[Python-ideas] clear() method for lists

[Gerald Britton]

On Wed, Feb 10, 2010 at 12:10 PM, Raymond Hettinger
<raymond.hettinger at gmail.com> wrote:
>
> On Feb 10, 2010, at 6:54 AM, Gerald Britton wrote:
>> Yes, I plan to ask for copy() as well, when the bug tracker opens up
>> for 3.3, 3.4, etc.  The issue is not, "Is there already a way to do
>> this?"  but rather, "Can we have consistent interfaces in the sequence
>> types and collections where possible and appropriate?"
>
> Use the copy module.
>
>> dict() and set() already support both clear() and copy() methods.
>> Previous posters have pointed to the disconnect and showed the problem
>> of having to test if a given iterable supports the clear() method
>> before calling it, in functions that take any iterable.
>>
>> Also, for what it's worth:
>>
>> s1 = set()
>> s2 = s1.copy()
>>
>> is faster than
>>
>> s1 = set()
>> s2 = set(s1)
>
> I question your timing skills.  Both call the same routine to do the work
> of copying entries:
>
>   set_copy() calls make_new_set() which calls set_update_internal()
>   set_init() calls set_update_internal()
>
> If there is any difference at all, it is the constant overhead of passing
> an argument to set(), not the implementation itself.  The actual
> set building work is the same.
>
>> (and also for dict()) probably because the first is
>> specifically-written for the copy operation whereas the second
>> actually iterates over s1, one item at a time.  (At least I think
>> that's what's going on).  I suppose that a list().copy() method might
>> also be faster than the other two approaches to copy a list.
>
> You need to read some code, learn about ref counts, etc.
> There's more to list copying than a memcpy().
> If list.copy() were added, it would use that same underlying
> code as list(s) and s[:].  There would be no speed-up.
>
>> Lastly, for completeness,  I suppose copy() might be appropriate for
>> both tuple and deque as well.
>
> Tuples?  Really?
> An immutable collection is its own copy.
>
>
> Raymond
>
>

Thanks for the feedback.  I also question my timing skills (and most
other skills that I think I have).  That's what's good about bouncing
ideas around here.  Silly ones get shot down, and rightly so!

-- 
Gerald Britton