-1 length
 -1 0 1 2 3 4 5 6 7 8 9      index
    - - - - - - - - -
    4 3 0 7 3 9 2 3 5        entires
       
Search the array for a number.
What if the number is in the array several times? What answer do you give? first, last, all
What if the number is not in the array? What answer do you give? -1, length
       
 "mississippi".indexOf("s")
    
In Python, if an array a has length 9, then
    a[-1] == a[8]
    a[-2] == a[7]
So in Python, -1 is not an invalid index.
So in Python, -1 in not a good answer for "not found".