Why does replace not work for one element?

Question

I did the quiz on the Python Crash Course on Google. But in the first version of my code, when I used the replace function, I did not get the result for the sentence "It is raining cats and cats". It gave me "It is raining dogs and dogs". But the correct result should be "It is raining cats and dogs". When I used formatting instead of using replace, it worked perfectly. Can you explain why replace doesn't work right in my example?

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(sentence) - len(old)
        new_sentence = sentence.replace(sentence[i:],new)
        return new_sentence

    # Return the original sentence if there is no match 
    return sentence
    
print(replace_ending("It's raining cats and cats", "cats", "dogs")) 
# Should display "It's raining cats and dogs"
print(replace_ending("She sells seashells by the seashore", "seashells", "donuts")) 
# Should display "She sells seashells by the seashore"
print(replace_ending("The weather is nice in May", "may", "april")) 
# Should display "The weather is nice in May"
print(replace_ending("The weather is nice in May", "May", "April")) 
# Should display "The weather is nice in April"

It's raining dogs and dogs
She sells seashells by the seashore
The weather is nice in May
The weather is nice in April

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(sentence) - len(old)
        new_sentence = "{}{}".format(sentence[:i],new)
        return new_sentence

    # Return the original sentence if there is no match 
    return sentence
    
print(replace_ending("It's raining cats and cats", "cats", "dogs")) 
# Should display "It's raining cats and dogs"
print(replace_ending("She sells seashells by the seashore", "seashells", "donuts")) 
# Should display "She sells seashells by the seashore"
print(replace_ending("The weather is nice in May", "may", "april")) 
# Should display "The weather is nice in May"
print(replace_ending("The weather is nice in May", "May", "April")) 
# Should display "The weather is nice in April"

It's raining cats and dogs
She sells seashells by the seashore
The weather is nice in May
The weather is nice in April

Answer 1

You start along the right lines with the slicing but then pretty much throw it away by putting it back inside a call to replace which replaces all instances

Your second snippet doesn't do this, it just joins together the old string up to the slice, as well as the new string you wish to replace with appended to the end