WEBVTT

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as I mentioned towards the end of the

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last video you can also create your own
as I mentioned towards the end of the

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exceptions their just objects after all
last video you can also create your own

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so we can provide more immediate
exceptions their just objects after all

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feedback to the developers using our duck
so we can provide more immediate

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module so let's go back and have a look
feedback to the developers using our duck

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at the add_duck method you
module so let's go back and have a look

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can see that their on line 50 now we
at the add_duck method you

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provided a hint that it should be
can see that their on line 50 now we

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passing a duck or subclass of duck now
provided a hint that it should be

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if we extended the duck class to create
passing a duck or subclass of duck now

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a mallard class perhaps then a mallard is
if we extended the duck class to create

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also a duck but we can do better than
a mallard class perhaps then a mallard is

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this we can check the type of the
also a duck but we can do better than

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argument that's past and show you how to
this we can check the type of the

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do that so we're going to type in here
argument that's past and show you how to

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in the add duck method...
do that so we're going to type in here

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...
in the add duck method...

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...if we go
...

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back to our file again and run it this
...if we go

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time we get all seven ducks taking off
back to our file again and run it this

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and there's no exception
time we get all seven ducks taking off

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now that's because Percy isn't a dark so
and there's no exception

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this test on line 51 fails and Percy
now that's because Percy isn't a dark so

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doesn't get added to the list and
this test on line 51 fails and Percy

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incidentally make sure you use the
doesn't get added to the list and

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lowercase D in the first duck this part
incidentally make sure you use the

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here on line 51 if you used a capital D
lowercase D in the first duck this part

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there then you checking the type of the
here on line 51 if you used a capital D

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duck class which is a class so the
there then you checking the type of the

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type function returns the type
duck class which is a class so the

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of an object so here we are testing that the
 type function returns the type

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duck that's been passed to our method
of an object so here we are testing that the

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really is a duck never do this
duck that's been passed to our method

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now not only is this is not python
really is a duck never do this

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it's also wrong
now not only is this is not python

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now it's an obvious thing to do and if
 it's also wrong

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you used to programming in java you
now it's an obvious thing to do and if

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probably expected to work but to see why
you used to programming in java you

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it doesn't we're going to create a mallard
probably expected to work but to see why

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class that extends duck i'm just going
it doesn't we're going to create a mallard

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to put it just after the penguin
class that extends duck i'm just going

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class....
to put it just after the penguin

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...
class....

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....if we were really
...

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creating a mallard then you add some of
....if we were really

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the properties of a mallard but here we're
creating a mallard then you add some of

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just demonstrating that the previous
the properties of a mallard but here we're

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code fails with sub classes so going back
just demonstrating that the previous

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to the migration .py file we need to
code fails with sub classes so going back

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change your definition and will actually
to the migration .py file we need to

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change for percy here and we'll put
change your definition and will actually

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Percy equals duck . mallard this time
change for percy here and we'll put

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let's run this and you can see that we
Percy equals duck . mallard this time

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still only get the seven ducks flying
let's run this and you can see that we

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so this type function that we're talking
still only get the seven ducks flying

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about now on line 55 returns the exact
so this type function that we're talking

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type of an object and we check to see if
about now on line 55 returns the exact

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that type is duck which isn't is mallard
type of an object and we check to see if

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in this case of the one that we set up
that type is duck which isn't is mallard

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for percy here on line 11 now if
in this case of the one that we set up

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you want to do this there are
for percy here on line 11 now if

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occasionally times when you need to then
you want to do this there are

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use is instance instead of checking the
occasionally times when you need to then

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type so let me change this duck method
use is instance instead of checking the

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now so instead of type...
type so let me change this duck method

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...
now so instead of type...

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....first one is the lowercase
...

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version for the variable the second one
....first one is the lowercase

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is the actual class name and if we do
version for the variable the second one

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that now that we've done that if we
is the actual class name and if we do

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go back to our code run it again and what I
that now that we've done that if we

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should have done is remove this last
go back to our code run it again and what I

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part so all we really want to do at this
should have done is remove this last

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point is if is instance and then in
part so all we really want to do at this

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parentheses duck , duck now if i try
point is if is instance and then in

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run that this time you can see
parentheses duck , duck now if i try

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we've actually got all 8 ducks in the
run that this time you can see

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air
we've actually got all 8 ducks in the

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the reason for that is this is
air

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is instance function so again if you must
the reason for that is this is

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do this type of checking always uses this is
is instance function so again if you must

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instance or there's also is instance
do this type of checking always uses this is

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method if you're dealing with classes
instance or there's also is instance

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rather than instances of classes and
method if you're dealing with classes

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don't check the type like we did earlier
rather than instances of classes and

