The replace_ending function replaces the old string in a sentence with the new string, but only if the sentence ends with the old string.

replace-string.py

def replace_ending(sentence, old, new):
    # Check if the old string is at the end of the sentence
    if sentence.endswith(old):
        # Using i as the slicing index, combine the part
        # of the sentence up to the matched string at the 
        # end with the new string
        i = len(old)
        new_sentence = sentence[:-i]+new
        return new_sentence
    # Return the original sentence if there is no match
    return sentence