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an important part of object-oriented
programming is the concept of

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encapsulation so the idea here is that
objects contain the data and the methods

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that operate on that data and don't
expose the actual implementation to the

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outside world so it's worth noting that
object oriented programming is not the

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only way to achieve this and
languages such as modular 2 aimed to

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provide encapsulation by using modules rather
than objects modular 2 enhance to

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become modular 3 which was really quite influential in the design

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of Java, C++ and also Python although the
object approach to encapsulation was

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the only one adopted by Java Python allows us to encapsulate data methods

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using either an object orientated
paradigm or a modular approach and we

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saw the modular approach when using the
pytz module to handle time zone

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information earlier in this course so
you don't have to know how pytz was

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implemented in order to use it

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the method that provides and the time zone information it uses are encapsulated

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within the pytz module in fact pytz also
uses classes internally so it's possible

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to mix both methods quite happily now
I repeatedly pointed out that everything

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in Python is an object so it may seem
strange that a language that is really

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so heavily based on objects doesn't restrict
us to using an object approach this is

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one of the important design decisions of
the Python language the basic principle

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being that we're all responsible adults
at least when programming now I mention this

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because many languages work by imposing
restrictions on what can be done with

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encapsulated data Java and C++ for
example restrict access to class fields

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by allowing them to be marked as private
in Python you can indicate that a

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method or attribute should be
considered non public but it's extremely

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difficult to fully enforce this once
you've been introduced to the idea of

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encapsulation it can be tempting to
spend a lot of time trying to enforce it

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in Python

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especially if he used 2 other languages
such as Java that do a reasonably good

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job of preventing access to private data
but you be more productive if you accept

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that Python takes a different approach

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neither approach is necessarily better they are just different and each has its own

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advantages and disadvantages so let's
continue our introduction to classes in

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Python with another example we are going to look at
the concept of a bank account and

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the classes are going to enable money to be
deposited withdrawn and will hopefully

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anyway maintain the correct balance for
the account so I'm going to use the same

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file we use previously but I'm just going to create a new
Python file so we are going call this one

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accounts....so
let's start first by creating the class

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so we are gonna call the class account so.... and it's going to be a simple

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....

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....

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....and incidentally the convention is to
start class names with a capital letter and to use

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camel case being that each time you
got a different word you capitalized

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just that word so if we're calling this the account number class we would type something like that

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capitalize each

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word but basically start with a capital letter so that is the convention in Python so

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ok will continue on with our init method so we want to passed name and balance to

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it and we are going to assign the attributes....

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.....

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....

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...and we are going to just check the amount that is passed

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to make sure that its greater than 0...and only add it to our balance if its greater than 0 and we will add the amount to what

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the current balance was like so....next
let's do a withdraw method....

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...

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....

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.....ok so there's our class
and as I just mentioned the convention is to

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start class names with a capital letter and
and to use camel case with that said you'll find

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many the built-in classes in Python do
use lowercase nevertheless IntelliJ will

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give you a warning if you use lowercase or
separate paths of the class name with

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underscores...another warning as you can see should only use case convention it's not

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happy if we use underscores now I've included a dot string on line 2 at the start of the class and it's a good idea to do

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this with all your classes although it can appear to be
a bit redundant with simple classes like

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this one which he can really figure out
the functionality just by glancing it's

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a good habit to get into and once your class structures get more complicated

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you will be glad that you did do this and so
will anyone else who uses your classes

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and I'll be talking more about doc strings later when I show you what they used for in

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more detail now the init method as you recall is used to initialize the

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class and it's extremely rare to create
a class without providing an init method

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now in an earlier video I used the term
constructor to described the init method but

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really that's not strictly true in the
new style Python classes that will be

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using for most of this section the Class
creation process involves two steps so

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the first method to be called when a
class instance is created is new and this

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takes care of the actual creation the
init method then customizes the instance

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performing tasks such as giving
values to the data attributes for example so

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technically the class constructor is the
new method which is actually calling

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underscore underscore new underscore
underscore as you probably expect

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generally speaking you don't need to
define new except in special cases

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when subclassing certain types of
classes so I won't be discussing you

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much in this section as a result now old style classes were the only type

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of class available in Python 2 up until 2.2
when the new style classes are

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introduced so in Python 2.2
onwards you could use both types of

