About some problems due to passing function's parameters by reference

Python Help
1

Hi there!
In the below code, I’m trying to change Python list inside functions through passing it by reference.

def ListInitialization(aList):
aList = list(range(10))
print("“ListInitialization” aList print: ", aList, “\n”)

def ListEdit(aList):
aList[2] = 2222
print("“ListEdit” aList print: ", aList, “\n”)

if name == “main”:
aList =
ListInitialization(aList)
print("“Main” after “ListInitialization” aList print: “, aList, “\n”)
aList = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
ListEdit(aList)
print(”“Main” after “ListEdit” aList print: ", aList, “\n”)

The output of the above code is:

“ListInitialization” aList print: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
“Main” after “ListInitialization” aList print:
“ListEdit” aList print: [0, 1, 2222, 3, 4, 5, 6, 7, 8, 9]
“Main” after “ListEdit” aList print: [0, 1, 2222, 3, 4, 5, 6, 7, 8, 9]

Why didn’t the ListInitialization function changes to aList go beyond the ListInitialization function? But the ListEdit function changes made to aList remained after the ListEdit was completed?

Many thanks!

1 Like
2

Your ListInitiallization function assigns to the name aList, so it is a local name. It does not re-assign the variable you called the function with.

In ListEdit, you don’t assign to the name aList. Assigning to aList[2] is equivalent to calling a method on in: aList.__setitem__(2, 2222). So it’s mutating the object that was passed in.

My talk Python Names and Values has much more detail about this.

1 Like
3

Thank you very much for your detailed explanation, sir. May I ask about your article “Names and Values ​​of Python”? Python is a living organism that is constantly evolving and changing. Did these changes affect the relevance of your article for the last years?

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4

Many things have changed in Python since I wrote that talk, but the things I describe in that talk have stayed the same.

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