Strange range

Fri Apr 1 09:26:09 EDT 2016

On Sat, 2 Apr 2016 12:15 am, Marko Rauhamaa wrote:
>> This seems sane:
[...] 
> This not so much:
>> >>> it = range(10)
> >>> for i in it:
> ... if i >= 3:
> ... break
> ...
> >>> list(it)
> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>> Note to self: range(10) is an iterator factory, not an iterator.

Incorrect. range is a lazy sequence.
The only difference between [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] and range(10) is
that the list [0, ..., 9] is calculated eagerly, ahead of time, while
range(10) only generates the values on demand. You can think of range as
equivalent to something close to this:
class Range(object):
 def __init__(self, start, end, step=1):
 self.start = start
 self.end = end
 self.step = step
 def __getitem__(self, index):
 value = self.start + (index-1)*self.step
 if value < self.end:
 return value
 raise IndexError
 def __iter__(self):
 try:
 index = 0
 while True:
 yield self[index]
 index += 1
 except IndexError:
 return
except with more error checking, better bounds checking, support for the
`in` operator, etc.
-- 
Steven