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Python OOP Tutorial 2: Class Variables



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hey there how's it going everybody in

the last video we learned how to create

a simple class and how to create

instances of that class we learned a lot

about instance variables which are used

for data that is unique to each instance

so instance variables are these here

that are set using the self argument

that we saw before so for example in the

employee class that we created we set

the names the email and the pay in our

Annette method and those are set for

each instance of the employee that we

create and I briefly mentioned class

variables in the last video but we

didn't go into detail and that's what

we're going to learn about in this video

so class variables are variables that

are shared among all instances of a

class

so while instance variables can be

unique for each instance like our names

and email and pay class variables should

be the same for each instance so if you

look here at our employee class what

kind of data would we want to be shared

among all employees well there's a lot

of different ideas that we could

probably come up with but for this

example let's say that our company gives

annual raises every year now the amount

can change from year to year but

whatever that amount is it's going to be

the same for all employees so that would

be a good candidate for a class variable

now before we actually create that class

variable let's first hard code this in

and see why the class variables would be

a better use case so I'm going to create

a method down here called apply raise

and remember our methods automatically

take in the instance which we are going

to call self now within this apply raise

I'm going to do a self dot pay and I'm

going to set this equal to an integer so

that we have a whole number and I'm

gonna do self dot pay times 1 point

let's just make this 4% so now if I was

to test this down here on an instance

then I can print out the employee 1 dot

pay and let me go ahead and copy this

twice and between here I'm going to do

an employee 1 dot apply raise now if I

go ahead and run this Oh actually forgot

to put in the parenthesis there so now

if I go ahead and run this

you can see that I printed out the pay

then we applied the raise and it added

4% onto our pay so we can see that it

worked but there are a couple of things

wrong here so first it would be nice if

we could access the raise amount by

doing something like employee 1 dot

raise amount or since it would apply to

the entire class we should also be able

to get the raise amount by doing

employee dot raise amount now that raise

amount attribute doesn't currently exist

so we can't see that it is 4% and also

what if I wanted to easily update that

4% amount so right now it's kind of

hidden within this method and for all I

know it could be in multiple places

throughout our code so we don't want to

have to manually go in if we wanted to

update this 4% we wouldn't want to have

to manually go in and change these and

multiple locations

so let's instead pull this 4% out into a

class variable and that's as easy as

going up here to the top of the class

and just saying that we want a raise

amount equal to one point and we'll just

do that at 4% still so now instead of

hard-coding this 4% down here and our

apply raise method now let's go ahead

and use this class variable now you

might expect us to just be able to type

in raise amount here but if I save that

and I'm going to comment out these lines

here so if I save that and run it you

can see that I got a name error and it

says that raise amount is not defined

and that's because when we access these

class variables we need to either access

them through the class itself or an

instance of the class so within the

apply raise I could either say employee

not raise amount and if I save that and

run it then you can see that that works

or I can also access through the

instance so I can do self dot raise

amount and if I run that then you can

see that that works as well now that

might be a little confusing to you

because if these are class variables

and why can we access them from our

instance so let me print out a few lines

here to get a better idea of what's

going on so I'm going to go ahead and

remove all of these lines here actually

I'm going to keep this and I'm going to

print out the employee one dot rays

amount and I'm also going to print the

employee that rays amount and also just

to see all of our instances here I'm

going to also do the employee two trays

amount so that we can see all of them

together so now if I go ahead and print

these out you can see that I can access

this class variable from both my class

itself as well as from both instances

now what's going on here

is that when we try to access an

attribute on an instance it will first

check if the instance contains that

attribute and if it doesn't then it will

see if the class or any class that it

inherits from contains that attribute so

when we access rays amount from our

instances here they don't actually have

that attribute themselves they're

accessing the class's raiseamount

attribute now there's a little trick

that we can do here to get a better idea

of what's going on so I'm going to go

ahead and print out the namespace of

employee one and we can do that by

printing out employee one double

underscore vicked

it's now if I run this if I were to

access these names or email or pay then

these are the values that they would

return but you can see that there's no

raise amount here in this list now if I

print it out the employee dict and run

that now we're gonna get a few things

here that we don't necessarily care

about but if we look down here

and we can see that the class does

contain this raise amount attribute and

that is the value that our instances see

when we access that raise amount

attribute from our instances now let me

show you an important concept here so

I'm going to go ahead and comment this

out and I'm going to take this employee

raise them out and I'm going to

set this equal to one point zero five

and now I'm going to uncomment out our

print statements here and rerun this

code and you can see that it changed the

Rays amount for the class and all of the

instances now what if I was to set the

Rays amount using an instance instead of

using the class so instead of doing

employee dot rays amount equals five

percent I'm going to say employee one

dot rays amount equals five percent so

if I run this now now this might be a

little unexpected you can see that it

only changed the Rays amount for

employee one it's the only one that has

this five percent so why did it do that

well when we made this assignment it

actually created the Rays amount

attribute within employee one so if I go

back up here and print back out employee

ones namespace and I'm going to do this

under the assignment so now if I run

that now you can see that employee one

has raised amount within its name space

equal to five percent and it finds this

within its own namespace and returns

that value before going and searching

the class and we didn't set that raise

amount on employee two so that still

falls back to the classes value now

that's an important concept to

understand because up here and our apply

rays method we can see that we could get

different results depending on whether

we did the self which is the instance

raised amount or the employee class

raise amount so in this case I think I'm

going to go ahead and leave this as self

dot raise amount because that will give

us the ability to change that amount for

a single instance if we really wanted to

so if I wanted to change employee ones

raise amount then I could go ahead and

do that and when I did apply raise then

it would use the employee ones raise

amount instead of the classes raise

amount and also using self here will

allow any subclass to override that

constant if they wanted to and we'll

look at sub classing and

future video so now let's look at

another example of a class variable

where it wouldn't really make sense to

use self so let's say that we wanted to

keep track of how many employees that we

have so the number of employees should

be the same for all instances of our

class so if I created a class variable

up here I'm just gonna go ahead and call

this num of employees is equal to zero

for now and each time we create a new

employee I'm going to increment that by

one and I can do that within the knit

method since the knit method runs every

time we create a new employee so within

here I'm going to do employee dot number

of employees plus equals one now I'm

definitely gonna use employee that

number of employees here instead of self

dot number of employees because with the

raises it's nice to have that constant

class value that can be overridden per

instance if we really need it to be but

in this case there's no use case I can

think of where we would want our total

number of employees to be different for

any one instance so for now I'm going to

go down here to the bottom and go ahead

and delete all of this and I'm going to

print out employee dot number of

employees so now if I go ahead and run

that you can see that it returned two

because it was incremented twice when we

instantiated both of our employees here

if I was to put this print statement

above where we instantiated those

employees then you can see that it was

zero and we created two employees and

then it printed out - okay so I think

that is going to do it for this video

now I know that that was a lot to take

in but hopefully now you better

understand the difference between

instance variables and class variables

and when you would use each one so one

of the obvious next questions is well if

we have class variables then are there

also class methods and the answer is yes

there are these things called static

methods and also class methods and we'll

look at the difference between those in

the next video but if you do have any

questions about what we covered here

then feel free to ask and

section below and I'll do my best to

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