String Replacement Combinations

by

How about:

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))

which gives

>>> filler("1xxx1", "x", "5")
<generator object <genexpr> at 0x8fa798c>
>>> list(filler("1xxx1", "x", "5"))
['1xxx1', '1xx51', '1x5x1', '1x551', '15xx1', '15x51', '155x1', '15551']

(Note that you seem to be missing 15x51.)
Basically, first we make a list of every possible target for each letter in the source word:

>>> 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',)]

And then we use itertools.product to get the Cartesian product of these possibilities and join the results together.

For bonus points, modify to accept a dictionary of replacements. :^)

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