如何优雅地在Python中创建双向for循环？

Question

As in with out outer scope variables, how do you loop getting the first element forward with one var and another var for the last element backward with the same loop? Is there a function similar to enumerate that returns two vars

Answer 1

you could zip one iterator and its reverse:

z = range(20,30)

for x,y in zip(z,reversed(z)):
    print(x,y)

results in:

20 29
21 28
22 27
23 26
24 25
25 24
26 23
27 22
28 21
29 20

With generators, though, you have to force iteration into a list since reverse expects a sequence

TypeError: argument to reversed() must be a sequence

list , tuple or range are OK.

g = (x for x in somefunc() if x > 0)
lst = list(g)
for x,y in zip(lst, reversed(lst)):
   ...