[Tutor] Deleted dictionary length not reporting

[Steven D'Aprano]

On 17/07/13 16:48, Jim Mooney wrote:
> On 16 July 2013 20:42, Steven D'Aprano <steve at pearwood.info> wrote:
>
>> Here's a question for you, to test your Python knowledge:
>
> What knowledge? I've been fooling around in this awful AZ heatwave and
> only got to chap 5 in the Lutz book. ;')
>>
>> Assuming that some_list is already a list, what's the difference between
>> these two lines, and under what circumstances why would you want to use the
>> second one?
>>
>> some_list = []
>> some_list[:] = []
>>
> However, fooling around helps, so I'd say:
>
>>>> p = [1,2,3]
>>>> id(p)
> 34586456
>>>> p[:] = []
>>>> id(p)
> 34586456
>>>> p = []
>>>> id(p)
> 34586752
>
> The slice will pass an "is" test, the equal will not.

Correct, but you may not have worked out the consequences that follow from that.

[Aside: ID numbers may be reused. So it is possible, although unlikely, that id(p) will remain the same by chance after p = []. That can't happen with Jython or IronPython, as they never re-use IDs, and I don't think it can happen for CPython, but it would be permitted behaviour for Python implementations. So if you wish to test for "is", you should actually use "is", and not just eyeball the ID.]

p = [1, 2, 3]
q = p
# confirm that q is not a copy of p, but they are
# two names for the same object
assert p is q

p[:] = []
assert p is q  # p and q remain the same object
assert q == []

p = q = [1, 2, 3]
p = []
assert p is not q  # p is a new list, q is still the old one


This is not just of theoretical interest. Sometimes you will have a function that accepts a list argument, and you might wish to clear the list. Depending on how you do so, the change may or may not propagate outwards to the caller:


mylist = [1, 2, 3, 4]
process(mylist)
print mylist

What gets printed depends on what process() does.

If process() clears the list using the slice version, then it will be cleared on the outside too. But if instead it assigns a new empty list to it, then the old list object remains untouched and mylist will not change.



> No fair making me think.This is as bad as being back in school.

Heh :-)


-- 
Steven