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Figure 6 . Output from the code in Listing 10.
6 Modify class var using class 7 Display class var using objectsTestClass object at 0x02091DD0: [1, 2, 3, 4, 'a', True] TestClass object at 0x0218C710: [1, 2, 3, 4, 'a', True]8 Display class var using class: [1, 2, 3, 4, 'a', True]

Both objects see the change that was made to the list even though that change was made independently of either object.

A word of caution is in order

This sort of behavior can lead to programming errors that are very easy to make and very difficult to find and fix. In my opinion, you should usemutable class variables very rarely if at all. On the other hand, class variables that are not mutable, such as tuples, can be very useful and are muchsafer to use. Even in that case, however, one immutable object can be replaced by a different immutable object so even the use of immutable objects is nottotally safe.

Shadowing or hiding a class variable

If an instance variable belonging to an object has the same name as a class variable in the class from which the object was instantiated, the instancevariable will "shadow" or hide the class variable insofar as that object is concerned. This is illustrated by the program shown in Listing 17 . The output from the program is shown in Figure 12 .

Class definition with a shadowing method

The class definition in Listing 11 contains a class variable named classVar . The class definition also contains a method named shadowClassVariable . If that method is executed on an object instantiated from the class, it will add a new instance variable to the objectnamed classVar . From that point forward, the class variable named classVar won't be visible insofar as that object is concerned unless it accesses the class variable using the name of the class, TestClass .

Listing 11 . Class definition with a shadowing method.
class TestClass(object): classVar = 1234#Note, the following code adds a new instance variable named classVar # to the object, which "shadows" or hides the actual class variable# named classVar insofar as this object is concerned. def shadowClassVariable(self):self.classVar = "ABCD" def printClassVar(self):print(str(self)[10:40] + ": " + str(self.classVar))

Listing 11 also defines a method named printClassVar . This method concatenates and prints a portion of the object identifier (self) with the contents of classVar (as seen by the object) .

Instantiate and display classVar in two objects

The code in Listing 12 instantiates two different objects from the class named TestClass and saves the objects' references in the variables named ref01 and ref02 .

Listing 12 . Instantiate and display classVar in two objects.
print("1 Instantiate two objects") ref01 = TestClass()ref02 = TestClass() print("2 Display classVar using objects")ref01.printClassVar() ref02.printClassVar()

The code in Listing 12 also uses those references to call the method named printClassVar on each object. This causes the contents of classVar to be printed as seen by each object at this point in the program. The results are shown in Figure 7 .

Questions & Answers

what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Itse 1359 introduction to scripting languages: python. OpenStax CNX. Jan 22, 2016 Download for free at https://legacy.cnx.org/content/col11713/1.32
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