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+5 votes

In this question, 4 functions are defined, which take a list as an input, and do NOT return anything, which I thought meant that the list which is input to them could not be changed by them, since they only change the list locally. This works for the first 2 functions, but the second 2 functions, which look very similar, change theList. Why? how can changing myLIst within a function change theList without using return?

asked in CSC201 Spring 2021 by (8 points)

3 Answers

+3 votes
Best answer

Yes, there is an important distinction between re-assigning a variable (which causes it to point to a different object), and modifying the object that a variable refers to.

Since lists are mutable, it's possible to modify the list object. e.g.

myList[0] = "new item"

If myList was a parameter in a function, then whatever list was passed in for myList will also be modified, because myList is referring to that same object, and we are modifying that object.

However, if you do:

myList = ["whole", "new", "list"]

then you are NOT modifying the original list object, you are just changing myList to point to a new list object that you created. Re-assigning the variable (or parameter) myList in one function will NOT affect the value of the list object that was passed in.

I realize this is a rather subtle distinction, but it is an important one, about how Python variables and assignment statements really work.

If we ignore functions and parameters for a moment, we can see the same issue comes up just using assignment statements:

x = [1, 2, 3]
y = x
y[0] = 100   #changes the first element of the list that both x and y refer to
print(f"x={x}  y={y}")

y = [4,5,6]  #changes y to point to a new/different list object
             # but does NOT change the list that x refers to (and y used to refer to)
print(f"x={x}  y={y}")

Now, since actual parameters are passed to functions "by assignment", the similar issue happens here:

def changeItem(y):
     y[0] = 100

def tryToChangeWholeList(y):
     y = [4,5,6]

def main():
    x = [1,2,3]
answered by (186 points)
selected by
+3 votes

I was confused by this as well, but as I understand it, it has something to do with the fact that the individual values in the list were being redefined new values added, rather than redefining the list as a whole. I'm not 100% sure, but that's what I found to be different.

answered by (8 points)
+2 votes

You could also write an if statement with repeating elif statements to produce a different output for each input.

answered by (8 points)