A question on modification of a list via a function invocation

[Mok-Kong Shen]

Am 14.08.2017 um 21:53 schrieb Ned Batchelder:
> On 8/14/17 3:21 PM, Mok-Kong Shen wrote:
>> Am 14.08.2017 um 20:50 schrieb Ned Batchelder:
>>> 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
>>
>> I could more or less understand that in test() alist is interpreted as
>> local but in the extended program below in test2() I first write the
>> same as in test1(), after which I logically assume that the name alist
>> is now known as global and then I write alist=[30,60,90] but that
>> doesn't have any effect globally, since I get the output:
>>
>> [1, 2, 3]
>> [3, 6, 9]
>> [3, 6, 9]
>>
>> Could you please explain that?
>>
>> M. K. Shen
>> ---------------------------------------------------------
>>
>> def test(alist):
>>    alist=[3,6,9]
>>    return
>>
>> def test1(alist):
>>    alist[0],alist[1],alist[2]=3,6,9
>>    return
>>
>> def test2(alist):
>>    alist[0],alist[1],alist[2]=3,6,9
>>    alist=[30,60,90]
>>    return
>>
>> ss=[1,2,3]
>> test(ss)
>> print(ss)
>> test1(ss)
>> print(ss)
>> test2(ss)
>> print(ss)
> 
> Your test2 function first mutates the caller's list by assigning
> alist[0]=3, then it rebinds the local name alist to be a new list.  So
> the caller's list is now [3, 6, 9].

Sorry for my poor knowledge. After the line alist[0]..., what is the
status of the name alist? It's now a global name, right? So why in the
line following that the name alist would suddenly be interpreted as
local? I can't yet fully comprehend the logic behind that.

M. K. Shen


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