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I mentioned in the last video

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there's just a couple more things we

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need to look at with exceptions

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I guess I said a couple of things but I

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really meant 4 so there's two optional

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clauses that we can add to a try

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statement else and finally so we'll have

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a look at what they are next

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now it can be important to consider the

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order that you catch exceptions using

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accept and i'm going to explain about

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that as well and the fourth thing is

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raising our own exceptions which you

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sometimes want to do in your functions

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if there's really no sensible action they

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can take your functions have to catch

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the exception

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otherwise your program will crash

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as we've seen but if you can't take any

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sensible action then sometimes the

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best approach is to pass the exception

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on to whatever called your function for

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it to deal with going to start with the

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finally Clause because our current

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example program provides quite a cool

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demonstration of the fact that a

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finally Claus will always be executed so

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a finally clause starts surprisingly

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with the keyword finally followed by the

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block of code that you want to execute

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whether an exception was raised or not

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let's go ahead and put the finaly clause

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into effect and do that on line 13 so...

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finally call on the next line as print

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...

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...now if you use

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a finaly Claus it must come after all

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your except causes right so let's just

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run this and i'm just going to add some

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valid input 10 and 5 notice the

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finally clause pop up there 4 and there's

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the finally executing again if we

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run this again and this time I do my

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contribute command trick and again

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ctrl D on windows or linux notice that

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the process still executed but the finally

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clause was still executed as well so the

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finally Claus really is executed

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regardless of whether an exception was

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handled or not that's particularly

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impressive considering what the code

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does when I terminate the input with

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control D

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so in the end of EOF or end of

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file error exception block on line 11 we

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we actually called sys.exit to

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terminate the program but even though

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the program's been told to terminate on

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line 12 the finally block on line

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fourteen is still executed first so if

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you think about it that really makes

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finally an ideal place for performing

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any tidying up that may be required

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before your code finishes now the sorts

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of things that you want to do in a

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finally block would be things like

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closing a database cursor or connection

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or closing any open files if your

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program is open any those types of

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things and that will actually use a

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finally clause when we get back to our

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database to make sure we close our

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cursors

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alright so moving on an else clause is

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executed if the code in the

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try block doesn't raise an exception

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so in that sense it's equivalent to the

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else clause in a four or a while loop if

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you remember the else

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in a while loop will execute if the loop

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terminates normally so it executed

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there's no break or return statement to

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execute the loop prematurely so the same

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is true of else when used with a try

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statement if no exception is handled the

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else clause will be executed

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otherwise it won't be now the use of the

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word else in these cases perhaps

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unfortunate it may help to pretend that

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a word like completion has been used

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instead but unfortunately it wasn't so

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we have to remember the two different

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uses of else

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an if statement and identifies a block

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that will be executed if no earlier

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condition is true and in for while and

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try it's executed if the previous

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code complete successfully so if you

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use the else clause in your try

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statements

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it must also come after all the except

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clauses but before any finally clause if

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you've got one

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alright so let's add an else clause to our

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program to see how it works

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we're going to print a message if the

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division was successful going to come

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down here after the code on line 23

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we're going to add an else on line 24

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...

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....okay

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so I run this again this time lets enter a

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couple of values so...we

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get 90 / 45 is 2.0 and division

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performed successfully so if we run it

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again

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90 this time but 0 for the second

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number you can't divide by 0

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and this time we haven't got the message

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division perform successfully on screen

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so how this is different to finally well

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the code in the else clause only execute

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if the triblock completed without raising

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an exception

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so when we attempted to divide by zero

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else code didn't execute if i change

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else on line 24 now to finally and run

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this again and repeat both tests so 90

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and 45 we correctly get the answer 2.0

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and division perform successfully run it

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again this time 90 and 0 as a

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second number and we get the message

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showing this time division performed

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successfully so finally is always executed

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even if there's an exception or as you saw

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earlier even if the program is

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terminated in the try block so code in

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the else clause only executed if the try

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completes it won't be executed if

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there's an exception or if the trial

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code returns or quits the program now

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you might be wondering this point why we

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didn't just put the division perform

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successfully message after the code on

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line 20 in other words after we printed

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out the result in this case we could

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have done just that

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that's because our print statement can't

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raise an exception so there's little

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point using else clause here else is

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really intended so that we don't

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accidentally catch an exception that

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wasn't raised by the code we're trying

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to protect with the try block so going

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back to our bank account example if you

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were protecting the updating of two

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transactions using a triblock would only

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want the code to roll back if there was

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an error saving the values to the two

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accounts if we add

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a load more code inside the try block

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and that code raises an exception then

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the transactions will be rolled back

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even if they completed successfully and

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will see that in a video coming up soon

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where it makes more sense to use an else

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clause so this last example is a bit

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contrived to demonstrate really how else

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and finally work and the difference

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between them

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now there's one last thing that this

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program is useful to demonstrate before

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we abandoned it and move on to something

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else

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now exceptions like everything else in

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python are objects in python three

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they must be derived from the base

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exception class or one of its subclasses

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that wasn't the case in python two

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although it was recommended and that's

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also mentioned at the start of the

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document we're looking at the browser

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earlier you can see here in Python

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all exceptions must be instance of a class

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instances of a class that derives from

