WEBVTT

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so just as a recap from the previous
video when we run this we do get an error

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and looking at the error we can see objects has no attribute power and the

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error is on line 45 which you can
see on the screen

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and the reason for that is the Hamilton
instance doesn't have a power

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attribute because we haven't created one by
assigning a value to it like we did for

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the Kenwood instance on line 43 and this is
the dynamic nature of Python that

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allows this kind of behaviour and you can
easily end up with instances that are

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created from the same class
template but which ultimately have

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different attributes and it can be
useful feature but can also cause

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problems if you make a typing error when
trying to assign a value to an existing

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data attribute for argument sake so it's
worth paying careful attention to the

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IntelliJ tool tips that will appear after
you type a do and to make sure that

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you're assigning it to attribute that you really expecting it to be so

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going back to the analogy of a class
definition being like house plan that

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can be used to create houses in the real
world theirs nothing to stop an extension

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being added to a house out after its built and
that's pretty much what is happening

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here now of course subclassing which is
where a new class is created from an

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existing one may be preferable to adding
attributes to instances and will be

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looking at sub classes a little later
there are ways to prevent this kind of

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behaviour and you can create classes in
such a way that additional

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attributes can't be added to instances
forcing classes to be sub class extra

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functionality is required but Python allow you to take either approach in now their is one aspect of

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classes that I want to mention before we
start practicing all this by creating

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some more useful classes now I introduced
to small talk term instance variable in

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the previous video and that's because it
includes the words variable whereas data

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attribute does not and this is important
because methods are also attributes of

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classes and you find the term attribute used to
refer to both in the documentation

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so the terms data attribute and method are
used to distinguish between the two

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types of attribute now the other aspect that I want to mention is that classes also

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have attributes and once again the
Smalltalk term instance variable is

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useful because it contains the word
instance now the data attributes in the

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kettle example as such as make and price have both been attributes of the instances and each

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instance has its own values for them so its
also possible for the class to have

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attributes which is shared by all the
instances so to stick with the analogy

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of a house plan each house that we built from the
plan will have different attributes such

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as its address however all the houses will share some attributes such as the type of

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house now all house are built from a plan for a four-bedroom lodge would be four

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bedroom lodges so our kettle class is modelling electric kettles so we can

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introduce a class attribute called power source
that all instances will share

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so lets go ahead and do that so we are gonna go back up to
the top of the definition for the kettle

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class and on line 32 I'm going to add....so

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that's a class attribute that we've added called power source

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so now we've done that we can demonstrate that all instances share a single version of this class attribute so we are gonna add the following

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at the end of the file but first I'm going to comment out line 47 to remove that error so we

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can try doing...

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...

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.....now if you run that...you could see that all three have got

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electricity as the power source so the two
instances kenwood and hamilton plus the class

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itself kettle have got this power source class
attribute so think of that if you know

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Java or C++ this is very similar to
static fields in those languages similar

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but not exactly the same but although its a
useful comparison if you are used to

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those other languages don't take this to
literally it is useful to examine the

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namespaces of the three objects to
verify that the two instances are

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sharing the same attribute which only
exists in the class and we can access the

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namespace via the dic attribute so lets go ahead and do that so...

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....

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....and

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I'm going to change or run it first move this as a
bit more space I'm gonna move to bottom we can see

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its a little bit better and looking at that
output down the bottom of the screen the

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kettle class namespace contains quite a
lot of items and we can see power

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source showing their and we can see some of the other functions we

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can see an init function and we
should be able to see our switch on

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method if we scroll over and their is the switch on

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kettle.switch_on so that's their and the 2
instances below that you can see that

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they've got far less in their namespace and in fact theirs only
the instance variables make price and on

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and you see Kenwood has got the extra power
attribute that's the second from

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bottom one that we added earlier in this
code but other than that

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their the same so what happens is that
when we try to access the power_source

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attribute for the instances Python checks
to see if the power source exists in the

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instance name space if it doesn't which
is the case here it then checks the class

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for the instance and finds power source
in the kettle class and that's why the

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reason it printed out because basically
it got it from the class attribute so if

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you close it down if you go ahead and make a
change with the power source if we do something like this

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so....

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....

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so if we run this again you can see what happens there is that atomic we are switching to atomic power and the three

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printouts are now all the same again as well and only updated the class attribute

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but it has changed the other two instances one automatically which is more or less sort of proof that the

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instances for Hamilton and kenwood are
looking at the class attribute at that

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point and this is probably not
surprising if Python is your first

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experience of object oriented
programming and will probably make

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perfect sense after our discussion of global
variables and functions that we've talk

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about previously so as long as we only access the class
data attribute via the class rather than via

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instance then when we want to assign a new value to it this works as expected so all 3 objects are still sharing their

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common power source in this scenario and
show the same value in this case atomic

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but that's not true if you try and
change this in different ways so if we

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come down to say the kenwood instance below their will do....

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...

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...so if we run that now...you can see in the bottom here we've got atomic for the class attribute we change kenwood to gas and that is showing

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as gas and Hamilton still showing
atomic

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and the class attribute is still showing
as atomic so this is probably not that

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surprising if Python is your first experience
of object-oriented programming and would

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probably make perfect sense after our
discussion in previous videos about

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global variables and functions remember
that as soon as we try to assign a value

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to a global variable

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Python created a new local variable that
shattered the global one so a

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similar thing is actually happening in
this scenario when we added the code to

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switch the Kenwood power source to gas the code on line 53 so although kenwood

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is now running on gas hamilton's is still
running on atomic power which is one of

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the ways that class attributes differ
from Java static fields and examining

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the source if we run this again and
look at the namespaces notice now that Kenwood second to

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bottom one has now got its own data attribute
called power_source that

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shadows the class attribute so this is
definitely something to watch out for as

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it's quite easy to make the mistake of
assigning a new value to a class attribute

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via an instance variable and Java will give you a warning if you try to access

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as a static field via an instance variable
but you won't get any such warnings in

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Python so that's the basic of classes
and their is a quite a bit of jargon and new concepts that we've thrown in

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there and it may not make complete sense
at the moment but don't worry because I'm

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gonna be using a lot of examples in the
next set of videos that will consolidate

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what's been covered so far a simple kettle class doesn't really do very much

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but as we create more class and using them to
encapsulate data and functionality it'll

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start making a lot more sense now
also we've introduced other object-oriented

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concepts such as inheritance and
composition so that we can build more

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complicated class structures and the
other thing I do is also introduce other

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object-oriented concepts such as
inheritance and composition so that we

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can build more complicated class
structures so we'll start off slow in

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the next few videos will look at some
more examples of using classes see you

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in the next video