Okay, so I have the following little function: def swap(inp): inp = inp.split() out = "" for item in inp: ind = inp.index(item) item = item.replace("i am", "you are") item = item.replace("you are", "I am") item = item.replace("i'm", "you're") item = item.replace("you're", "I'm") item = item.replace("my", "your") item = item.replace("your", "my") item = item.replace("you", "I") item = item.replace("my", "your") item = item.replace("i", "you") inp[ind] = item for item in inp: ind = inp.index(item) item = item + " " inp[ind] = item return out.join(inp) Which, while it's not particularly efficient gets the job done for shorter sentences. Basically, all it does is swaps pronoun etc. perspectives. This is fine when I throw a string like "I love you" at it, it returns "you love me" but when I throw something like: you love your version of my couch because I love you, and you're a couch-lover. I get: I love your versyouon of your couch because I love I, and I'm a couch-lover. I'm confused as to why this is happening. I explicitly split the string into a list to avoid this. Why would it be able to detect it as being a part of a list item, rather than just an exact match? Also, slightly deviating to avoid having to post another question so similar; if a solution to this breaks this function, what will happen to commas, full stops, other punctuation? It made some very surprising mistakes. My expected output is: I love my version of your couch because you love I, and I'm a couch-lover. The reason I formatted it like this, is because I eventually hope to be able to replace the item.replace(x, y) variables with words in a database.