Question about idioms for clearing a list

[Steven D'Aprano]

On Mon, 06 Feb 2006 09:39:32 -0500, Dan Sommers wrote:

> On Tue, 07 Feb 2006 01:01:43 +1100,
> Steven D'Aprano <steve at REMOVETHIScyber.com.au> wrote:
> 
>> On Mon, 06 Feb 2006 13:35:10 +0000, Steve Holden wrote:
>>>> I'm wondering why there is no 'clear' for lists. It feels like a common
>>>> operation for mutable containers. :-/
>>>> 
>>> Because it's just as easy to create and assign a new empty list (and 
>>> have the old unused one garbage collected).
>>> 
>>> l = []
>>> 
>>> is all you need!
> 
>> Not so. If that logic were correct, then dicts wouldn't need a clear
>> method either, because you could just assign a new empty dict with d = {}.
> 
>> But your own sentence tells us why this is not sufficient: because you
>> aren't emptying the list, you are reassigning (rebinding) the name. The
>> old list still exists, and there is no guarantee that it will be garbage
>> collected, because there is no guarantee that it isn't in use somewhere
>> else:
> 
>> L = [0,1,2]
>> D = {"key": L}
>> L = []  # rebinds the name L, but the list instance still exists
> 
> That is a red herring.  Consider this:
> 
> L = [object(), object()]
> O = L[1]
> L = [] # or insert your favorite list-clearing/emptying statement here
> 
> What damage is done now that O is still referring to one of the items
> that used to be in L?

What relevance is this? If there is one and only one reference to the list
L, then it will be garbage collected when L is rebound. I never denied
that. I pointed out that, in the general case, you may have multiple
references to the list (not all of which are bound to names), and
rebinding the name L will NOT have the side-effect of clearing the list.


> The trouble begins when references to "the list to which L refers" end
> up somewhere else.  Then we have to wonder if rebinding L will leave
> some other block of code with an outdated list.

Precisely, just as my example shows.


-- 
Steven.