WEBVTT

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so just running our application again to
see where we're at and you can see we've

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got the transaction log correctly
showing deposits and withdrawals and the

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way they were able to differentiate the
2 was by negating the amount in the

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withdrawal method so we put a minus and
whatever the amount is and consequently we

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can when we are in the transaction print out
we're actually checking to see if the

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amounts is greater than 0 if it is it's deposited
otherwise its withdrawn so that is

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how it knows how to figure out whether
it's an withdrawal or a deposit

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so looking at the transaction log in the
output if I run it again

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we can easily calculate that I had a
balance of $500 and that's only possible

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because we know that when we open the
count for myself on line 46 the account had

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a zero balance but lets open and another
account and by doing that we can see the

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problem if the initial balance isn't 0 and once we do that I'll set a mini

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challenged for you to fix the problem so what we
wanted to do is add some code down at the

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end so after the show transactions for
my balance lets create another one...

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....

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...lets just run that but before we do that one thing I

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meant to do at the start of the video is you noticed up here we got the warning here

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for IntelliJ saying it needs two lines
to blank lines and that's talking about

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the two lines above the class definition
so I'll add another line just to fix that problem

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up so after the imports normally allowed
two lines their to keep IntelliJ

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happy ok so if we run this application
now we made those code changes looking

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at the transaction log down the bottom we got a 100 deposited and 200 withdrawn so according

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to that transaction log steph is
overdrawn by a 100 and that because the show

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transactions method isn't taking into
account the initial deposit so it's time

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for that mini challenge and the mini challenge
is quite simple so just to change the class

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so that the amount used to open the account also
appears in the transaction list so no

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change needs to be made to the main
part of the program only the class itself can

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be changed so again what we're trying to
do is change the class so that the

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amount used to open the account also
appears in the transaction list so

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go away and make that changed when
you're ready to see our solution come back

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and I'll take you through....

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ok so how did you get on did you figure out how to do that
let's go and have a look at our way of fixing

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this and the change that we need to make is in the init methods so lets go back to the init on line 13 we got our attributes set on lines

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14 15 and 16 but what we wanted to do is make
a change to transaction list so instead of

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starting with an empty one what we want
to do is we want to set up and put that initial

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transaction in and we can do that easily by
within the initialization on lines

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16 we are going to change that to.....

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and what we can also do then is put a self....at that stage so now if we run it we can see

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in the case of steph now $800 deposited which was
the initial deposit a 100 deposited and in

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200 withdrawn so just as when logging the deposits and withdrawals

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the date and balanced are enclosed in
parentheses as you saw be put on line 16 doing that

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way so their added as single tuple rather than separate
items in the list now running the

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program we do get a more complete
transaction history however theirs no mention on

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the fail withdrawn of 2000 from my
account and scrolling down this on here I'm talking

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about on line 56 but anything that can
affect the balance is logged or is it

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so before finishing with the accounts
classes theirs a couple more things I like to

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cover concerning underscores the first
is that the naming of the data

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attributes for class now although we can't
prevent them from being changed by the

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main program we should really give clients an indication that they're not

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intended to be modified as that bypasses
the transaction logging so if what I mean is if I

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just closing this down

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in the case of steph here we could
there's nothing stopping us from putting

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steph....

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.....and so we
can run that as you can see now the balance

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doesn't match the transaction history so again theirs no indication that the

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client accessing this class shouldn't
directly modify the balance data

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attribute so the least that we should
do is really rename all 3 data attributes so

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that their names start with an underscore
now IntelliJ makes it easy to refactor

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the attributes when they are defined in
the init method so close that down come back

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up to init method and we just refactor this so make this
an underscore you can see its made the changes for us

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underscore balanced and just to be complete will also do
it for transaction list as well

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ok it has been changed throughout now
strangely enough on line 61 as you can see

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their IntelliJ is not actually giving us
a warning when we assign a new value to the

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balance we've got a warning when using
underscore module variables but with

