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

Steven D'Aprano steve at pearwood.info
Tue May 31 10:16:25 EDT 2016

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 ?


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 

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.


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