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
--
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