这是一个简单的凯撒密码算法。它适用于下面的函数docstrings中的测试用例。但对于给定的加密“寓言”，它很快就会停止递归。什么失败了？
以下是相关功能。至于apply_shift和is_word，它们完全听起来像它们。转变是角色的常规凯撒转变。轮班模板看起来像'abcdefghijklmnopqrstuvwxyz'，大写字母也是如此。
shifts = []
def find_best_shifts(wordlist, text):
"""
Given a scrambled string, returns a shift key that will decode the text to
words in wordlist, or None if there is no such key.
Hint: Make use of the recursive function
find_best_shifts_rec(wordlist, text, start)
wordlist: list of words
text: scambled text to try to find the words for
returns: list of tuples. each tuple is (position in text, amount of shift)
Examples:
>>> s = random_scrambled(wordlist, 3)
>>> s
'eqorqukvqtbmultiform wyy ion'
>>> shifts = find_best_shifts(wordlist, s)
>>> shifts
[(0, 25), (11, 2), (21, 5)]
>>> apply_shifts(s, shifts)
'compositor multiform accents'
>>> s = apply_shifts("Do Androids Dream of Electric Sheep?", [(0,6), (3, 18), (12, 16)])
>>> s
'JufYkaolfapxQdrnzmasmRyrpfdvpmEurrb?'
>>> shifts = find_best_shifts(wordlist, s)
>>> print apply_shifts(s, shifts)
Do Androids Dream of Electric Sheep?
"""
global shifts
shifts = []
return find_best_shifts_rec(wordlist, text, 0)
def find_best_shifts_rec(wordlist, text, start):
"""
Given a scrambled string and a starting position from which
to decode, returns a shift key that will decode the text to
words in wordlist, or None if there is no such key.
Hint: You will find this function much easier to implement
if you use recursion.
wordlist: list of words
text: scambled text to try to find the words for
start: where to start looking at shifts
returns: list of tuples. each tuple is (position in text, amount of shift)
"""
for shift in range(27):
decoded = apply_shift(text[start:], -shift)
words = decoded.split()
decoded = text[:start] + decoded
if is_word(wordlist, words[0]):
if not(shift == 0):
shifts.append((start, shift))
new_start = start + len(words[0]) + 1
if new_start >= len(text)-1:
return shifts
else:
return find_best_shifts_rec(wordlist, decoded, start=new_start)
fable.txt
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输出：
>>> decrypt_fable()
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No shift found