python中有办法在给定范围内往复循环一个变量吗？

Question

Hi，在python中有简单的写法可以让我在给定范围内往复循环一个变量吗？如，给定range(4)，我希望变量像这样变化：0,1,2,3,2,1,0,1,2,3....直到变化指定的次数或者其它条件满足为止。

Answer 1

pythonimportitertools
defbounce(interval):
"""Infinitebounceforspecificrangelist."""
down=reversed(interval[1:-1])
returnitertools.cycle(itertools.chain(interval,down))
#>>>list(itertools.islice(bounce(range(5)),20))
#[0,1,2,3,4,3,2,1,0,1,2,3,4,3,2,1,0,1,2,3]
                            

Answer 2

itertools.cycle(iterable)
Makeaniteratorreturningelementsfromtheiterableandsavingacopyofeach.Whentheiterableisexhausted,returnelementsfromthesavedcopy.Repeatsindefinitely.Equivalentto:
defcycle(iterable):
#cycle('ABCD')-->ABCDABCDABCD...
saved=[]
forelementiniterable:
yieldelement
saved.append(element)
whilesaved:
forelementinsaved:
yieldelement
Note,thismemberofthetoolkitmayrequiresignificantauxiliarystorage(dependingonthelengthoftheiterable).