range() is not the best way to check range?

[Antoon Pardon]

On 2006-07-20, Paul Boddie <paul at boddie.org.uk> wrote:
> Alex Martelli wrote:
>> Paul Boddie <paul at boddie.org.uk> wrote:
>> >
>> > Well, range is a function in the current implementation, although its
>> > usage is similar to that one would get if it were a class, particularly
>> > a subclass of list or one providing a list-style interface. With such a
>> > class, you could provide a __contains__ method which could answer the
>> > question of what the range contains based on the semantics guaranteed
>> > by a range (in contrast to a normal list).
>>
>> You'd also have to override just about every mutating method to switch
>> back to a "normal" __contains__ (or change self's type on the fly) -- a
>> pretty heavy price to pay.
>
> A subclass of list is probably a bad idea in hindsight, due to various
> probable requirements of it actually needing to be a list with all its
> contents, whereas we wanted to avoid having anything like a list around
> until the contents of this "lazy list" were required by the program. If
> we really wanted to subclass something, we could consider subclassing
> the slice class/type, but that isn't subclassable in today's Python for
> some reason, and it doesn't really provide anything substantial,
> anyway. However, Python being the language it is, an appropriately
> behaving class is quite easily written from scratch.

Except that if you write your own class from scratch, you can't use
it as a slice. For a language that is supposed to be about duck typing
I find it strange that if I make my own class with a start, stop and
step attribute, that python barfs on it when I want to use it as a
slice.

>>> class sl(object):
...   def __init__(self, start = None, stop = None, step = None):
...     self.start = start
...     self.stop = stop
...     self.step = step
... 
>>> lst = range(20)
>>> s1 = slice(3,13)
>>> s2 = sl(3,13)
>>> s1.start
3
>>> s2.start
3
>>> s1.stop
13
>>> s2.stop
13
>>> s1.step
>>> s2.step
>>> lst[s1]
[3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
>>> lst[s2]
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: list indices must be integers
>>> 

-- 
Antoon Pardon