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Then the code in the function creates a new list and attempts to use the second incoming parameter to replace the original list pointed to by that parameter withthe new list. The last line of text in Figure 1 confirms that this was not successful . The copy of the incoming parameter points to the new list but this does not cause the original reference to point to the new list.

A non-technical explanation

This same situation occurs in the Java courses that I teach. I often attempt to explain it this way. Assume that I write my home address on a piece of paperand make a copy of that piece of paper. Then I lose the copy. If a burglar finds the copy, he has a pointer to my house. He could use that pointer in an attemptto break into my house. However, even if he were to scratch out the address on the copy and write a new address on the copy, that would not change the addresswritten on the original. The original would still point to the same old address.

This is essentially what happens in Listing 1 when the code creates a new list and assigns its value to the copy of the original reference. The copy points tothe new list inside the function but the original reference outside the function continues to point to the original list.

In effect, the copies that are received as parameters by a function become local variables within the function. Those variables, and in this case the newlist that is pointed to by one of those variables, all cease to exist when the function terminates unless returned by the function.

Visualization of the code in Listing 1

Figure 2 shows a visualization of the code in Listing 1 after all of the code has been executed.

Figure 2. Visualization of the code in Listing 1.

Visualization of the code in Listing 1

I recommend that you create a visualization for the code in Listing 1 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 concept of "passing by value."

A modified approach to the same objective

Listing 2 shows a modification to the code from Listing 1 that is designed to accomplish what Listing 1 attempted to do but was unable to do.

Listing 2 . Another list-modifier function.
# Illustrates pass by value or reference #---------------------------------------------------def listModifier(listA,listB): """Illustrates pass by value or reference"""print("In listModifier") print("Use incoming parameter to append to listA")listA.append(3.14159) print("New listA = " + str(listA))print("Assign a new list to listB") listB = ["A","new","list"]print("New listB = " + str(listB)) return listB#End function definition #Call the functionprint("Create two lists") aList = ["ab","cd","ef"]bList = ["The","old","list"] print("aList = " + str(aList))print("bList = " + str(bList)) print("Call listModifier")bList = listModifier(aList,bList) print("Back from listModifier")print("aList = " + str(aList)) print("bList = " + str(bList))

Questions & Answers

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
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
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
How can I make nanorobot?
Lily
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
how can I make nanorobot?
Lily
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
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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