Python’s insert returning None?

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Mutating-methods on lists tend to return None, not the modified list as you expect — such metods perform their effect by altering the list in-place, not by building and returning a new one. So, print numbers instead of print clean will show you the altered list.

If you need to keep numbers intact, first you make a copy, then you alter the copy:

clean = list(numbers)
clean.insert(3, 'four')

this has the overall effect you appear to desire: numbers is unchanged, clean is the changed list.

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