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classes I will discuss the Python 2 classes later on

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so that your aware of the differences and
can recognize them but in Python 3

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there are only the new style
classes and as a results I'm going to be

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concentrating on those as we move
forward so getting back to the account

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class the init method takes two
parameters in addition to self and these

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are used to set the values of the data attributes name and balance now notice that all

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the methods takes self as their 1st parameter and you saw IntelliJ

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automatically adding that for us and
whenever we want to refer to the data

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attributes we have to use the form self. and whatever the name of the attribute is

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again as you see me type in this class in
Python there is no shorthand and self

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must be specified if your used to java you can omit the word this before

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a field if their references is not ambiguous
but in Python you can't do that so you need

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to get into the habit of doing that each
time I just wanna remember i just fixed that

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those warnings and add the extra space to keep IntelliJ happy
now the positive withdrawal methods

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obviously a pretty simple and they adjust
the balance by the amount passed to the parameter

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they also check that the amounts are positive
although as withdrawal its just a negative

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deposit and a deposit is just a negative
withdrawal we could have used a single

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method to handle both cases now there are
extra steps that are really sure of

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implemented in the withdrawal method to
make sure there is enough money in the

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account though so it makes sense to keep
the two transaction type separate the

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show balance method again very simple it just
prints the balance data attribute so now the

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that the classes been defined we
can create accounts with people and start

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looking out for their money so
I'll start by creating account for myself

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of course with a zero balance and then deposit
withdraw some cash let's go ahead and do

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that

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....

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.....and lets

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run this to make sure its workings account
created for Tim the original balance was

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0 which would expect balances is a thousand
when we deposited 1000 and we've

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withdrawn 500 and shows a balance of 500
so that's working fine

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so this is a very simple example as I said and theirs plenty of room for improvement

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now rather than having to call the show
balance method after each transaction

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will probably be helpful if the balance if
the deposit and withdrawal methods

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did this for us automatically I will also reluctantly
fix the problem that lets me take as much

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money as I want regardless of the actual
balance so for both methods I'm just gonna

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call the show balance method after
adjusting the balance preventing

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unlimited withdrawals easily done just to
confirm the way the withdrawal method is

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written at the moment I could quite
happily withdraw 50,000 and it would

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work all be with a $49,000 overdrawn balance
but the point is it would work and we need

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to restrict that so that when we are
withdrawing money we can only withdraw the

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amount to the maximum amount we've got currently in the
account so lets go ahead and do that so

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we are going to add to the deposit method so.....

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...

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....

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....

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....

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....so we'll leave the
initial show balance call on line 27

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but will comment out the other one because they can be called automatically

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when we call those methods and lets put a valid withdrawal if we went back into 500

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then we will put a invalid one in.....so lets run this one

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you can see balance is 0 account was created for

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Tim balance is 0

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balance is 1000 it is calling the show balance method
now as we are calling the deposit

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and withdrawal methods balance is deducted to 500 and we got a message

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here when we called line 34 when line 34 was executed the amount must be

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greater than 0 no more than your account
balance and it also reprinted the

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balance you can see the balance is still
the same as it was when that method

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was called so this seems to be working
fine and it does show that you can

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call methods from other methods as I
mentioned earlier that you must prefix

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all attributes with self and that applies to method calls in this class and you

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probably saw when I was typing one of the
show balance methods that I forgot to put the self their and

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IntelliJ was flagging as an error so
this may catch Java programmers out for

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a while and it shouldn't be a huge problem
because as you saw IntelliJ will let you

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know as soon as you tried to use attribute or a
method call within a method without using

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self again just to confirm what I'm talking
back we can delete that we then get an error un-resolve

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reference so we have to use self and likewise for the attributes we get an error

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unless you specifically do it so It will probably
catch you out if your coming from another

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language such as Java but after you

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done a bit of Python coding
it becomes pretty straightforward and

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again IntelliJ is helping you by flagging the error so you need to fix the

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errors to run the program so sooner or
later you'll figure it out anyway but

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with that said there is a fundamental problem
with the code for this account class and

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although fixing it is easy their more
than one way to do so

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I'm gonna cover that later but for now I'm
gonna see if you can spot the problem

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until we get to that section where our
I will show you how to get around it

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so we'll start working on that in the
next video