list.pop(0) vs. collections.dequeue

[Dave Angel]

Steve Howell wrote:
> On Jan 22, 12:40 pm, Christian Heimes <li... at cheimes.de> wrote:
>   
>> Steve Howell wrote:
>>     
>>> Is that really true in CPython?  It seems like you could advance the
>>> pointer instead of shifting all the elements.  It would create some
>>> nuances with respect to reclaiming the memory, but it seems like an
>>> easy way to make lists perform better under a pretty reasonable use
>>> case.
>>>       
>>> Does anybody know off the top of their head if the "have-to-be-shifted-
>>> by-one" warning is actually valid?
>>>       
>> Why do you think the documentation has such obvious errors?
>>     
>
> I wasn't making any assumptions, hence the question mark.  The Python
> docs are very good, but even the best of projects make advances that
> aren't reflected in the docs.
>
>   
>> CPython's list type uses an array of pointers to store its members. The
>> type is optimized for the most common list operations in Python:
>> iteration and appending. Python code rarely changes the head or middle
>> of a list. The dequeue type is an optimized data structure for popping
>> and inserting data at both ends.
>>
>>     
>
> I disagree that Python code rarely pops elements off the top of a
> list.  There are perfectly valid use cases for wanting a list over a
> dequeue without having to pay O(N) for pop(0).  Maybe we are just
> quibbling over the meaning of "rarely."
>
>   
>> When you list.pop() or list.insert() the pointers in the internal array
>> must be shifted. appending is much faster because the internal array is
>> overallocated meaning it contains free slots at the tail of the data
>> structure. A linked list of pointers requires more memory and iteration
>> is slower since since it can't utilize the CPU's cache as good as an array.
>>
>>     
>
> I am not proposing a linked list of pointers.  I am wondering about
> something like this:
>
> p =p[1];
> (and then reclaim p[0] as free memory, I already said I understood
> that was the tricky bit)
>
> The pointer arithmetic for accessing each element would still work in O
> (1), right?
>
>   
I think it was Dijkstr (sp?) who said you can accomplish anything with 
just one more level of indirection.  Clearly each attribute (variable) 
that has a binding to a given list must point to a fixed piece of 
memory, that cannot safely be moved, because there's no efficient way to 
find all the attributes.   That fixed piece is the list object, and I 
expect it's 16 or 20 bytes, on a 32bit implementation.  It must in turn 
point to the actual malloc'ed block that contains pointers to all the 
elements of the list.  So that block will be 4*n where n is the number 
of reserved cells, at least as large as len().  This is the block where 
copying takes place when you insert or delete from the beginning.

The list object must contain a pointer to the beginning of this block, 
or it wouldn't be able to free() it later.  So you'd be suggesting a 
second pointer that actually points to the current 0th pointer.  And a 
pop would simply increment this second pointer.

Such an approach might be worthwhile if you expect lots of pops and 
pushes, with a minimal net change.  But of course each time you did a 
pop, you'd have to decide whether it was time to normalize/compact  the 
block.

As you say, reclaiming the 0th element of this block to the memory pool 
would be tricky.  Doubly so, because  1) the C memory allocator has no 
such notion as resizing the beginning of a block. and 2) it would have 
nothing useful to do with the 4 bytes freed.  The minimum allocated 
block in Python is probably 16 bytes of actual address space.  I'd guess 
that's 4 bytes for malloc's overhead, and 8 bytes for the minimum object 
header, and 4 bytes for data.  To check me, try:

 >>> a = 5.3
 >>> b = 49.6
 >>> id(a), id(b)
(11074136, 11074152)

Anyway, this could be done, where once the two pointers get some 
distance apart, you do a realloc, and copy everything.  But of course 
you'd want to build some hysteresis into it, to avoid thrashing.

There wouldn't be much of a performance hit, but it would increase every 
list size by 4 bytes minimum.  So I doubt it would be a list replacement.

This'd be an interesting project.to do as an addon module.

DaveA