WEBVTT

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I mentioned in the last video

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there's just a couple more things we
I mentioned in the last video

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need to look at with exceptions
there's just a couple more things we

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I guess I said a couple of things but I
need to look at with exceptions

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really meant 4 so there's two optional
I guess I said a couple of things but I

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clauses that we can add to a try
really meant 4 so there's two optional

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statement else and finally so we'll have
clauses that we can add to a try

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a look at what they are next
statement else and finally so we'll have

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now it can be important to consider the
a look at what they are next

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order that you catch exceptions using
now it can be important to consider the

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accept and i'm going to explain about
order that you catch exceptions using

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that as well and the fourth thing is
accept and i'm going to explain about

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raising our own exceptions which you
that as well and the fourth thing is

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sometimes want to do in your functions
raising our own exceptions which you

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that is really no sensible action they
sometimes want to do in your functions

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can take your functions have to catch
that is really no sensible action they

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the exception
can take your functions have to catch

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otherwise your program will crash
the exception

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as we've seen but if you can't take any
otherwise your program will crash

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sensible action then sometimes the
as we've seen but if you can't take any

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best approach is to pass the exception
sensible action then sometimes the

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on to whatever called your function for
best approach is to pass the exception

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it to deal with going to start with the
on to whatever called your function for

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finally Clause because our current
it to deal with going to start with the

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example program provides quite a cool
finally Clause because our current

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demonstration of the fact that a
example program provides quite a cool

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finally Claus will always be executed so
demonstration of the fact that a

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a finally clause starts surprisingly
finally Claus will always be executed so

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with the keyword finally followed by the
a finally clause starts surprisingly

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block of code that you want to execute
with the keyword finally followed by the

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whether an exception was raised or not
block of code that you want to execute

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let's go ahead and put the finaly clause
whether an exception was raised or not

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into effect and do that on line 13 so...
let's go ahead and put the finaly clause

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finally call on the next line as print
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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...now if you use
...

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a finaly Claus it must come after all
...now if you use

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your except causes right so let's just
a finaly Claus it must come after all

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run this and i'm just going to add some
your except causes right so let's just

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valid input 10 and 5 notice the
run this and i'm just going to add some

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finally clause pop up there 4 and there's
valid input 10 and 5 notice the

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the finally executing again if we
finally clause pop up there 4 and there's

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run this again and this time I do my
the finally executing again if we

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contribute command trick and again
run this again and this time I do my

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ctrl D on windows or linux notice that
contribute command trick and again

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the process still executed but the finally
ctrl D on windows or linux notice that

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clause was still executed as well so the
the process still executed but the finally

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finally Claus really is executed
clause was still executed as well so the

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regardless of whether an exception was
finally Claus really is executed

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handled or not that's particularly
regardless of whether an exception was

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impressive considering what the code
handled or not that's particularly

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does when I terminate the input with
impressive considering what the code

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control D
does when I terminate the input with

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so in the end of EOF or end of
control D

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file error exception block on line 11 we
so in the end of EOF or end of

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we actually called sys.exit to
file error exception block on line 11 we

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terminate the program but even though
we actually called sys.exit to

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the program's been told to terminate on
terminate the program but even though

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line 12 the finally block on line
the program's been told to terminate on

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fourteen is still executed first so if
line 12 the finally block on line

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you think about it that really makes
fourteen is still executed first so if

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finally an ideal place for performing
you think about it that really makes

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any tidying up that may be required
finally an ideal place for performing

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before your code finishes now the sorts
any tidying up that may be required

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of things that you want to do in a
before your code finishes now the sorts

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finally block would be things like
of things that you want to do in a

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closing a database cursor or connection
finally block would be things like

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or closing any open files if your
closing a database cursor or connection

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program is open any those types of
or closing any open files if your

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things and that will actually use a
program is open any those types of

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finally clause when we get back to our
things and that will actually use a

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database to make sure we close our
finally clause when we get back to our

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cursors
database to make sure we close our

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alright so moving on an else clause is
cursors

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executed if the code in the
alright so moving on an else clause is

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try block doesn't raise an exception
executed if the code in the

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so in that sense it's equivalent to the
try block doesn't raise an exception

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else clause in a four or a while loop if
so in that sense it's equivalent to the

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you remember the else
else clause in a four or a while loop if

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in a while loop will execute if the loop
you remember the else

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terminates normally so it executed
in a while loop will execute if the loop

