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<v ->Hi, and welcome back.</v>

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In this video we're looking at raising errors in Python.

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In the last lecture, I showed you the syntax real quickly,

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but we're just gonna go over it again in this video

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and give you a couple examples so that you know

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in which scenarios you should be raising errors.

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And also, I'm going to introduce a new piece

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of Python syntax that I think is pretty cool.

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So let's right-click our project that I've just created

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in pyshon and create a new Python file.

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And again I'm going to call it app.py.

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This is a pretty popular name for your first or your

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most important file in Python that you're going to run.

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So you can right-click it and run it.

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Normally, this file would be called app.py or run.py,

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it's just a name for your main, most important file.

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Let's say we have a code that is for a garage.

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It stores cars and you can add cars to it.

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Let's define a class garage that has an init method

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and it defines self.cars to be called cars.

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Sorry, to be equal to an empty list.

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By the way, notice how, in an attempt to reduce

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any indentation or tab errors, I have told pyshon

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to show me any hidden characters or invisible characters.

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You've got these small, tiny dots that

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you can barely see without it being in a video.

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You can certainly not see them very well on video,

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but you've also got these lines here.

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So you've got these dots, that are very small,

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I'm gonna zoom in in the video just so you can see it,

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and you've got this line here.

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The line tells you where an indentation level begins,

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or where it is maintained.

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The dots show you the spaces, so you know these

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are not tabs, they're spaces.

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So if we were to add another line here,

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notice how now we've got a three line,

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three line long grey line here that tells us that

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the indentation is maintained for all of this.

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So this garage has an init method.

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It has a len method.

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That returns len of self.cars.

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And also it's got an add_car method

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that takes in a car that we're going to add as an argument.

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However, we've not yet had the chance of working on this

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too much, so what we're gonna do, is we're gonna print,

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"This method is a work in progress."

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This is obviously not the right way to go about it,

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as you know now, but then let's say that

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we are using this garage, so we're gonna say

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ford garage is garage, and then ford.add_car

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Fiesta

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And then we wanna print the len of ford.

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Now, this method here is going to print out something

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onto the screen, but if we were not really paying attention,

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if we were, for example saving this thing here to a file

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or to a database, we may not realise that this is

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printing something out, we would just see that

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this is zero every time.

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Let's run our app.

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And as you can see here, it says,

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"This method is a work in progress,"

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First of all, and then it just says zero.

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No matter how many cars we add, all we're gonna see is,

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"This method is a work in progress"

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Over and over again.

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And if we were saving this length to a database or

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just to a file or somewhere, it would always be zero.

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If you run it, then you may wonder,

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"Hey, what's going on here?"

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And then you're going to have to actually

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run the programme manually like so in PyCharm

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to see the printouts, and then you'd see

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"Ah, okay this method, this code isn't working yet."

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So instead, of course, we can raise an error.

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We can raise and all you do is raise

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and then type a NotImplementedError.

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And then you got brackets, so you're creating a new

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NotImplementedError instance, this is a class,

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you're creating a new object

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of type NotImplementedError, and you're raising that.

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Inside the brackets, you can put a small message,

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We're gonna say, "We can't add cars to the garage yet."

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Now when you run the app, which I'm gonna do by pressing

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on the top right here, you get your nice trace back.

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Then it says,

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"NotImplementedError: We can't add cars to the garage yet."

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Then it tells you where that error happened.

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Raise NotImplementedError here, we did it manually.

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It says line 9, in add_car, so that's line 9 of our code

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inside the add_car function.

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Now, where was this function called?

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Where did this function originally happen?

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On line 13, here,

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and ford.add_car Fiesta

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So as you can see, error happened here, inside this function

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This function was called here.

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So it tells you all the functions and parts

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of your file where this error has gone through.

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From being raised up until it reached the file level.

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So that's two places right now.

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So this is much better, cause now when we run the programme,

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it's gonna crash, we're not gonna save

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anything to the database, and we're gonna say,

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"Oh, okay, it crashed, let's investigate."

