python 反向迭代器reserved

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)