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there's no break or return statement to
terminates normally so it executed

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execute the loop prematurely so the same
there's no break or return statement to

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is true of else when used with a try
execute the loop prematurely so the same

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statement if no exception is handled the
is true of else when used with a try

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else clause will be executed
statement if no exception is handled the

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otherwise it won't be now the use of the
else clause will be executed

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word else in these cases perhaps
otherwise it won't be now the use of the

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unfortunate it may help to pretend that
word else in these cases perhaps

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a word like completion has been used
unfortunate it may help to pretend that

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instead but unfortunately it wasn't so
a word like completion has been used

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we have to remember the two different
instead but unfortunately it wasn't so

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uses of else
we have to remember the two different

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an if statement and identifies a block
uses of else

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that will be executed if no earlier
an if statement and identifies a block

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condition is true and in for while and
that will be executed if no earlier

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try it's executed if the previous
condition is true and in for while and

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code complete successfully so if you
try it's executed if the previous

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use the else clause in your try
code complete successfully so if you

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statements
use the else clause in your try

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it must also come after all the except
statements

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clauses but before any finally clause if
it must also come after all the except

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you've got one
clauses but before any finally clause if

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alright so let's add an else clause to our
you've got one

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program to see how it works
alright so let's add an else clause to our

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we're going to print a message if the
program to see how it works

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division was successful going to come
we're going to print a message if the

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down here after the code on line 23
division was successful going to come

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we're going to add an else on line 24
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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....okay
...

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so I run this again this time lets enter a
....okay

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couple of values so...we
so I run this again this time lets enter a

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get 90 / 45 is 2.0 and division
couple of values so...we

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performed successfully so if we run it
get 90 / 45 is 2.0 and division

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again
performed successfully so if we run it

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90 this time but 0 for the second
again

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number you can't divide by 0
90 this time but 0 for the second

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and this time we haven't got the message
number you can't divide by 0

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division perform successfully on screen
and this time we haven't got the message

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so how this is different to finally well
division perform successfully on screen

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the code in the else clause only execute
so how this is different to finally well

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if the triblock completed without raising
the code in the else clause only execute

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an exception
if the triblock completed without raising

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so when we attempted to divide by zero
an exception

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else code didn't execute if i change
so when we attempted to divide by zero

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else on line 24 now to finally and run
else code didn't execute if i change

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this again and repeat both tests so 90
else on line 24 now to finally and run

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and 45 we correctly get the answer 2.0
this again and repeat both tests so 90

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and division perform successfully run it
and 45 we correctly get the answer 2.0

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again this time 90 and 0 as a
and division perform successfully run it

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second number and we get the message
again this time 90 and 0 as a

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showing this time division performed
second number and we get the message

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successfully so finally is always executed
showing this time division performed

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even if there's an exception or as you saw
successfully so finally is always executed

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earlier even if the program is
even if there's an exception or as you saw

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terminated in the try block so code in
earlier even if the program is

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the else clause only executed if the try
terminated in the try block so code in

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completes it won't be executed if
the else clause only executed if the try

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there's an exception or if the trial
completes it won't be executed if

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code returns or quits the program now
there's an exception or if the trial

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you might be wondering this point why we
code returns or quits the program now

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didn't just put the division perform
you might be wondering this point why we

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successfully message after the code on
didn't just put the division perform

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line 20 in other words after we printed
successfully message after the code on

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out the result in this case we could
line 20 in other words after we printed

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have done just that
out the result in this case we could

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that's because our print statement can't
have done just that

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raise an exception so there's little
that's because our print statement can't

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point using else clause here else is
raise an exception so there's little

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really intended so that we don't
point using else clause here else is

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accidentally catch an exception that
really intended so that we don't

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wasn't raised by the code we're trying
accidentally catch an exception that

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to protect with the try block so going
wasn't raised by the code we're trying

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back to our bank account example if you
to protect with the try block so going

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were protecting the updating of two
back to our bank account example if you

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transactions using a triblock would only
were protecting the updating of two

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want the code to roll back if there was
transactions using a triblock would only

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an error saving the values to the two
want the code to roll back if there was

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accounts if we add
an error saving the values to the two

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a load more code inside the try block
accounts if we add

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and that code raises an exception then
a load more code inside the try block

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the transactions will be rolled back
and that code raises an exception then

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even if they completed successfully and
the transactions will be rolled back

