A question on modification of a list via a function invocation

[Mok-Kong Shen]

Am 17.08.2017 um 00:39 schrieb Chris Angelico:
> On Thu, Aug 17, 2017 at 8:29 AM, Mok-Kong Shen
> <mok-kong.shen at t-online.de> wrote:
>> I have earlier learned some other (older) programming languages. For
>> these the formal parameters are either "by reference" or "by value".
>> In the first case, any modification of the formal parameter inside
>> a function affects the corresponding actual parameter of a function
>> call, while in the second case a copy of the actual parameter is
>> passed into the function so that any modification of the formal
>> parameter inside the function has no effect at all outside. This is
>> extremely clear-cut in comparison to Python, isn't it? Anyway, while
>> any new user of a programming language certainly can be expected to
>> take good efforts to learn a lot of new stuffs, I suppose it's good
>> for any practical programming language to minimize the cases of
>> surprises for those that come from other programming languages.
> 
> Python has a data model that is neither of the above, but it's simpler
> in that you have one pattern for everything. Whether you're looking at
> function parameters, return values, assignment, loops, function
> definitions, or anything else, the model is exactly the same. And that
> model is: objects exist independently of names, and names refer to
> objects. If you do "x = y", you're saying "figure out which object 'y'
> means, and make the name 'x' refer to it". If you do "x[1] = y",
> you're saying "figure out which object 'y' means, and tell the object
> that 'x' means that it should make [1] refer to that object". So if
> you have multiple names referring to the same object, any change you
> ask that object to do will be seen by every other name that also
> refers to it - because it's all about the object.

I may have misunderstood you. But I don't think what you wrote
above would explain why the program below produces the output:

[1, 2, 3]
[3, 6, 9]

M. K. Shen
-----------------------------------------------------

def test2(alist):
   alist[0],alist[1],alist[2]=3,6,9
   alist=[30,60,90]
   return

def test3(alist):
   alist=[30,60,90]
   alist[0],alist[1],alist[2]=3,6,9
   return

ss=[1,2,3]
test3(ss)
print(ss)
test2(ss)
print(ss)