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

[Steven D'Aprano]

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.


-- 
Steve