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will see that in a video coming up soon
even if they completed successfully and

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where it makes more sense to use an else
will see that in a video coming up soon

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clause so this last example is a bit
where it makes more sense to use an else

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contrived to demonstrate really how else
clause so this last example is a bit

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and finally work and the difference
contrived to demonstrate really how else

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between them
and finally work and the difference

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now there's one last thing that this
between them

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program is useful to demonstrate before
now there's one last thing that this

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we abandoned it and move on to something
program is useful to demonstrate before

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else
we abandoned it and move on to something

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now exceptions like everything else in
else

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python are objects in python three
now exceptions like everything else in

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they must be derived from the base
python are objects in python three

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exception class or one of its subclasses
they must be derived from the base

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that wasn't the case in python two
exception class or one of its subclasses

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although it was recommended and that's
that wasn't the case in python two

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also mentioned at the start of the
although it was recommended and that's

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document we're looking at the browser
also mentioned at the start of the

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earlier you can see here in Python
document we're looking at the browser

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all exceptions must be instance of a class
earlier you can see here in Python

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instances of a class that derives from
all exceptions must be instance of a class

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base exception so most exceptions will
instances of a class that derives from

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be subclasses of exception rather than
base exception so most exceptions will

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base exception which is the
be subclasses of exception rather than

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recommendation in the fourth paragraph
base exception which is the

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on that page programmers are encouraged to
recommendation in the fourth paragraph

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derive new exceptions from the exception
on that page programmers are encouraged to

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class or one of its subclasses and not
derive new exceptions from the exception

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from base exception so why is that
class or one of its subclasses and not

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important well the first reason is that
from base exception so why is that

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you can catch exception in your except
important well the first reason is that

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Clause to catch any exception that's
you can catch exception in your except

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derived from it so let's actually have a
Clause to catch any exception that's

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look at how that works in our code
derived from it so let's actually have a

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so go back to the IDE so on line
look at how that works in our code

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nine we've got this line except
so go back to the IDE so on line

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value error now we can
nine we've got this line except

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check that value area is a subclass of
value error now we can

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exception by holding down the control
check that value area is a subclass of

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key or on in my case on a Mac the
exception by holding down the control

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command key clicking on it to see the
key or on in my case on a Mac the

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code you can see their class value error
command key clicking on it to see the

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exception then click on that and have a
code you can see their class value error

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look
exception then click on that and have a

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so you can see here that value error
look

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inherits from exception and if we click
so you can see here that value error

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on exception
inherits from exception and if we click

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you can see that exception inherits from
on exception

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base exception so what that means is we
you can see that exception inherits from

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can come back to our code gonna
base exception so what that means is we

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close it down means that we can actually
can come back to our code gonna

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change this on line 9 and replace that
close it down means that we can actually

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with exception and if we run this we
change this on line 9 and replace that

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should find that still copes
with exception and if we run this we

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with invalid input 90 and 0
should find that still copes

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just bogus number valid number and then 0
with invalid input 90 and 0

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so still coping and basically
just bogus number valid number and then 0

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catching those exceptions
so still coping and basically

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now with that said I'm not recommending that
catching those exceptions

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you do this or suggesting you do that
now with that said I'm not recommending that

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it's generally a good idea to be
you do this or suggesting you do that

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specific in the exceptions you catch
it's generally a good idea to be

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rather than just catching everything one
specific in the exceptions you catch

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reason for being specific is that you
rather than just catching everything one

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can handle the different exceptions
reason for being specific is that you

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differently now if you can just catch
can handle the different exceptions

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all exceptions as we're doing here
differently now if you can just catch

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the code will react in the same way
all exceptions as we're doing here

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regardless of what exception was raised
the code will react in the same way

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that may often not be the appropriate
regardless of what exception was raised

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way to react and in fact we've got a
that may often not be the appropriate

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problem with our code now so it looks
way to react and in fact we've got a

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like it worked fine until you understand
problem with our code now so it looks

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this next bit you probably won't see
like it worked fine until you understand

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anything wrong by looking at the code now
this next bit you probably won't see

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IntelliJ is aware that
anything wrong by looking at the code now

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something is not quite right
 IntelliJ is aware that

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we actually got a warning on line 11
something is not quite right

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that's where we catch the end of file
we actually got a warning on line 11

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error you can see the exception
that's where we catch the end of file

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superclass of exception class EOF error
error you can see the exception

