To address the two examples you brought up:
import itertools
data1 = range(10)
# This creates a NEW list
data1[2:5]
# This creates an iterator that iterates over the EXISTING list
itertools.islice(data1, 2, 5)
data2 = [1, 2, 3]
data3 = [4, 5, 6]
# This creates a NEW list
data2 + data3
# This creates an iterator that iterates over the EXISTING lists
itertools.chain(data2, data3)
There are many reasons why you'd want to use iterators instead of the other methods. If the lists are very large, it could be a problem to create a new list containing a large sub-list, or especially create a list that has a copy of two other lists. Using islice()
or chain()
allows you to iterate over the list(s) in the way you want, without having to use more memory or computation to create the new lists. Also, as unutbu mentioned, you can't use bracket slicing or addition with iterators.
I hope that's enough of an answer; there are plenty of other answers and other resources explaining why iterators are awesome, so I don't want to repeat all of that information here.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…