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Figure 3 shows the output from the code in Listing 2 .
| Figure 3 . Output from the code in Listing 2. |
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Create two lists
aList = ['ab', 'cd', 'ef']bList = ['The', 'old', 'list']
Call listModifierIn listModifier
Use incoming parameter to append to listANew listA = ['ab', 'cd', 'ef', 3.14159]
Assign a new list to listBNew listB = ['A', 'new', 'list']
Back from listModifieraList = ['ab', 'cd', 'ef', 3.14159]
bList = ['A', 'new', 'list'] |
Listing 2 has two major modifications relative to Listing 1 . First, the function returns a copy of the reference to the new list instead ofreturning nothing. Second, the return value is assigned to the variable named bList . This does cause the variable named bList to point to a different list object. This is confirmed by the last line of text in Figure 3 .
Conclusion -- function parameters in Python are passed by value, but the copies that arepassed can often be used to access and modify the objects pointed to by the original reference variables of which they are copies.
In this program, the function's second argument is of no consequence. The function and the program would behave the same if the second argument wereeliminated entirely. I left it in for illustration purposes only.
Hopefully you can study these two programs along with the program output and the above discussion to understand the basic behaviorof Python function arguments.
Note that there is a subtle technical difference between arguments and parameters. However, I often tend to use the two terms interchangeably so you will simplyneed to grit your teeth and bear with me if you see me using one or the other term incorrectly.
When a function defines one or more arguments using the syntax shown in Listing 1 and Listing 2 , the code that calls the function must pass one parameter for each argument and must pass them in the correct order. That is why we call them required arguments . There isn't a lot more that needs to be said about required arguments.
A function can define one or more default arguments in addition to required arguments as shown in Listing 3 . Default values are defined for the default arguments when the function is defined using the syntax shown in Listing 3 .
The calling program can ignore some or all of the default arguments when calling the function. By this I mean that the calling program can simply notpass parameters for some or all of the default arguments. However, when default arguments are ignored, they must be ignored from right to left in the argumentlist. In other words, the calling program cannot ignore a default argument in the middle of a group of default arguments.
When the calling program ignores default arguments, the defined default values for those arguments are used by the code in the body of the function.
The program shown in Listing 3 illustrates a function with one required argument ( listA ) and three default arguments ( listB , listC , and listD ).
| Listing 3 . Illustration of default arguments. |
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# Illustrates default arguments
#-----------------------------------------------------------def listModifier(listA,listB=["B"],listC=["C"],listD=["D"]):"""Illustrates default arguments"""
print("In listModifier")listA.append(1.00001)
print("listA = " + str(listA))listB.append(2.00002)
print("listB = " + str(listB))listC.append(3.00003)
print("listC = " + str(listC))listD.append(4.00004)
print("listD = " + str(listD))return
#End function definitionaList = ["ab","cd","ef"]
bList = ["The","old","list"]cList = ["This old house"]
dList = ["is falling down"]print("aList = " + str(aList))
print("bList = " + str(bList))print("cList = " + str(cList))
print("dList = " + str(dList))print("Call listModifier")
listModifier(aList,bList)print("Back from listModifier")
print("aList = " + str(aList))print("bList = " + str(bList))
print("cList = " + str(cList))print("dList = " + str(dList)) |
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