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has already been caught so let's
superclass of exception class EOF error

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actually see the effect of that before
has already been caught so let's

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talking anymore about it i'm going to
actually see the effect of that before

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run it again going to do the ctrl D
talking anymore about it i'm going to

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command D on the mac you can see what's
run it again going to do the ctrl D

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happening here we're going into a loop
command D on the mac you can see what's

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our programs running wild again i'm
happening here we're going into a loop

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going to stop it and cancel out of that
our programs running wild again i'm

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the question at this point is what went
going to stop it and cancel out of that

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wrong
the question at this point is what went

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remember when I said that the order that
wrong

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you handle the exceptions is important
remember when I said that the order that

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what's happening here with our code is
you handle the exceptions is important

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that exception is a subclass of EOF
what's happening here with our code is

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error now subclass of a base class is
that exception is a subclass of EOF

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also an instance of its base class so
error now subclass of a base class is

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this case EOF error is an
also an instance of its base class so

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instance of exception and when we catch
this case EOF error is an

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exception on line 9 we're
instance of exception and when we catch

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actually catching all exceptions that
exception on line 9 we're

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inherit from it including the eof error
actually catching all exceptions that

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exception that has been defined on line
inherit from it including the eof error

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11 so what that means is that the code
exception that has been defined on line

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that runs when eof error is raised is the
11 so what that means is that the code

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actual code on line 10 so therefore the
that runs when eof error is raised is the

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program doesn't get to exit it just
actual code on line 10 so therefore the

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prints a message and loops or allows the
program doesn't get to exit it just

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loop to go around again and again and
prints a message and loops or allows the

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again because the input streams closed
loop to go around again and again and

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and we keep getting these
again because the input streams closed

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eof errors raised by line 7 IntelliJ is
and we keep getting these

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aware of this and that's why its giving
eof errors raised by line 7 IntelliJ is

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us a warning
aware of this and that's why its giving

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so when you're handling multiple
us a warning

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exceptions like we are here it is very
so when you're handling multiple

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important that you catch them in the
exceptions like we are here it is very

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right order
important that you catch them in the

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so the more specific exceptions have to
right order

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appear first so you can fix this program
so the more specific exceptions have to

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up by moving lines 11 and 12 up until
appear first so you can fix this program

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before we catch the exception on line 9
up by moving lines 11 and 12 up until

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so I'm gonna just copy those cut
before we catch the exception on line 9

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them out and pasted above the code that
so I'm gonna just copy those cut

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was on line 9 and if you actually run
them out and pasted above the code that

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this and enter some bogus text that isn't a
was on line 9 and if you actually run

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number here we get invalid number and
this and enter some bogus text that isn't a

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now if I come back here again and do a
number here we get invalid number and

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command on a Mac ctrl D on windows or
now if I come back here again and do a

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linux we actually get it to exit
command on a Mac ctrl D on windows or

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correctly with exit code one so you can see
linux we actually get it to exit

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the code that we generated for the
correctly with exit code one so you can see

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exception eof error is now being
the code that we generated for the

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executed correctly when we do the
exception eof error is now being

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control d e so it's not often
executed correctly when we do the

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that you specify exception as the
control d e so it's not often

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exception to handle your except Clause
that you specify exception as the

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but this applies whether you're catching
exception to handle your except Clause

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multiple exceptions and some of the ones
but this applies whether you're catching

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you want to catch derive from others and
multiple exceptions and some of the ones

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by the way we are getting this other error here
you want to catch derive from others and

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about two broad exception clause this
by the way we are getting this other error here

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warning is that perhaps we should be
about two broad exception clause this

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more specific
warning is that perhaps we should be

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alright so the last thing i want to
more specific

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mention is related to this now remember
alright so the last thing i want to

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from the documentation that not all
mention is related to this now remember

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exceptions have to be derived from
from the documentation that not all

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exception the requirement is that they
exceptions have to be derived from

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00:12:32.900 --> 00:12:35.300
derived from base exception or one of
exception the requirement is that they

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00:12:35.300 --> 00:12:37.820
its subclasses so if you really do want
derived from base exception or one of

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00:12:37.820 --> 00:12:40.490
to catch any exception then there's a
its subclasses so if you really do want

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way to specify the sort of wild card
to catch any exception then there's a

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exception meaning everything to do that
way to specify the sort of wild card

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00:12:45.500 --> 00:12:47.990
just leave off the exception type so
exception meaning everything to do that

