really can't just write

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code without reading documentation has

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lots of it but with that said I'm also

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the first to admit that program

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documentation isn't the most shall we

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say riveting of reads so there's

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another way to find out what we need to

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know i'll jump that as well so i'm going

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to comment all that out for now and run

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this little program so let's just

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comment out the code the code we want to

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run temporarily.....

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....

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alright so let's run this and we're

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going to enter a number obviously with a valid

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number we get no problems but we try

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to run it with some garbage input you

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see that the exception that we've raised

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here by putting in something invalid is

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a value error invalid literal for int with

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base 10 switching back to the

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documentation if we look for our value

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exception

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sorry value error now obviously search for it

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but its a fair way down the list so

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chances are you might have to do a favor

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of reading to find it so either way you

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could actually read this or you could

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have also gone back to the code and did

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what I did was try and get the program

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to crash without using try and except to

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see what errors what exceptions are

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actually generated in this case the

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value error either way we'll find out

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what we need but when dealing with

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exceptions

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I find personally it's often easier to

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just cause the problem and see what

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exceptions raised and then to add the try

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except Clause after that

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alright let's delete that line that i

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added temporarily and go ahead and

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uncomment our function and just

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carry on explaining more about what it

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does

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so if a valid number is entered on line

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4 the code is going to continue and

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the number will just be returned but if

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invalid input has entered a value error

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exception will be raised as we've just

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seen so we handle that error in our except

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Clause now we don't do a great deal is

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not really much we can do apart from

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asking the user to type another number

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then we could perhaps make the message a

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bit more helpful or mention that only

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digits are allowed

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so you've got an extra point their gold

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star if you manage to do that in your

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message now once that message in the

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exception blocks printed the code

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continues so we'll go around the loop

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the while loop again and that will keep

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happening until a valid number is entered

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so we can be really confident this that

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this function will always return a valid

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int value

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alright so how do we make use of this

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function well let's add get the two

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numbers from the users i'm going to put

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..

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...

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...

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...let's finish this

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off by performing the division now

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we've seen in the previous video that

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division by zero can raise a 0 division

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error exception i'm going to put the

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code to perform the division in a try

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block and catch the exception i'm gonna

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come down here and then try...

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...

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...

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ok now using the same prompt twice isn't

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really very user-friendly a user might

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wonder what they've done wrong to get

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exactly the same prompt again so let's

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say actually easily fixed by passing the

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text we want to prompt the user want to

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prompt the user with as an argument to our

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get int function so let's change this a

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little bit i'm going to come up here and

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make this prompt and then what we're going to

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do number equals...instead of

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putting a hard-coded string there

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we're going to put prompt the prompt is

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going to be the message that we want

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displayed when we're getting a number

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input and in terms of using the get int

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method now are the function now

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we're going to put...

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...

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...

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so let's try running this so

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please enter the first number we put some

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rubbish numbers in there

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invalid number entered please try again

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and you can see I'm typing it again it's

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going to continue asking for that first

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number until we put something valid in

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there so let's put a valid number like 455

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in there now it's asking for the second

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number same deal there with invalid input

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is going to keep asking until I put

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something valid in there

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let's put 0 in as the value for the

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second number and we get the error can't

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divide by 0 so this point I think we've

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got a fairly robust program that the

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user can't crash by entering invalid

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input so if your program behaves in the

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same way congratulations

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now there are a number of ways that this

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could be written and you can even do

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away with the second try except block

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and use an if statement to check his

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second number 0 instead that's also

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fine and in this program is very little

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to choose between the two

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methods now there's still one way the

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user can crash the program and that's to

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do a control D which may be

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control z on earlier versions of

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Windows now on operating systems derived

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from unix that includes mac OS x and

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linux ctrl-d triggers an end-of-file

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eof and Windows 10 behave in the same

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way i'm going to show you what i mean by

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that by running it again and down here

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if I do control d the first number now

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when I press ctrl-d on a Mac actually

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get a different output

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that's because the controlled d is map

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to another process so what i'm going to do is

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cancel out of that and run this again so

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on a linux machine or Windows machine

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you can try ctrl D from a mac what I have

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to do a command D simulate something

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similar so i'm going to do that now when

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i do that you can see I get this EOF

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error the end of file when reading a

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line so that's another exception that's

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been raised

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however if you're doing this on a window

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machine or linux machine you might find

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that the program starts going a little

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bit nuts and that you have to start

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looking at manually stopping it

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because at this stage the mac has

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actually gracefully shut down the

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program but you might find that it's

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looping continually windows and linux

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that's the case come over here and click

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on stop process to stop it then cancel

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the program if you happen to be running

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this from the command line like a

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terminal session or from Windows 10 come

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from a windows command prompt and

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control C should probably stop as well

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the bottom line here is that control

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d signals to python that there's no

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input left so consequently pythons

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close the input stream so our code then

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catches the exception and in the case of

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windows at least and linux loops

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back around but the input streams

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already been closed so the input

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function returns immediately with

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another end of file error our code

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catches the end of file error and loops back around

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and so on and so forth so we definitely
catches the end of file error and loops back around

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don't want to do that even if the mac

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has only come up with one error

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it's still not something not a

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00:11:01,260 --> 00:11:03,900
situation we want to get into so if your

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user terminates input in this way

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respect their choice and allow the

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program to terminate there's nothing you

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can really do about it anyway if you're

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using python input function to read

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from the user right so let's actually

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have a look at what we can do is close

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down and what we can do in the case of

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an end of file error lets actually trap

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that and down here what we will do is up

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here would be the place to do it after

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the accepted value error will get

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another one except this time must be eof

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error we can do assist.....

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we've got an error there and the error is

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that we need to basically it's

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unresolved reference so we need to

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import sys so lets go ahead and do that so

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...

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...and sys . exit

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terminates the program with the exit

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code that's used or past as its

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argument now for normal program

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termination is quite usual to return 0

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and if you decide that the user boarding

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the program isn't normal termination

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then you can return a different value

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i'll do that next let's just try running

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this first to make sure that works

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I've been here and going to do on

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a mac command d you would do a ctrl

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D on windows or linux and we now just

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got the program terminating without

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anything else happening now if you are

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writing a python script to be used

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from the terminal possibly called from a shell

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script or Windows command file that it

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may be really useful to use different

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error codes or exit codes so we can

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return a exit code of one if the user

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terminate the program and perhaps 2 if they attempt

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a division by 0

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let's go ahead and add that code so

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going to come up here to line 12 going

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to change that to assists

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sys.extone let's also put an

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error in here for the division

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by zero close the program now...

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....you can see here process finished

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with exit code zero so let's try

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running this again so now if we do my

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command d on a Mac or control d on windows

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or linux we get process finished with

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exit code one running it again this time

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we'll put some valid numbers in 10 the

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second number put zero to ensure the 0 division

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error comes up can't divide by 0 this

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time we got process finished with exit code 2

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...

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that's really good demonstration of

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reacting in a different way when

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handling different types of operations

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alright let's finish the video here and

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we're still got a couple more things we

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need to look at with exceptions

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let's start on that in the next video