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Welcome to lesson 11 which roughly covers pages 72 and 73 of automate the boring stuff with Haiphong

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textbook right now getting an error or exception in your play on program means that the entire program

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will crash.

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You don't want this to happen in real world programs.

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Instead you want the program to be able to detect errors handle them and then continue to run.

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We can make error happen easily.

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So in a new file entry the following

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def div 42 by Divide by this will be a function that just returns the value 42 divided by whatever argument

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we passed in we can call this here print to 42 by dividing by two

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divided by 12.

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Divide by zero and divided by 1

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and I'll just save this as an example that Y will run this program.

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So we'll see.

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This is caused an error.

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This line worked just fine it called div div 42 by and it passed to.

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So it returned 42 divided by two which was just 21 that backup printed to the screen and we also called

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it by passing 12 so 42 divided by 12 returned 3.5.

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But an error happened here when we passed in 0 42 divided by 0.

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Cause this 0 division error computers don't really know how to handle dividing a number by zero.

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So whenever that happens it says well I don't want to continue running the instructions in this program

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because I've just come across this area that I don't know how to handle.

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So instead it crashes and terminates the program.

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That's why this function call never gets made because the program crashed.

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At this point.

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But you can handle an error with a try.

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Except statement.

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The program.

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So it looks like this.

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Try colon

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return 42 divided by Divide by and accept zero division error that's this errors name right here.

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Colon print where you try to divide by zero.

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Now when we run this program in C it divides by two it divides by 12.

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It tries to divide by zero which causes this error to happen.

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And it runs this code.

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And since there is no return statement this function just ends up returning the none value which gets

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printed here and then the program continues on it.

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Calls div 42 by and passes 1 to it.

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So 42 divided by 1 is 42.

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And the program just ran to completion.

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It didn't have any sort of ugly error message that appeared.

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This happens because when code in a try clause causes a 0 division error the program execution immediately

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moves to the code inside the except clause and after running that code except the execution continues

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running as normal.

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This keeps the program from crashing entirely.

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You can also have a simple accept statement without specifying the type of error it catches and it will

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catch all types of errors.

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This code can be useful for input validation.

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Say I have a program where I enter the number of cats I own.

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Might look like this

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.

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So here it asks how many cats do you have.

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The user can type in how many they have it's assigned to numb cats and since input returns a string

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value we have to convert that to an integer and then that's compared to see if it's greater than or

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equal to 4.

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And if so print out.

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That's a lot of caps.

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Otherwise it prints out.

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That's not that many cats so let's run this program.

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How many cats do you have.

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I'll just enter 5 says that is lot like cats.

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Let's run that again.

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How many cats do you have.

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Zero.

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It's not that many cats.

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There's a possible problem with this code though.

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User can type in anything they don't have to type in a number.

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So when I run this program I could type in 6 and then that causes an error.

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This error happens because the string 6 assigned to the variable name cats is then passed to the function

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which is expecting a string that has numeric digits in it.

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It works just fine if you have a string like 6.

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Soon as you try to pass it a string that doesn't have numeric digits in it like the word 6 then it causes

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a value error.

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So we can add a try except block in sight here

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.

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To handle this error except value for

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print.

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You did not enter page number

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.

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Now when I run this program I can type anything I want and it will give me an error message saying you

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did not enter a number but that's a lot better looking than just this ugly looking Python error message

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.

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And this is what we mean by input validation.

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It validates the input that the user has given us so that it doesn't cause the program to crash just

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for extra credit.

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You might want to change this program because technically a user could enter something like negative

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for cats which is technically not greater than or equal to 4 so it prints out that it's not that many

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cats.

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See if you can figure out some code to add to this program that will change it so that it detects when

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the user enters a negative number and displays a different error message.

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To recap.

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Divide by zero error.

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Happens when ever the python program tries to divide a number by zero.

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Errors usually cause the program to crash entirely because Python doesn't know how to continue on after

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that.

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It doesn't want to run the instructions with bad data.

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So we just Holts the entire program and displays this error message but an error that happens inside

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a try block will cause code in the accept block to execute that code can handle the error or display

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a message to the user or whatever you want it to do so that the program can keep going.