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00:12:47.990 --> 00:12:49.910
the except Clause is just the keyword
just leave off the exception type so

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accept and a colon so this example is
the except Clause is just the keyword

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going to delete the exception on line 11
accept and a colon so this example is

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and just leave the colon there before I
going to delete the exception on line 11

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00:12:59.150 --> 00:13:01.340
do this just to be clear I'm going to
and just leave the colon there before I

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00:13:01.340 --> 00:13:03.650
change the finally here on line 24 to an else
do this just to be clear I'm going to

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00:13:03.650 --> 00:13:04.160
again
change the finally here on line 24 to an else

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00:13:04.160 --> 00:13:07.190
so that when I make the code
again

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00:13:07.190 --> 00:13:08.870
available in the resources it's showing
so that when I make the code

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00:13:08.870 --> 00:13:10.340
how it should be done rather
available in the resources it's showing

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00:13:10.340 --> 00:13:12.860
demonstrating how not to do it right the
how it should be done rather

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00:13:12.860 --> 00:13:14.540
other thing I'm going to do here is on
demonstrating how not to do it right the

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00:13:14.540 --> 00:13:17.060
line 11 is just to make it clear here
other thing I'm going to do here is on

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with a comment
line 11 is just to make it clear here

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00:13:18.180 --> 00:13:23.370
we should really be except...
with a comment

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00:13:23.370 --> 00:13:27.990
...
we should really be except...

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00:13:27.990 --> 00:13:29.490
alright let's test the program a few
...

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00:13:29.490 --> 00:13:33.029
times let's try a number or partial
alright let's test the program a few

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00:13:33.029 --> 00:13:34.770
numbers i will test for invalid input
times let's try a number or partial

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00:13:34.770 --> 00:13:39.240
using numbers and letters invalid number
numbers i will test for invalid input

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00:13:39.240 --> 00:13:43.380
will try just letters that works and try
using numbers and letters invalid number

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00:13:43.380 --> 00:13:52.050
76.....that works ok running
will try just letters that works and try

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00:13:52.050 --> 00:13:56.040
again and again we're getting the errors
76.....that works ok running

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00:13:56.040 --> 00:13:58.350
caught successfully invalid numbers by
again and again we're getting the errors

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00:13:58.350 --> 00:14:00.660
the Wildcard accept on line 11 so that's
caught successfully invalid numbers by

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00:14:00.660 --> 00:14:03.089
good right second time lets do a divided
the Wildcard accept on line 11 so that's

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00:14:03.089 --> 00:14:07.500
by 0 so 32 and 0 the finally clause always
good right second time lets do a divided

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00:14:07.500 --> 00:14:09.899
execute and you can't divide by 0 and
by 0 so 32 and 0 the finally clause always

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00:14:09.899 --> 00:14:11.850
we done a assist.exit 2 as you can see
execute and you can't divide by 0 and

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00:14:11.850 --> 00:14:14.730
there the code on line 23 was executed and
we done a assist.exit 2 as you can see

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00:14:14.730 --> 00:14:17.490
let's do the control d check again run
there the code on line 23 was executed and

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00:14:17.490 --> 00:14:20.820
that and do a control command D on a mac
let's do the control d check again run

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00:14:20.820 --> 00:14:22.560
and control d on a windows
that and do a control command D on a mac

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00:14:22.560 --> 00:14:25.380
or linux and the finally clause
and control d on a windows

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00:14:25.380 --> 00:14:27.390
always execute and we're finishing the
or linux and the finally clause

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00:14:27.390 --> 00:14:30.540
code on line 10 is executed
always execute and we're finishing the

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00:14:30.540 --> 00:14:31.320
correctly
code on line 10 is executed

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00:14:31.320 --> 00:14:33.480
so everything's working so
correctly

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00:14:33.480 --> 00:14:35.279
that's exceptions and how to handle them
so everything's working so

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00:14:35.279 --> 00:14:37.650
in python so in the next video we can
that's exceptions and how to handle them

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00:14:37.650 --> 00:14:38.940
have a quick look at how to raise our
in python so in the next video we can

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00:14:38.940 --> 00:14:41.880
own exceptions before we ultimately get
have a quick look at how to raise our

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00:14:41.880 --> 00:14:43.890
back to databases so see you as
own exceptions before we ultimately get

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00:14:43.890 --> 00:14:50.000
always in the next video
back to databases so see you as