[Python-ideas] Proposal to change List Sequence Repetition (*) so it is not useless for Mutable Objects

[Terry Reedy]

On 5/31/2016 10:16 AM, Steven D'Aprano wrote:
> On Tue, May 31, 2016 at 01:36:31PM +0000, Joseph Martinot-Lagarde wrote:
>>> I can only agree here. Even today, despite knowing the fact, it's
>>> causing some headaches in some cases.
>>
>> How about raising an exception if mutable objects are in the list ?
>
> -1
>
> It's a gratuitous breakage that cannot solve the problem, because you
> cannot tell in advance which objects are mutable and which are not. The
> best you can do is recognise known built-ins.
>
> ("list is mutable, tuple is not, except when it contains a mutable
> item, but mymodule.MySequence may or may not be, there's no way to
> tell in advance.")
>
>
>> - maybe there are useful use cases of duplicating the reference ?
>
> Absolutely. That's a standard way of grouping items taken from an
> iterable:
>
> py> it = iter("hello world!")
> py> for a, b, c in zip(*[it]*3):  # groups of three
> ...     print(a, b, c)
> ...
> h e l
> l o
> w o r
> l d !
>
>
> This wouldn't work if the iterators were three independent copies.

-1 also.  As Steven has pointed out several times in other threads, 
Python is consistent in not copying objects unless requested.  Thus

a = [1,2,3]
b = a  # a reference copy, not an object copy, as some expect
a[1] = 4
print(b[1])
# 4 -- a surprise for those who expect '=' to copy the list object

Sequence * is an elaboration of the same issue.

-- 
Terry Jan Reedy