A question on modification of a list via a function invocation
Ned Batchelder
ned at nedbatchelder.com
Mon Aug 14 14:50:51 EDT 2017
On 8/14/17 2:21 PM, Mok-Kong Shen wrote:
> I ran the attached program and got the following output:
>
> [1, 2, 3]
> [3, 6, 9]
>
> I don't understand why the modification doesn't work in the case of
> test() but does work in the case of test1().
>
> Thanks for your help in advance.
>
> M. K. Shen
>
> ------------------------------------------------------------
>
> def test(alist):
> alist=[3,6,9]
> return
>
> def test1(alist):
> alist[0],alist[1],alist[2]=3,6,9
> return
>
> ss=[1,2,3]
> test(ss)
> print(ss)
> test1(ss)
> print(ss)
This reassigns the name alist: alist = [3, 6, 9]. That changes the
local variable, but cannot affect the caller's variables.
This leaves alist as the same object, but reassigns its elements,
mutating the list: alist[0] = 3
This talk has more details: https://nedbatchelder.com/text/names1.html
--Ned.
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