怎么样：
from itertools import product
def filler(word, from_char, to_char):
options = [(c,) if c != from_char else (from_char, to_char) for c in word]
return (''.join(o) for o in product(*options))
这使
>>> filler("1xxx1", "x", "5")
<generator object <genexpr> at 0x8fa798c>
>>> list(filler("1xxx1", "x", "5"))
['1xxx1', '1xx51', '1x5x1', '1x551', '15xx1', '15x51', '155x1', '15551']
（请注意，您似乎丢失了15x51。）基本上，首先，我们为源单词中的每个字母列出每个可能的目标：
>>> word = '1xxx1'
>>> from_char = 'x'
>>> to_char = '5'
>>> [(c,) if c != from_char else (from_char, to_char) for c in word]
[('1',), ('x', '5'), ('x', '5'), ('x', '5'), ('1',)]
然后，我们使用itertools.product这些可能性的笛卡尔积，并将结果结合在一起。
对于奖励积分，进行修改以接受替换字典。：^）
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