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Instructor: Error handling allows us

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to let the script continue running

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even if there is an error.

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Let's try an example

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to show you how we can do error handling in Python.

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Let's say we wanted to create a program

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that allows us to take a users input.

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Let's say that we ask the user for an age.

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So, we use the input prompt and say, what is your age?

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And then here,

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what we're going to do is just print the age.

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Nice and simple program.

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If I run this

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and I type in let's say 10,

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there you go, everything is working.

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But what if I didn't enter a number and I go like this?

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Well, that still works

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but is that what I wanted to do with my program?

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Maybe I wanted to enter the age

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and calculate if they can play the game or not,

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whether they are 18 and over.

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How can we fix this?

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Well, one thing we can do

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is just to wrap this in an int, right?

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So, we convert whatever the user gives us into an integer.

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So that if I run this

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and I type this,

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uh-oh, I get an error, a value error,

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invalid literal for int.

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I'm trying to convert this into an integer

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but it's telling me that's an error, I can't do that.

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So how can we fix this with error handling?

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In Python, we can do it this way.

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We have the try.

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Oops.

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We have the try keyword which says,

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hey, I want you try this piece of code

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and we have to make sure we indent it.

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So, we try this piece of code.

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Now, anything that happens inside of the try block,

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I'm able to handle.

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See, where handling is coming from errors

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with the except keyword.

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And except says,

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hey, if whatever's in here in the try block,

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if anything errors out, I want you to say something.

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So instead of having this red error

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that exits out of the program,

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do something else, do this.

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So I'm going to say,

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print please enter a number.

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Let's see what happens now.

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If I click run, what is my age?

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I'm going to just write gibberish.

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And look at that.

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I get, please enter a number.

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I am handling my errors here

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by wrapping all of my code in a try block.

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So I could have just copy it my entire file here

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put it inside of a try block.

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And what's going to happen

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is instead of my program erroring out

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before my program crashes,

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it's going to run this code and then it's going to say,

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hey, if within this block anything happens,

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well, catch it in here with the except block

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and I can hear and do whatever I want.

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In my case, I can say, hey, please enter a number.

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But we have our problem here, right?

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The program just ended and we want this to keep running.

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I have to click run again and ask for the age once again.

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So, how can we make sure that we keep running this

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until the user actually enters something valid?

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Well, we've seen this before.

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We can do a while loop and say, while true.

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Run this.

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And if this works,

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well, we're gonna, hopefully, get the age.

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Let's see that.

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If I click run, what's my age?

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Let's say...

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Nope, please enter a number.

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What's your age?

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Let's try to break it again.

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Nope, please enter a number.

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All right, fine.

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My age is 10.

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There you go. I get the name 10, but...

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Uh-oh, it's still asking me what's my age.

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Ideally, I wanna break out of this true statement, right?

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So, how can I do that?

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Well, I can use what we call in here an else block,

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which we've seen before

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but this works with a try except block.

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We put it at the end.

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It says, hey, try this.

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If there's an error, do this.

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However, if there is no error, else, do this.

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So in our case, we can just say, break.

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And you know what?

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We can even print, thank you, just so we're nice .

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If I stop this and run again and I give them my age,

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there you go.

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We're done. We broke out of the loop and everything is good.

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And this try, except, else block can go anywhere.

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We can wrap our entire file in a try, except, else block.

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We can just add it into our functions,

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inside of our each individual functions.

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Whatever we want, we can use.

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This is just another tool that Python gives us,

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to handle errors.

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But let me ask you something.

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What if I did, instead of print age,

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I did let's say, 10 divided by the age.

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If I run my program

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and I type in here zero,

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and I enter,

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uh-oh, I get an error, please enter a number.

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But I did enter a number.

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This is a buggy program.

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Although I'm typing a number,

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it's still giving me the same error message.

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So, how do we handle this?

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How do we handle different errors?

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One where it is a value error

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and another one that is obviously, a zero division error.

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Well, in the except block

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I can simply give it what type of error I wanna handle.

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And remember, these are built-in errors in here

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that I can just copy and paste.

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So in my case, I wanna say this is for a value error.

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So that if I run my program

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and I type in zero,

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uh-oh, I get my error.

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This except block only accepts value errors.

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Any other errors, it does not care about.

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Hmm. So what can we do here?

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Well, one option is to copy here and say,

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hey, let's add another except.

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And in this case, I want a zero division error.

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In case a user enters zero, you can say,

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Please enter age higher than zero.

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So that if I run this now and I do zero,

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I get, please enter age higher than zero.

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If I do some gibberish...

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Oops.

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If I do some gibberish, I get, please enter a number.

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If I finally enter a number, look at that.

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Thank you. Everything is working.

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Now, here's my question.

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What happens if we enter another value error here?

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And this value error is just going to say exclamation marks.

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What happens now?

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I get please enter a number.

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because this except block is going to run only once.

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As soon as it catches the error it's looking for,

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it's going to run and then come back to the while loop.

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So only one of the errors will be caught.

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Okay, let's take a break

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and learn more about error handling in the next video.

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Bye-bye.