Negative array indicies and slice()

[Andrew Robinson]

On 10/29/2012 05:02 PM, Steven D'Aprano wrote:
> On Mon, 29 Oct 2012 08:42:39 -0700, Andrew Robinson wrote:
>
>>>> But, why can't I just overload the existing __getitem__ for lists and
>>>> not bother writing an entire class?
> You say that as if writing "an entire class" was a big complicated
> effort. It isn't. It is trivially simple, a single line:
>
> class MyList(list):
>      ...
No, I don't think it big and complicated.  I do think it has timing 
implications which are undesirable because of how *much* slices are used.
In an embedded target -- I have to optimize; and I will have to reject 
certain parts of Python to make it fit and run fast enough to be useful.

>>> You can just overload that one method in a subclass of list.  Being
>>> able to monkey-patch __getitem__ for the list class itself would not be
>>> advisable, as it would affect all list slicing anywhere in your program
>>> and possibly lead to some unexpected behaviors.
>> That's what I am curious about.
>> What unexpected behaviors would a "monkey patch" typically cause?
> What part of "unexpected" is unclear?
>
Ahh -- The I don't know approach!  It's only unexpected if one is a bad 
programmer...!
> Let me see if I can illustrate a flavour of the sort of things that can
> happen if monkey-patching built-ins were allowed.
>
> You create a list and print it:
>
> # simulated output
> py>  x = [5, 2, 4, 1]
> py>  print(x)
> [1, 2, 4, 5]
<snip>

Finally you search deep into the libraries used in your code, and *five
days later* discover that your code uses library A which uses library B
which uses library C which uses library D which installs a harmless
monkey-patch to print, but only if library E is installed, and you just
happen to have E installed even though your code never uses it, AND that
monkey-patch clashes with a harmless monkey-patch to list.__getitem__
installed by library F. And even though each monkey-patch alone is
harmless, the combination breaks your code's output.

Right, which means that people developing the libraries made 
contradictory assumptions.

> Python allows, but does not encourage, monkey-patching of code written in
> pure Python, because it sometimes can be useful. It flat out prohibits
> monkey-patching of builtins, because it is just too dangerous.
>
> Ruby allows monkey-patching of everything. And the result was predictable:
>
> http://devblog.avdi.org/2008/02/23/why-monkeypatching-is-destroying-ruby/
>

I read that post carefully; and the author purposely notes that he is 
exaggerating.
BUT Your point is still well taken.

What you are talking about is namespace preservation; and I am thinking 
about it. I can preserve it -- but only if I disallow true Python 
primitives in my own interpreter; I can't provide two sets in the memory 
footprint I am using.

 From my perspective, the version of Python that I compile will not be 
supported by the normal python help; The predecessor which first forged 
this path, Pymite, has the same problems -- however, the benefits 
ought-weigh the disadvantages; and the experiment yielded useful 
information on what is redundant in Python (eg: range is not supported) 
and when that redundancy is important for some reason.

If someone had a clear explanation of the disadvantages of allowing an 
iterator, or a tuple -- in place of a slice() -- I would have no qualms 
dropping the subject.  However, I am not finding that yet.  I am finding 
very small optimization issues...

The size of an object is at least 8 bytes.  Hence, three numbers is 
going to be at least 24 bytes; and that's 24 bytes in *excess* of the 
size of slice() or tuple () which are merely containers.  So -- There 
*ARE* savings in memory when using slice(), but it isn't really 2x 
memory -- its more like 20% -- once the actual objects are considered.

The actual *need* for a slice() object still hasn't been demonsrated.  I 
am thinking that the implementation of __getitem__() is very poor 
probably because of legacy issues.

A tuple can also hold None, so ( 1, None, 2 ) is still a valid Tuple.
Alternately:  An iterator, like xrange(), could be made which takes None 
as a parameter, or a special value like 'inf'.
Since these two values would never be passed to xrange by already 
developed code, allowing them would not break working code.

I am only aware of one possible reason that slice() was once thought to 
be necessary; and that is because accessing the element of a tuple would 
recursively call __getitem__ on the tuple.  But, even that is easily 
dismissed once the fixed integer indexes are considered.

Your thoughts?  Do you have any show stopper insights?