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now keep in mind though it's still not
don't check the type like we did earlier

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pythonic though in Python remember we're
now keep in mind though it's still not

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not interested in whether percy is a
pythonic though in Python remember we're

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duck or not
not interested in whether percy is a

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we're only interested in wether percy
duck or not

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can fly if it flies like a duck it's a
we're only interested in wether percy

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duck now checking like this again
can fly if it flies like a duck it's a

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is sometimes appropriate if your
duck now checking like this again

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function or method requires and int for
is sometimes appropriate if your

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example it does make sense to check if
function or method requires and int for

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you've got an int or subclass of int and
example it does make sense to check if

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yes in Python you can subclassing if
you've got an int or subclass of int and

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you want to it wouldn't be practical to
yes in Python you can subclassing if

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check all the behavior of an argument to
you want to it wouldn't be practical to

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see if it quacks like an int but when
check all the behavior of an argument to

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dealing with the most other objects the
see if it quacks like an int but when

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pythonic way is to focus on the behavior
dealing with the most other objects the

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you want not on the type of the object so
pythonic way is to focus on the behavior

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what we really want to do is check to
you want not on the type of the object so

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see if the arguments gotta fly method if
what we really want to do is check to

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it can fly then we can add it to the
see if the arguments gotta fly method if

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flock so there's two steps to this first
it can fly then we can add it to the

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what we want to do is check if there's
flock so there's two steps to this first

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an attribute called fly the next we
what we want to do is check if there's

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check to see that this is a method and
an attribute called fly the next we

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not a data attribute so let's add the
check to see that this is a method and

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code for that instead
not a data attribute so let's add the

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so again we're doing this code on that
code for that instead

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line 55 going to change that line again
so again we're doing this code on that

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so the first line of the add_
line 55 going to change that line again

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duck method we're going to put....
so the first line of the add_

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...
duck method we're going to put....

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...
...

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....so the get
...

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attributed method checks an objects
....so the get

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dictionary to see if it contains the
attributed method checks an objects

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attribute we specify fly in this case
dictionary to see if it contains the

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now if the attribute wasn't present
attribute we specify fly in this case

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would get an error so it works very
now if the attribute wasn't present

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like a dictionaries get method by
would get an error so it works very

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specifying a default value we get none
like a dictionaries get method by

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returned instead of the error
specifying a default value we get none

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next we're using this callable
returned instead of the error

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function to check if the fly method is
next we're using this callable

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callable now functions and methods are
function to check if the fly method is

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callable data attributes aren't now that's
callable now functions and methods are

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not strictly true because you can't have
callable data attributes aren't now that's

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a callable data attribute in python but
not strictly true because you can't have

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if we call the fly attribute
a callable data attribute in python but

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then our code will work so this test is
if we call the fly attribute

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perfectly fine and just to confirm if we
then our code will work so this test is

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go back and run it again we are getting
perfectly fine and just to confirm if we

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all 8 ducks flying
go back and run it again we are getting

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alright so let's change our percy now
all 8 ducks flying

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back to being a penguin again and to
alright so let's change our percy now

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confirm this time you run it again we
back to being a penguin again and to

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now getting seven ducks taking off
confirm this time you run it again we

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so clearly the fact that we have
now getting seven ducks taking off

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change percy back to penguin means that
so clearly the fact that we have

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the code that we've defined here is
change percy back to penguin means that

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correctly figuring out that the penguin
the code that we've defined here is

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class hasn't got a fly method and
correctly figuring out that the penguin

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not trying to call it and and
class hasn't got a fly method and

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consequently not trying to add it to our
not trying to call it and and

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list so now that we've done that this is
consequently not trying to add it to our

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the pythonic way to do the test we're
list so now that we've done that this is

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not interested in the type of the duck
the pythonic way to do the test we're

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argument we are only interested in whether it
not interested in the type of the duck

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can fly or not
argument we are only interested in whether it

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now for our purposes if it flies like a duck
can fly or not

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then its a duck
now for our purposes if it flies like a duck

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now sometimes this will be an
then its a duck

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appropriate solution as it is often you
now sometimes this will be an

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want to notify the caller that
appropriate solution as it is often you

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something's going wrong and the Penguins
want to notify the caller that

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been excluded from the flock now this is
something's going wrong and the Penguins

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really a programming error the user of
been excluded from the flock now this is

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our module shouldn't be passing
really a programming error the user of

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flightless birds to a module that deals
our module shouldn't be passing

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with bird migration so this may be
flightless birds to a module that deals

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another place where raising an exception
with bird migration so this may be

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is actually appropriate let's go ahead
another place where raising an exception

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and do that so if we don't add the duck
is actually appropriate let's go ahead

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to the flocks list let's raise an
and do that so if we don't add the duck

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exception at that point we're going to
to the flocks list let's raise an

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do is put an else to the if callable
exception at that point we're going to

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...
do is put an else to the if callable

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....
...

