Why is this function not returning a pyg-latin value?

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Python

My code:

#Problem: Move the first letter of each word to the end of it, then add #"ay" to the end of the word. Leave punctuation marks untouched.

#Example: "The train is fast !" --> "heTay raintay siay astfay !"

def pig_it(text):
    import string
    words = text.split()
    for word in words:
        first_letter = word[0]
        word += first_letter
        word = word[1:]
        if word not in string.punctuation:
            word += ("ay")
    new_words = ((" ").join(words))
    return(new_words)


pyg_latin = pig_it("The train is fast !")
print(pyg_latin)

The output returns "The train is fast !" when it should return the pyg latin value.

What I have tried:

I tried debugging and I got some results. during the for loop, it looks like each word (element) gets changed to its respective pyg-latin value correctly, but after the for loop, the words list gets changed back to its original value.