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In Python, when your code runs into an error, it does this

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thing called raising an exception,

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for the most part. In other languages, this is also called

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throwing an exception.

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Now, in Python, we have this little 'try-except' block. So

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we could 'try' some code, and then we could 'except' an error.

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So if our code fails for whatever reason, we can 'except' the

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error and gracefully handle it.

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Now, a good example of this is '1/0', you get

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a 'ZeroDivisionError'.

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Simply put, in mathematics, 1 cannot be divided by 0.

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And so if we do this, 'print("HELLLOOOOO")',

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notice that our 'print' statement does not show up.

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It caught an error on here and died.

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Now, like we see our 'print' statement in the error itself,

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but we don't actually see it printed out to the page.

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And so this is a really good reason why we want to catch

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errors and then gracefully handle them so that our program

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doesn't totally bug out and die, because that's a terrible

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user experience, and the first person they're going to blame

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is the programmer.

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So why is it important to be able to catch this particular

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error or any sort of error that happens?

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Well, (a) I don't want you to get blamed for anything, but (b) in

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Python, when it runs into an error it usually just dies. And

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see, errors are going to happen.

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There's just no way around it. If it has to do with working with humans,

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so maybe you're making a website, or maybe you're working

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with data, or you have something that has to do in relation

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to a human in any way, shape or form,

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you're going to get errors,

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and that's because humans come with user input, and user input,

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always, always, always is a string, and users, a lot of us,

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including myself, can be quite malicious.

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We try to break things just because we know we can.

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And so it's your job as a programmer to say, "Hey, yep, human,

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you are allowed to try to break this, but I'm going to catch

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that problem and I'm going to work with it anyways".

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So let's go ahead and show another error here.

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So let's say we've got a dictionary, and it's got a key in

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here 'name', and the value is going to be "Kalob".

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And then we try to do something like 'print(d[])',

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and then my 'hobby', which does not exist, this key in the

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dictionary simply does not exist.

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We're going to see a 'KeyError'.

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This one's got a 'ZeroDivisionError', and this one is a 'KeyError'.

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And there's all different types of errors.

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So when it comes to catching errors, there are a few keywords

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we want to keep our eyes out for. 'try' is one of them, 'except'

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is another one, and 'finally' is the last one.

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Now, what this is going to do, is it's going to 'try' to execute the code,

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if it runs into an error,

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if it runs into some sort of exception, it's going to exit

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the 'try' block immediately, and it's going to try to handle

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the error, and then 'finally' will always run no matter what.

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So let's go ahead and give this a quick little demo.

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Let's 'try: total = 10/0', and we're

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going to 'except' that there could possibly be an error in

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there, a 'ZeroDivisionError',

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so we're going to 'except: ZeroDivisionError',

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and if that happens, we're going to say the 'total = 'N/A'',

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it's not applicable, just can't be done,

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and 'finally:' we will say 'print(

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"The total is", total)',

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[no audio]

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and it says, "The total is N/A".

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So it tried this, failed, and then went over here and said,

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"Okay, we'll set the variable.

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total = 'N/A'",

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and then finally it ran some code.

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Now, if you're doing something like this, technically, you

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don't really need the 'finally' in there. You can run this,

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and it works just as fine.

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But I'm going to undo that, because that is a good little

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example. Let's try another one.

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Let's say we've got 'number1 = 50', and

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'number2' is going to be user input.

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So 'input("Enter a number:")', any number, and then we're going

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to print the total.

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So the 'total' is going to be 'number1 + number2',

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[no audio]

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and 'print(total)'.

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"Enter a number", 123, and we get a different type of error.

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We get a 'TypeError'.

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Now, the reason for that is because 'number1', this is an

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int, 'number2', this is a string.

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Well, how are you going to add numbers to letters?

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That's really hard to do.

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Now, even though I entered a proper number, again a program

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does not know that this came back as a string.

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And so Python said, "You are trying to add letters to a number,

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simply cannot do that".

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But what we can do instead, is we can 'try' to do this, and

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then we can 'except' that there is some sort of error in here,

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and we can say 'print("There was an error")'.

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And let's go ahead and move that 'print(total)' up here, and rerun this.

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"Enter a number: 456, and it just says, "There was an error".

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And so now our code isn't complaining.

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It's not doing this.

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It's actually saying, "Hey, this doesn't work".

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The total was not printed, but the error was printed.

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And that means, 'print(

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"The program can continue without problems")'.

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And sure enough, "The program can continue without problems".

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Now, in our first example, that 'print' statement no longer

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worked. It died right here.

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Python just said, "I can't figure this out",

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and so this was never executed. Down here

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Python said, "Hey, I can't figure this out",

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it broke out of this

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'try' block, did not execute this, and executed the 'except' code

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instead. Now, we've actually seen a lot of different error

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types throughout this course, and I think we've seen three

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of them in this particular lesson.

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So in the next lesson, I'm going to teach you how to discover

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what the error types are and how you can handle them.