list - Python's insert returning None?

Question

#!/usr/bin/python

numbers = [1, 2, 3, 5, 6, 7]

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

print clean
# desire results [1, 2, 3, 'four', 5, 6, 7]

I am getting "None". What am I doing wrong?

Answer 1

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.