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base exception so most exceptions will

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be subclasses of exception rather than

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base exception which is the

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recommendation in the fourth paragraph

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on that page programmers are encouraged to

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derive new exceptions from the exception

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class or one of its subclasses and not

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from base exception so why is that

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important well the first reason is that

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you can catch exception in your except

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Clause to catch any exception that's

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derived from it so let's actually have a

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look at how that works in our code

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so go back to the IDE so on line

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nine we've got this line except

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value error now we can

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check that value area is a subclass of

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exception by holding down the control

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key or on in my case on a Mac the

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command key clicking on it to see the

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code you can see their class value error

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exception then click on that and have a

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look

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so you can see here that value error

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inherits from exception and if we click

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on exception

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you can see that exception inherits from

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base exception so what that means is we

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can come back to our code gonna

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close it down means that we can actually

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change this on line 9 and replace that

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with exception and if we run this we

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should find that still copes

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with invalid input 90 and 0

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just bogus number valid number and then 0

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so still coping and basically

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catching those exceptions

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now with that said I'm not recommending that

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you do this or suggesting you do that

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it's generally a good idea to be

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specific in the exceptions you catch

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rather than just catching everything one

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reason for being specific is that you

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can handle the different exceptions

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differently now if you can just catch

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all exceptions as we're doing here

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the code will react in the same way

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regardless of what exception was raised

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that may often not be the appropriate

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way to react and in fact we've got a

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problem with our code now so it looks

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like it worked fine until you understand

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this next bit you probably won't see

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anything wrong by looking at the code now

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IntelliJ is aware that

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something is not quite right

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we actually got a warning on line 11

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that's where we catch the end of file

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error you can see the exception

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superclass of exception class EOF error

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has already been caught so let's

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actually see the effect of that before

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talking anymore about it i'm going to

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run it again going to do the ctrl D

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00:10:05,720 --> 00:10:10,700
command D on the mac you can see what's

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happening here we're going into a loop

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our programs running wild again i'm

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going to stop it and cancel out of that

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the question at this point is what went

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wrong

256
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remember when I said that the order that

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you handle the exceptions is important

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what's happening here with our code is

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that exception is a subclass of EOF

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error now subclass of a base class is

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also an instance of its base class so

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this case EOF error is an

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instance of exception and when we catch

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exception on line 9 we're

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actually catching all exceptions that

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inherit from it including the eof error

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exception that has been defined on line

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11 so what that means is that the code

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that runs when eof error is raised is the

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actual code on line 10 so therefore the

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program doesn't get to exit it just

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prints a message and loops or allows the

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loop to go around again and again and

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again because the input streams closed

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and we keep getting these

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eof errors raised by line 7 IntelliJ is

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aware of this and that's why its giving

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us a warning

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so when you're handling multiple

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exceptions like we are here it is very

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important that you catch them in the

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right order

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so the more specific exceptions have to

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appear first so you can fix this program

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up by moving lines 11 and 12 up until

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before we catch the exception on line 9

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so I'm gonna just copy those cut

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them out and pasted above the code that

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was on line 9 and if you actually run

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this and enter some bogus text that isn't a

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number here we get invalid number and

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now if I come back here again and do a

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command on a Mac ctrl D on windows or

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linux we actually get it to exit

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correctly with exit code one so you can see

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the code that we generated for the

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exception eof error is now being

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executed correctly when we do the

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control d e so it's not often

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that you specify exception as the

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exception to handle your except Clause

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but this applies whether you're catching

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multiple exceptions and some of the ones

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you want to catch derive from others and

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by the way we are getting this other error here

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about two broad exception clause this

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warning is that perhaps we should be

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more specific

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alright so the last thing i want to

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mention is related to this now remember

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from the documentation that not all

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exceptions have to be derived from

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exception the requirement is that they

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derived from base exception or one of

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its subclasses so if you really do want

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to catch any exception then there's a

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way to specify the sort of wild card

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exception meaning everything to do that

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just leave off the exception type so

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the except Clause is just the keyword

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accept and a colon so this example is

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going to delete the exception on line 11

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and just leave the colon there before I

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do this just to be clear I'm going to

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change the finally here on line 24 to an else

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again

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so that when I make the code

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available in the resources it's showing

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how it should be done rather

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demonstrating how not to do it right the

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other thing I'm going to do here is on

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line 11 is just to make it clear here

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with a comment

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we should really be except...

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...

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alright let's test the program a few

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times let's try a number or partial

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numbers i will test for invalid input

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using numbers and letters invalid number

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will try just letters that works and try

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76.....that works ok running

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again and again we're getting the errors

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caught successfully invalid numbers by

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the Wildcard accept on line 11 so that's

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good right second time lets do a divided

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by 0 so 32 and 0 the finally clause always

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execute and you can't divide by 0 and

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we done a assist.exit 2 as you can see

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there the code on line 23 was executed and

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let's do the control d check again run

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that and do a control command D on a mac

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and control d on a windows

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or linux and the finally clause

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always execute and we're finishing the

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code on line 10 is executed

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correctly

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so everything's working so

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that's exceptions and how to handle them

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in python so in the next video we can

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have a quick look at how to raise our

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own exceptions before we ultimately get

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back to databases so see you as

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always in the next video