Python's insert returning None?

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.

The insert method modifies the list in place and does not return a new reference. Try:

>>> numbers = [1, 2, 3, 5, 6, 7]
>>> numbers.insert(3, 'four')
>>> print numbers
[1, 2, 3, 'four', 5, 6, 7]

The list.insert() operator doesn't return anything, what you probably want is:

print numbers