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classes it is down to us to remember this
convention so the rule is attributes

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whose name start with a single
underscore are for internal use only

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there's nothing to prevent you from
messing with them but things will

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probably break if you do mess with
them so I've been using the term non public

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rather than private and that's because
private implies that it's enforced and

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the second thing is something I
mentioned in the previous section

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that applies to classes and that's the
convention of using 2 underscores at

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the start of a name now that is really
intended for use when subclassing which

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we're going to be looking at later in this section and the idea of that is to prevent name clashes

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and it causes Python to perform name
mangling

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on the attribute or method name so that its not accidentally
reused by sub class so to demonstrate

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what I'm talking about lets refactor the balanced attribute to start with two underscores

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we will do that in the init method....and run it....800

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800 deposit initially then a 100
makes 900 200 deducted balances is 700 so

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the balance agrees with the transaction
histories and what's interesting is it

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the balance attribute wasn't changed by the code
on line 61 this code here even though the

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name __balance is correct so that
strange and obviously for it not to

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change something funny is going on you know
what's what's going on why is it doing

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that well the answer is that

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Python as I mentioned Mangles the
names of class attributes both methods

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and variables that start with two
underscores and the easiest way to see whats

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going on is to print out the dic
so let's go ahead and do that so if go under the

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show balance lets do....so if we do that we can see what's going on

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so theirs couple of things to know here firstly as you can see at the start steph has an attribute called underscore underscore balance with a

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value of $200 now this data attribute was created when
we assign the value of 20o to it on line 61

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Python didn't find an attribute with that name
in steps namespace so it looked at the

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name space from the class and didn't find
it there either now as a result it

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created a new data attribute and we saw that
happening in an earlier video when we

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created a power attribute for the kenwood kettle and the second thing which is also

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the reason why python didn't find the
balance attribute is it there is a data

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attribute called account balance and again that's
got the expected value of 700

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so by prefixing the name balanced
with two underscores we are asking Python to

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perform name mangling

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and it automatically renames the attributes to start with
an underscore and the name of the classes

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as you can see and this is done behind the scenes our source code isn't

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changed now whenever we refer to the balance
attribute within the class Python mangles

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the name for us when we use the same
name outside the class well it doesn't so in

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this way the attribute is hidden so that's not accidentally changed

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now if you really wanted to and we also need to remember to be always responsible so Python won't stop us doing something that we

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really want to do so if you really want to do
that then we can change the mangled attribute

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and just to see how you go about doing that lets make a change to that so we could do something

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like here by something on line 67 so.....

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...so run that and

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you can see the balanced now has been
affected by that so this mechanism is

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really intended to prevent accidental

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accidental shadowing of attributes when
creating sub classes and although it can

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be used to indicate a sort of private access to the variables this used is

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discouraged and if you think about it
there really isn't much point using

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double underscores merely to try and force
private access it doesn't work for one

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thing and it's still possible to access
the real attribute using the form

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underscore class underscore underscore attribute name as we've done here on

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line 67 it also doesn't provide any
great indication that the attribute is

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non-public that a single underscore does
now you're unlikely to accidentally

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access a non-public attribute by
typing a lead underscore anyway so

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there's really no need to make access
even more difficult

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now if this absolutely horrifies you it's possible
it's actually possible to read and write to

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private class fields in Java and C++ in Java you
can use a technique known as reflection

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to do this it's not easy but it can be
done so Python just makes it easy to do it if

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you are certainly need to do it in both
cases it's really not something that

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you'll end up doing

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accidentally now the final comment on
this is that ending an attribute name with

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2 underscores doesn't result in the
name being magnled so names are only

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mangled if they start with two
underscores and if they end with no more than a

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single underscore so init method
for example is not mangled and we are talking about

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the init method which has got a underscore underscore init underscore underscore all right so I'm going to end

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this video here now in the next video we
gonna start talking about doc strings

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and continue our discussion on classes so see you in the next video