Question

I understand using 0 and 1 for true and false evaluation, but can someone please explain the use of -1 to me in the following. s is a file name string.

if s.find(".py")==-1:
    return False

Answer 1

The s.find() method returns the index of the given substring in s. It is a number between 0 and len(str) - 1 in case the substring was found. In case it was not found, -1 was chosen as return value because it does not collide with any valid return value for the case that it was found.

Note that this line would be better written as

if ".py" not in s:
    return False

or — with a slightly different but probably intended meaning — as

if not s.endswith(".py"):
    return False