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Would be almost worthless in Java or C++

Although it would be technically possible to define an empty class and instantiate objects from that class in Java and C++, such objects probablywouldn't be very useful in their own right.

In Python, you can define an empty class and instantiate objects from that class. You can add instance variables to the object at runtime after itis instantiated, which might be useful in some situations. As you will see in a future module, you can even add class variables to a class after it has beenused to instantiate objects. That might also be useful in some situations.

Add an instance variable to one object

The code in Listing 2 uses the variable named ref01 to add a new variable with a value of 1234 to the object referred to by ref01 .

Listing 2 . Add an instance variable to one object.
print("Add an instance variable to one object and display it") ref01.iVar = 1234print(ref01.iVar)

Listing 2 also uses the reference variable named ref01 to print the value of the new variable belonging to the object referred toby ref01 . Figure 2 shows the output produced by the code in Listing 2 .

Figure 2 . Output from the code in Listing 2.
Add an instance variable to one object and display it 1234

Try to display the same instance variable in the other object

The code in Listing 3 uses the variable named ref02 in an attempt to display the same instance variable on the object referred to by ref02 .

Listing 3 . Try to display the same instance variable in the other object.
print("Try to display the same instance variable in the other object") print(ref02.iVar)

However, the code that added a new instance variable to one object in Listing 2 did not add the same instance variable to both objects that were originally instantiated from the same class. Therefore, at this point, thetwo objects have different structures. The code in Listing 3 produced the error message shown in Figure 3 .

Figure 3 . Output from the code in Listing 3.
Try to display the same instance variable in the other object Traceback (most recent call last):File "1359-1430-13.py", line 18, in<module>print(ref02.iVar) AttributeError: 'TestClass' object has no attribute 'iVar'

Visualize the code in Listing 9

I recommend that you create a visualization for the code in Listing 9 and step through the program one instruction at a time. As you do that, pay attention tothe movements of the red and green arrows on the left, the diagram on the right, and the printed material at the bottom. That should help you to betterunderstand the behavior of empty objects in Python.

Object can modify itself at runtime

The next program that I will discuss is a minor modification to the previous program but ends up with the same result. A complete listing of the program isprovided in Listing 10 and the output from the program is shown in Figure 6 . I will only discuss those things that are different between this program and theprevious program.

The definition of a class named TestClass

The first difference is shown in Listing 4 , which shows the definition of a class named TestClass . Instead of being empty as in the previous program, this class provides a method named addInstanceVar by which an object can add and initialize a new instance variable named iVar to itself at runtime.

Questions & Answers

Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
hi
Loga
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
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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