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....
....

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....so again what we're
....

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doing here is if we don't add the duck to
....so again what we're

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the flocks list because we established
doing here is if we don't add the duck to

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that because we established that it hasn't got an
the flocks list because we established

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attribute called fly and that it's not
that because we established that it hasn't got an

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callable then we're going to raise this
attribute called fly and that it's not

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typeerror we saw earlier that there are
callable then we're going to raise this

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a lot whole load of exceptions available
typeerror we saw earlier that there are

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so you should try and choose the most
a lot whole load of exceptions available

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appropriate exception to raise here
so you should try and choose the most

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they're passing something that can't fly
appropriate exception to raise here

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so a type error is a good choice and is
they're passing something that can't fly

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also shows a valid use of the type
so a type error is a good choice and is

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function so we're not going to check what type
also shows a valid use of the type

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something is but to provide
function so we're not going to check what type

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information on the type of object that
something is but to provide

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caused the problem in Python if you find
information on the type of object that

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yourself using type to check what type
caused the problem in Python if you find

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something is then step back and consider
yourself using type to check what type

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if there's another way to achieve what
something is then step back and consider

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you're trying to do so now
if there's another way to achieve what

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I've done that we go back to migration
you're trying to do so now

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and run it again you can see we've got
I've done that we go back to migration

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an exception here cannot add duck are you
and run it again you can see we've got

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sure it's not a penguin and probably
an exception here cannot add duck are you

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got an extra space that I could fix up just to
sure it's not a penguin and probably

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be clear run again
got an extra space that I could fix up just to

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okay so we are now getting an exception
be clear run again

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raised before the migration is started
okay so we are now getting an exception

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at the time the flightless penguin is
raised before the migration is started

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added to the list now the stack trace
at the time the flightless penguin is

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here is useful and shows where the
added to the list now the stack trace

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exception was raised as well as the call
here is useful and shows where the

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that originated the error now this error
exception was raised as well as the call

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would be picked up straightaway testing
that originated the error now this error

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and the programmer can modify the code
would be picked up straightaway testing

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to remove the bug so it's really
and the programmer can modify the code

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easy to stick to the Python principle of
to remove the bug so it's really

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if it quacks like a duck while still
easy to stick to the Python principle of

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making functions and methods type safe
if it quacks like a duck while still

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for one of a better expression now
making functions and methods type safe

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there's another reason you may want to
for one of a better expression now

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raise exceptions and that's in order to
there's another reason you may want to

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test your exception handlers now to see
raise exceptions and that's in order to

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this working we're going to fix the
test your exception handlers now to see

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code so that Percy does have a fly
this working we're going to fix the

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method in fact i'm going to give him an
code so that Percy does have a fly

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aviat method instead and it'll be
method in fact i'm going to give him an

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clear why shortly
aviat method instead and it'll be

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let's go back and do that we're going to
clear why shortly

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add another method down here for the
let's go back and do that we're going to

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penguin class...
add another method down here for the

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....
penguin class...

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....
....

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now going to explain a bit more about
....

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that but first i'm going to run the
now going to explain a bit more about

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program to make sure that works let's do
that but first i'm going to run the

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that you can see that that's work fine
program to make sure that works let's do

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we've got I won the lottery and bought
that you can see that that's work fine

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a lear jet and haven't got an exception
we've got I won the lottery and bought

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anymore and going back to our code
a lear jet and haven't got an exception

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looking at the init method here on our
anymore and going back to our code

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line 35 we created a new data
looking at the init method here on our

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attribute called fly and we assign it to our
line 35 we created a new data

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reference to the aviate method and I
attribute called fly and we assign it to our

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mentioned not to include the opening and
reference to the aviate method and I

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closing parenthesis after aviate and
mentioned not to include the opening and

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noting that IntelliJ actually tried to
closing parenthesis after aviate and

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add them for us which would have
noting that IntelliJ actually tried to

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prevented this from working properly so
add them for us which would have

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by removing the parentheses we're adding
prevented this from working properly so

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just a reference to the aviate method that
by removing the parentheses we're adding

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this code here on line 36 now everything
just a reference to the aviate method that

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in python is an object even functions
this code here on line 36 now everything

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and methods so you can assign a method
in python is an object even functions

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to a variable then call that variable
and methods so you can assign a method

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self.fly in this case just like you
to a variable then call that variable

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called a method that points to now we
self.fly in this case just like you

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saw that when we created the deal_
called a method that points to now we

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_dealer and deal_player
saw that when we created the deal_

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functions and assign them to the command
_dealer and deal_player

