问Python反转了元组的某些部分

EN

a = (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20)
new_a = list(a)
for c, i in enumerate(range(0, len(a), 4)):
  if c%2: 
     new_a[i:i+4] = new_a[i:i+4][::-1]

print(tuple(new_a))

(1, 2, 3, 4, 8, 7, 6, 5, 9, 10, 11, 12, 16, 15, 14, 13, 17, 18, 19, 20)

a = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]

for n in range(0, len(a), 8):
    a[n+4:n+8] = a[n+7:n+3:-1]

print(a)

from itertools import zip_longest as zipl
from itertools import cycle, chain

_marker = object()

def cycle_map(iterable, func_iterable):
    funcs = cycle(func_iterable)
    for func, item in zip(funcs, iterable):
        if func:
            yield func(item)
        else:
            yield item

def reverse_filter(iterable, remove=_marker):
    t = tuple(i for i in iterable if i is not remove)
    return reversed(t)

def reverse_alternating(iterable, length=4):
    chunks = zipl(*[iter(iterable)]*length, fillvalue=_marker)
    funcs = (None, reverse_filter) 
    return chain.from_iterable(cycle_map(chunks, funcs))

a = (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20)
list(reverse_alternating(a))
# [1, 2, 3, 4, 8, 7, 6, 5, 9, 10, 11, 12, 16, 15, 14, 13, 17, 18, 19, 20]