Python 如何反方向迭代一个序列

>>> a = [1, 2, 3, 4]
>>> for x in reversed(a):
...   print(x)
...
4
3
2
1

# Print a file backwards
f = open('somefile')
for line in reversed(list(f)):
  print(line, end='')

class Countdown:
  def __init__(self, start):
    self.start = start

  # Forward iterator
  def __iter__(self):
    n = self.start
    while n > 0:
      yield n
      n -= 1

  # Reverse iterator
  def __reversed__(self):
    n = 1
    while n <= self.start:
      yield n
      n += 1

for rr in reversed(Countdown(30)):
  print(rr)
for rr in Countdown(30):
  print(rr)