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attribute or our tk buttons all the way back in
functions and assign them to the command

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the blackjack game in Section 11
attribute or our tk buttons all the way back in

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alright so back to raising exceptions for
the blackjack game in Section 11

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testing we've now got a working program
alright so back to raising exceptions for

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that contains exception handlers so how
testing we've now got a working program

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do we test that the exception handlers
that contains exception handlers so how

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are working correctly
do we test that the exception handlers

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well with a program like this we could
are working correctly

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do it by passing inappropriate arguments
well with a program like this we could

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to the add_duck method and
do it by passing inappropriate arguments

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we've seen that working but sometimes
to the add_duck method and

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getting real exceptions isn't that easy
we've seen that working but sometimes

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simulating something like
getting real exceptions isn't that easy

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perhaps a disk full exception that
simulating something like

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prevents writing data to a file is possible
perhaps a disk full exception that

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but very fiddly to set up
prevents writing data to a file is possible

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you may also want to respond to
but very fiddly to set up

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exceptions if the network link goes down
you may also want to respond to

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while you're downloading data and if you
exceptions if the network link goes down

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have a test team then probably they will do
while you're downloading data and if you

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things like switching off the router
have a test team then probably they will do

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while downloading to make sure your
things like switching off the router

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program kept gracefully like situations
while downloading to make sure your

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like that but before it gets to the test
program kept gracefully like situations

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team you can test your exception
like that but before it gets to the test

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handling by raising your own exception in
team you can test your exception

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the code that's protected by a try block
handling by raising your own exception in

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so going back to our migrate method of
the code that's protected by a try block

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the flock in here let's now raise an
so going back to our migrate method of

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exception inside the for loop i'm coming
the flock in here let's now raise an

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down here to the try then the ducks duck
exception inside the for loop i'm coming

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. fly and after here what we want to do is
down here to the try then the ducks duck

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you want to put raise attribute error
. fly and after here what we want to do is

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...
you want to put raise attribute error

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....
...

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....if we test this now you can see that
....

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i'm basically getting this exception
....if we test this now you can see that

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its raise after each duck flies and we
i'm basically getting this exception

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can see the one duck down message after
its raise after each duck flies and we

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each duck
can see the one duck down message after

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once all the ducks has been process the
each duck

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exception is raised and then get the
once all the ducks has been process the

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stack trace that's a useful technique
exception is raised and then get the

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for testing your exception handlers just
stack trace that's a useful technique

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raise an exception that you're handling
for testing your exception handlers just

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in the code inside the try block and
raise an exception that you're handling

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again that's this code here on line 73 now
in the code inside the try block and

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one final thing before I end this video
again that's this code here on line 73 now

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if you forget about that test exception
one final thing before I end this video

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and release your code things aren't going to
if you forget about that test exception

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go well so what you can do is get
and release your code things aren't going to

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IntelliJ to help make sure you remember
go well so what you can do is get

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to remove the test code and the way to
IntelliJ to help make sure you remember

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do that would be to put a comment on the
to remove the test code and the way to

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line hash...
do that would be to put a comment on the

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....adding a comment that contains
line hash...

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the word to do and it's not
....adding a comment that contains

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case-sensitive causes IntelliJ to track
the word to do and it's not

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it as an item that needs to be done
case-sensitive causes IntelliJ to track

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note that the comment from the to do
it as an item that needs to be done

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here appears in a different color depending
note that the comment from the to do

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on your choice of theme it'll be
here appears in a different color depending

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probably blue or in my case it's sort of
on your choice of theme it'll be

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a yellow so that's the to do but
probably blue or in my case it's sort of

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also what we can do is we can bring up a
a yellow so that's the to do but

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tab to see all the to dos in our
also what we can do is we can bring up a

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projects i'm close down the run window
tab to see all the to dos in our

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this is to do down here that down the
projects i'm close down the run window

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bottom left click on that and that
this is to do down here that down the

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actually shows us all that to do entries
bottom left click on that and that

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and then clicking on them to be taken to
actually shows us all that to do entries

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00:15:33.400 --> 00:15:35.110
the relevant bit of code so even though
and then clicking on them to be taken to

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it's on the screen i can click on there
the relevant bit of code so even though

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00:15:36.910 --> 00:15:38.710
and it just shows on the screen pretty
it's on the screen i can click on there

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cool
and it just shows on the screen pretty

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alright so that's the end of our
cool

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00:15:41.170 --> 00:15:43.660
discussion on exceptions in python in
alright so that's the end of our

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the next video we're going to get back
discussion on exceptions in python in

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to databases but we are gonna start making
the next video we're going to get back

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use of exception handling in our code
to databases but we are gonna start making

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see you in the next video
use of exception handling in our code