Fade-out sound in Python

This is my fade_out function.

def fade_out (snd, fade_length):
    new_snd = sound.copy(snd)
    factor = 1.0/fade_length
    fade_o = 1
    for sample in new_snd:
        sound.set_left(sample, int(sound.get_left(sample)*fade_o))
        sound.set_right(sample, int(sound.get_right(sample)*fade_o))
        if fade_o > 0:
            fade_o = fade_o - factor
    return new_snd

The first problem I have is with this part:

    if fade_o > 0:
       fade_o = fade_o - factor

I don't understand why face_o is only subtracted from once. I thought fade_o would continue to be subtracted from until it reaches 0.

The second problem I have is that I don't know how to make the fade-out start on the time I want. For example, if the sound length is 600 000 and the fade length is 100 000, I want to start fade out at an offset of 500 000 (which is 600 000 - 100 000). I am not sure how t开发者_运维知识库o do this.

I thought of something along these lines:

    for sample in new_snd:
        if sound.get_index(sample) > (len(snd)-fade_length):
            sound.set_left(sample,........)

But, this doesn't seem to be working.