Top and Bottom Values [PEP: 326]

[Antoon Pardon]

On 2006-09-27, Peter Otten <__peter__ at web.de> wrote:
> Antoon Pardon wrote:
>
>> I had written my own module, which works similarly but
>> is somewhat extended. Here is an example of how it can
>> be used and how I would like to use it but get stuck.
>> 
>> from extreme import Top
>>>>> Top
>> Top
>>>>> Top + 1
>> Top
>>>>> Top - 30
>> Top
>>>>> Top > 1e99
>> True
>>>>> lst = range(10)
>>>>> lst[:Top]
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in ?
>> TypeError: slice indices must be integers or None
>> 
>> So this is where I am stuck. I need this Top value in
>> a context where it can be used as a start or stop value
>> in a slice. My idea was that since a stop value greater
>> than the length of lst in this context would simply return
>> a copy of lst, using Top in such a way should behave so
>> too. However I don't see a way of doing that.
>> 
>> So can someone provide ideas to get this behaviour?
>
>>>> import sys
>>>> class Top(object):
> ...     def __index__(self):
> ...             return sys.maxint
> ...
>>>> Top = Top()
>>>> range(5)[:Top]
> [0, 1, 2, 3, 4]
>
> This can of course fail in many interesting ways...

To begin with this already fails:

>>> for i in xrange(Top):
...   print i
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: an integer is required


What bothers me a bit about the rejection of PEP 326 is that one of the
reasons stated is:

  http://mail.python.org/pipermail/python-dev/2004-January/042306.html

  - it is easily implemented when you really need it

Well I thought it would simplify some things for me, so I tried an
implementation and then found that some of the things that I would
want to do with it wont work. So the "is easily implemented" bit
seems not to be correct.

-- 
Antoon Pardon