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

[Yongsheng Cheng]

simplicity matters

Bar Harel <bzvi7919 at gmail.com>于2016年6月1日周三 上午3:11写道：

> -1 as well.
>
> I have used the generator multiplication many times before. The use of the
> same object helps a lot in certain cases.
> Changing that will cause a confusion among those who know the phenomenon
> and would break compatibility.
>
> Regarding your matrix example, maybe we can make use of the new '@'
> operator for copying new objects, what do you say?
>
> -- Bar Harel
>
> On Tue, May 31, 2016 at 8:01 PM Terry Reedy <tjreedy at udel.edu> wrote:
>
>> 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
>>
>>
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