Python - Why this list doesn't change

Question

I want to change a part of a list and save the result. I would like to know why this method is not working. And Thank you!

Code:

def Test(L):
    for i in range(len(L)):
        L[i] = L[i][1:]

L = ["-stackoverflow", "-Python", "-C++"]
Test(L[1:])
print(L)

Ouput:

['-stackoverflow', '-Python', '-C++']

Expected:

['-stackoverflow', 'Python', 'C++']

your function needs to return the modified list and reassign it at the caller. — user20396240
– user20396240, Commented Dec 8, 2022 at 15:18
Does this answer your question? Does a slicing operation give me a deep or shallow copy? — luk2302
– luk2302, Commented Dec 8, 2022 at 15:24

Daniel Walker · Accepted Answer · 2022-12-08 15:20:32Z

2

Whenever you use [:] on a list, it constructs a new list. When you called Test(L[1:]), you didn't pass it L but rather a completely new List unrelated to L. There are two things you can do here: Either return the new list for reassignment or pass L into Test() and not L[1:].

edited Dec 8, 2022 at 15:20

Daniel Walker

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answered Dec 8, 2022 at 15:18

Anthony Garza

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2 Comments

luk2302 Over a year ago

"a completely new List unrelated to L" - no exactly: the strings are still the same instances. If OP would mutate L[i] instead of assign to it then it would work.

Daniel Walker Over a year ago

@luk2302, you can't mutate strings. It is the list itself the the OP wants to change.

Gandhi · Accepted Answer · 2022-12-08 15:18:01Z

2

You call the Test() function with L[1:], but this is only a copy of the list and not the original L list. So when you modify the list in your function, you modify the copy.

answered Dec 8, 2022 at 15:18

Gandhi

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Comments

user20396240 · Accepted Answer · 2022-12-08 15:19:19Z

2

your function needs to return the modified list and reassign it at the caller.

def Test(L):
    for i in range(len(L)):
        L[i] = L[i][1:]
    return L

L = ["-stackoverflow", "-Python", "-C++"]
L = Test(L[1:])
print(L)

answered Dec 8, 2022 at 15:19

user20396240

Comments

attacker88 · Accepted Answer · 2022-12-08 15:37:22Z

2

you just need to write Test(L) and not Test(L[1:]) as the function is already doing the operation for you.

 def Test(L):
   for i in range(len(L)):
     L[i] = L[i][1:]

L = ["-stackoverflow", "-Python", "-C++"]

Test(L)
print(L)

edited Dec 8, 2022 at 15:37

answered Dec 8, 2022 at 15:22

attacker88

216 bronze badges

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