List copying idiom was Re: [Python-Dev] implementation of copy standard lib

[Steve Holden]

Tom Anderson wrote:
> On Tue, 16 Aug 2005, Ron Adam wrote:
> 
> 
>>Simon Brunning wrote:
>>
>>
>>>On 8/14/05, Martijn Brouwer <e.a.m.brouwer at alumnus.utwente.nl> wrote:
>>>
>>>
>>>>I can imagine that *a lot* of libs/scripts use the copy library,
>>>
>>>I think that copy is very rarely used. I don't think I've ever imported it.
>>
>>I use copy.deepcopy() sometimes, and more often [:] with lists. 
>>Dictionary objects have a copy method.  All non mutable objects are 
>>copies.
> 
> 
> When you say [:], do you mean that you copy lists like this:
> 
> l = someList()
> m = []
> m[:] = l
> 
> ?
> 
> That's what i've been doing. The other day, i realised that i could just 
> do:
> 
> l = someList()
> m = list(l)
> 
> The same works for copying dicts. Whilst it's still not utterly obvious 
> that what this is about is making a copy, it's more obvious, cleaner, 
> shorter and probably faster than the slice assignment, which is really 
> kludgy (almost intercalorific, in fact - or even perlish!).
> 
Not really: it used to be the preferred mechanism, but now it seems a 
little old-fashioned is all.

> 
>>I too have wondered why copy isn't a builtin,
> 
> 
> Same here. It seems like a sort of obvious thing to have, and could 
> probably implemented much more simply and quickly in the interpreter. 
> You'd probably want a __copy__ hook for classes which want special 
> handling, and just do a normal deep copy for everything else.
> 
Well yes, but given that module copy now exists (and will therefore have 
to continue ti exist until Py3) that would introduce some redundancy.

regards
  Steve
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