A question on modification of a list via a function invocation

[Mok-Kong Shen]

Am 17.08.2017 um 02:41 schrieb Steve D'Aprano:
> On Thu, 17 Aug 2017 08:29 am, Mok-Kong Shen wrote:
> 
>> I have earlier learned some other (older) programming languages. For
>> these the formal parameters are either "by reference" or "by value".
> 
> By reference and by value are not the only two conventions.
> 
> Perhaps if you go back to the 1950s you might be able to argue that "reference"
> and "value" are the only two conventions, but alternatives have existed for
> many decades, since *at least* 1960 when Algol introduced "call by name".
> 
> Python's evaluation strategy has existed for at least 43 years since Barbara
> Liskov named the calling convention used by CLU "call by sharing" in 1974.
> 
> (It actually is much older than CLU, it goes back all the way to Lisp.)
> 
> https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_sharing
> 
> 
>> In the first case, any modification of the formal parameter
> 
> Technically, you cannot modify the formal parameter, because the formal
> parameter is just a name unbound to any value. It's just the label in the
> function definition. You need to have actually passed a value as argument to
> the function before there is anything to modify.
> 
>> 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?
> 
> Python (and Ruby, Scheme, Ocaml, etc) are very clear-cut too. Just different.
> 
> This may help:
> 
> http://import-that.dreamwidth.org/1130.html
> 
> 
>> 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.
> 
> Indeed. And call by value is surprising: why should passing a giant array of a
> million values make a copy of the array just because I pass it to a function?
> 
> And call by reference is even more surprising: if I assign a value to a local
> name inside a function, why should it modify names in the caller's namespace?
> 
> Python's evaluation strategy is the least surprising of all the calling
> strategies I've used.

Sorry for my poor capacity to comprehend. I read in the web page you
cited: "In Python, the parameters to a function or method are always
local to that function or method. Any assignments to local variables
inside the function only affect the local variable, not the caller's
variable." But then why in my test2 the global list does get modified?
(Would alist[0]=3 etc. be "in-place modification" while alist=[30,60,90]
is deemed not an "in-place modification"? If yes, where is that term
clearly defined? (That term is new for me.)

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