Replace substring in a string using a list

Question

I'm looking for a more pythonic way to replace a known length of placeholders in a string with the values found in a list. They should replace in order and only be used once. For example with the values:

replaceVals = ['foo', 'bar']
origStr = 'I went to the ? to get ?'

I am looking to get:

newStr = 'I went to the foo to get bar'

I was able to get the desired result with the following loop, but I feel like there should be a better way to go about this than using a loop like so.

for i in range(len(replaceVals)):
 origStr = origStr.replace('?', replaceVals[i], 1)

• roganjosh: origStr = 'I went to the {} to get {}'.format(*replaceVals)
• Travis Griggs: Do people get points for trying to point out duplicates? Just curious. Seems to be a real desire of late to not allow anything near a duplicate. I certainly don't want SO filled with the exact same question 100 times, but I don't mind 3 or 4 of pretty similar questions, just because it helps others with similar problems triangulate their own better.
• Travis Griggs: I guess I don't see @roganjosh. Like coding, I reduce/combine/refactor common patterns, once they've matured and I can prove that they really are. Premature "this looks like it's gonna be that" before it's matured, doesn't work for me.
• Travis Griggs: I like your answer, just been burned too many times on SO lately where I post a question and someone slaps down the "looks like a duplicate folks" and often kills the chance of anyone looking at it any further. The "no, I went and looked at it and let me clarify why my case is different" ends up being too late.
• Indominus: @TravisGriggs, maybe SO can have 3 types of dups: 1. easy-to-find exact dups, should be downvoted and closed; 2. hard-to-find exact dups, should be just closed with a reference link with no penalty to OP, some of us come to SO because we lack a certain knowledge, and due to lack of that very same knowledge, we couldn't recognize our questions were dups.; 3. similar questions, still worth discussing with reference links to similar questions under it, think it as teachers giving students practice problems of slight variations but testing the same concept.

Answer 1

You can use the replace and format methods of the string as shown below:

origStr.replace('?','{}').format(*replaceVals)
Out[334]: 'I went to the foo to get bar'

Answer 2

Here's an idea using generators:

replaceVals = iter(['foo', 'bar'])
origStr = 'I went to the ? to get ?'
(' ').join(next(replaceVals) if i == '?' else i for i in origStr.split())

Output:

'I went to the foo to get bar'

---

The advantage of doing it this way, is that the amount of items in replaceVals does not have to match the amount of items to be replaced in origStr:

replaceVals = iter(['foo', 'bar', 'other'])
origStr = 'I went to the ? to get ?'
(' ').join(next(replaceVals) if i == '?' else i for i in origStr.split())
#'I went to the foo to get bar'

However using string formatting would cause errors under these circumstances.

Answer 3

@roganjosh 's answer in comments is possibly the best, though the OP is abstract enough that it's not clear what his real case is. I was curious whether one could do this with f-strings which showed up in Python3. What makes an f-string less appealing than @roganjosh's, is that it's just so easy to unpack the replacement sequence through the .format() call. That said, IF you wanted to try an f-string, something like this would work:

replaceVals = ['foo', 'bar'] 
stream = iter(replaceVals)
f'I went to the {stream.next()} to get {stream.next()}'

Answer 4

f string

r=["foo","bar"]
origStr = f'I went to the {r[0]} to get {r[1]}'
origStr
Out[21]: 'I went to the foo to get bar'

Answer 5

You can split the string with '?', then use itertools.zip_longest to pair substrings in the resulting list with replacement strings in replaceVals with an empty string as a fill value, and join the pairs of strings after flattening them with a generator expression:

from itertools import zip_longest
''.join(i for p in zip_longest(origStr.split('?'), replaceVals, fillvalue='') for i in p)