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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