A question on modification of a list via a function invocation

[Mok-Kong Shen]

Am 14.08.2017 um 22:10 schrieb oliver:
> It is not a global because accessing an item of a list does not change
> whether it is global or local. It would only be global if you declared it
> global via a "global" statement. Can you give an example, that would help
> determine the issue.

If without a global statement, a name is local, then alist[0]=3 should
not work globally, if it works at all, in my layman's logic.

M. K. Shen
> 
> On Mon, 14 Aug 2017 at 16:06 Mok-Kong Shen <mok-kong.shen at t-online.de>
> wrote:
> 
>> 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
>>
>>
>>    .
>>
>> --
>> https://mail.python.org/mailman/listinfo/python-list
>>