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We would see that a NotImplementedError was raised.

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And that's all good.

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So this is how you raise an error.

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You use a key word, then you create a new object, you can

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put something inside the brackets which will be the message.

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Let's say we add another class.

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That is gonna be a class of type car.

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And that's take a make and a model

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as parameters.

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And then it's also gonna have a repr function of course,

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because you always want these things.

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And it's going to return a car with

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self.make and self.model like so.

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So this is just a representation of a car.

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Remember the repr dunder function,

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you can print out the car object

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and it's gonna print something that looks like this.

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Okay.

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Now, the add_car function here should

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only accept

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the parameter being a car object.

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So how do you do that?

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Well pretty straightforward actually,

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there is a built-in function in python called isinstance.

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Isistance tells you whether the variable that you give it

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is an instance of the type that you give it.

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So this is the variable, car.

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This is the type, which is the class, Car with a capital C.

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So if not isinstance, so if car is not an instance of Car,

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we're gonna raise a type error.

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We're gonna say, "Tried to add a

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"car class

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"name

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"to the garage,

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"but you can only

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"add Car objects."

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So it's bit of a longer message there.

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And all I've done is formatted the string

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and said car._class_._name_

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That's the name of the class of the variable.

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So if this was a string, this would print out str.

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If this was a class,

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if this was a car, it would print out car.

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This is just a nice bit of construct

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that you can access for essentially any variable.

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This gives you the class of the variable,

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and this gives you the name of the class.

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So we're trying to add that to the garage,

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but you can only add car objects.

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And then of course we're not gonna

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raise a NotImplememntedError.

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Now here we can say self.cars.append(car)

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And you may wonder, "Well, why do you

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"not put this in an else?"

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You don't have to.

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If the car is not an instance of the Car class,

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we're gonna raise the error,

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and that's gonna crash our programme.

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We are never going to reach this line if that happens.

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If however, it is an instance, it's not going to go in here,

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and it's just going to go here.

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So you don't need the else for anything,

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because the only path--

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there are only two paths.

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One where this doesn't run because you raised the error,

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and the other one where this runs,

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and that's where you want it to run.

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So that's sufficient.

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So once again, let's try to add this string car.

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And notice how now, we get a nice trace back here

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that says, "tried to add an str to the garage,

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"but you can only add Car objects."

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And I think that's a pretty nice error message.

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So, what can you do?

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Well, you can create a new car object.

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Let's say car, this is gonna be a Ford Fiesta,

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and then you're gonna add that car,

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which is a car object, into the add_car function.

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Again, the value of the car variable is a Car object.

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And that's what you are teleporting

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over to this car variable.

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So the value of the car variable is now

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being assigned to this car variable here.

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And then you're going to use that inside here.

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So now we can run it.

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And it all works.

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We can print the len of ford.

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And you'll see that one gets printed out,

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which is pretty nice.

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This is raising errors in Python.

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It's really straightforward.

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All you have to do is, when you want an error to happen,

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raise it, create an object of it, remember the brackets,

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and inside the brackets, if you want, it's totally optional,

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you can put a nice message for when the error gets raised,

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the person who's writing the code knows what happened.

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Errors are most useful for developers.

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Of course, if your users--

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if somebody was downloading this code

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over the internet saying, "I'm gonna run my garage."

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Of course this is a bit basic for that,

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but if somebody was downloading your code and using it,

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they encountering a type error, may not be

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terribly useful for them because

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it doesn't tell them anything, they're not developers.

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So errors are most useful for developers.

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When you are writing code, or when somebody else is

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helping you write code, working on a team,

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having nice errors can be really helpful because then

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when you make a mistake or you do something incorrectly,

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you'll quickly get an error and you'll be able to fix it.

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If you don't get errors and instead you print things out

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like we did earlier on, you may never find out that

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this is not working until you see that the programme

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doesn't do what you were expecting.

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So, raise errors, use them.

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They'll be your best friend in no time.

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So that's it for this video, raising errors in Python.

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I'll see you in the next one.