string - Python replace function [replace once]

Question

I need help with a program I'm making in Python.

Assume I wanted to replace every instance of the word "steak" to "ghost" (just go with it...) but I also wanted to replace every instance of the word "ghost" to "steak" at the same time. The following code does not work:

 s="The scary ghost ordered an expensive steak"
 print s
 s=s.replace("steak","ghost")
 s=s.replace("ghost","steak")
 print s

it prints: The scary steak ordered an expensive steak

What I'm trying to get is The scary steak ordered an expensive ghost

Answer 1

I'd probably use a regex here:

>>> import re
>>> s = "The scary ghost ordered an expensive steak"
>>> sub_dict = {'ghost':'steak','steak':'ghost'}
>>> regex = '|'.join(sub_dict)
>>> re.sub(regex, lambda m: sub_dict[m.group()], s)
'The scary steak ordered an expensive ghost'

Or, as a function which you can copy/paste:

import re
def word_replace(replace_dict,s):
    regex = '|'.join(replace_dict)
    return re.sub(regex, lambda m: replace_dict[m.group()], s)

Basically, I create a mapping of words that I want to replace with other words (sub_dict). I can create a regular expression from that mapping. In this case, the regular expression is "steak|ghost" (or "ghost|steak" -- order doesn't matter) and the regex engine does the rest of the work of finding non-overlapping sequences and replacing them accordingly.

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Some possibly useful modifications

• regex = '|'.join(map(re.escape,replace_dict)) -- Allows the regular expressions to have special regular expression syntax in them (like parenthesis). This escapes the special characters to make the regular expressions match the literal text.
• regex = '|'.join(r'{0}'.format(x) for x in replace_dict) -- make sure that we don't match if one of our words is a substring in another word. In other words, change he to she but not the to tshe.