Python: Swapping two strings within a list

The following code works. I used word1 (currentWord) and word2 (newWord) variables as user input

sentList = ['i like math', 'i am a cs expert']

word1 = 'math'
word2 = 'cs'

assert word1, word2 in sentList
sentList = [s.replace(word1, word2) if word1 in s
           else s.replace(word2, word1) if word2 in s
           else s for s in sentList]

If we break it down to steps, it looks like

for i, s in enumerate(sentList):
    if word1 in s:
        sentList[i] = s.replace(word1, word2)
    elif word2 in s:
        sentList[i] = s.replace(word2, word1)

You can achieve it by simple swap function. Don't need to complicate code :)

def swap(str, x, y):
    words = []
    for w in str.split():
        if w == x:
            words.append(y)
        elif w == y:
            words.append(x)
        else:
            words.append(w)
    return ' '.join(words)

if __name__ == '__main__':
    sentList = ['i like math', 'i am a cs expert']
    word1 = 'math'
    word2 = 'cs'
    new_list = [swap(x, word1, word2) for x in sentList]
    print(new_list)

A single liner using list comprehension.

[sent.replace(currentWord,newWord) if sent.find(currentWord)>=0 else sent.replace(newWord,currentWord) for sent in sentList]

So,

IN: sentList = ['i like math', 'i am a cs expert']
IN : currentWord = "math"
IN : newWord = "cs"
OUT : ['i like cs', 'i am a math expert']

Here, if sent.find('math')>=0 will find out if the string conntains 'math' , and if so, replaces it with 'cs', else it replaces 'cs' with 'math'. In case the string contains neither, then too it will print the original, as replace only works if the substring is found.

Edit : as @Rawing pointed out, there were a few bugs in the above code. So here is the new code that will handle every and all cases.

I have used re.sub to handle replacement of only words, and the replacement algorithm is that of how you would swap two variables, say for x and y, where we introduce a temporary variable t to help with the swap: t = x; x = y; y = t. This approach was chosen since we have to do Multiple substitution at the same time.

from re import sub

for s in sentList:

  #replace 'math' with '%math_temp%' in s (here '%math_temp%' is a dummy placeholder that we need to later substitute with 'cs')
  temp1 = sub(r'\bmath\b','%math_temp%' , s)

  #replace 'cs' with 'math' in temp1
  temp2 = sub(r'\bcs\b','math', temp1)

  #replace '%math_temp%' with 'cs' in temp2
  s = sub('%math_temp%','cs', temp2)

  print(s)

So this time on a bigger test case, we get :

IN : sentList = ['i like math', 'i am a cs expert', 'math cs', 'mathematics']
IN : currentWord = "math"
IN : newWord = "cs"

OUT : i like cs
      i am a math expert
      cs math
      mathematics

Assuming that each sentence only contains one word (a sentence can't contain both math and cs) just make a new list and append new strings to it:

newSentList = []
for sent in sentList:
    assert(currentWord in sent != newWord in sent)
    if currentWord in sent:
        newSentList.append(sent.replace(currentWord, newWord))
    else:
        newSentList.append(sent.replace(newWord, currentWord))

You can try like this,

In [20]: currentWord = 'cs'

In [21]: newWord = 'math'

In [22]: [i.replace(currentWord,newWord) if  currentWord in i else i.replace(newWord,currentWord) if newWord in i else i for i in sentList]
Out[22]: ['i like cs', 'i am a math expert']

It's a simple combination of list comprehension and if else conditions.

This is a naive answer (words can be substrings of other words, etc), but following your approach, we can swap wa with wb by

wa=input....
wb=input....
new = [s.replace(wa,"_").replace(wb,wa).replace("_",wb) for s in sentList]

this way a sentence with both is transformed the following way:

"cs is older then math" --> "_ is older then math" --> "_ is older then cs" --> "math is older then cs"