Simple del[] list question

[Gregoire Welraeds]

In reply to the message of Martijn Faassen sent on Apr 26 (see below) :

> Note that another strategy which tends to avoid these troubles is to
> construct a new list if you modify one. That is, usually if you're going

Isn't it time and memory consuming when working on big list (eg with 1000
or above elements) ?


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Gregoire Welraeds
greg at perceval.be
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On 26 Apr 2000, Martijn Faassen wrote:

> Date: 26 Apr 2000 18:07:44 GMT
> From: Martijn Faassen <m.faassen at vet.uu.nl>
> To: python-list at python.org
> Newsgroups: comp.lang.python
> Subject: Re: Simple del[] list question
> 
> Matthew Hirsch <meh9 at cornell.edu> wrote:
> [snip problem description]
> > Why did a change?  Why suddenly when you go to two dimensions does the 
> > behavior change?
> 
> It's because [:] only does a shallow copy; that is, it copies the list,
> the individual elements are *not* copied; only new references are made.
> So when you delete an element of a sublist, both lists will now have 
> a reference to the same sublist which now misses an element.
> 
> Python is really very consistent about this, but tricky sometimes, indeed.
> 
> In order to make a deep copy of the list, use the deepcopy() function from
> the standard copy module.
> 
> Note that another strategy which tends to avoid these troubles is to
> construct a new list if you modify one. That is, usually if you're going
> to alter some list, you're going to alter multiple elements, or perhaps
> not include multiple elements, and not just a single one. In that case it's
> easier and possibly even faster to do something like this:
> 
> mylist = [...] 
> newlist = []
> for element in mylist:
>     if some_criterion(element):
>         newlist.append(element)
> mylist = newlist
> 
> By constructing a new list, you avoid the potential problems of some other
> variable still pointing to your modified list.
> 
> Regards,
> 
> Martijn
> -- 
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