How can I truncate an mp3 audio file by 30%?

Question

I am trying to truncate an audio file by 30%, if the audio file was 4 minutes long, after truncating it, it should be around 72 seconds. I have written the code below to do it but it only returns a 0 byte file size. Please tell me where i went wrong?

def loadFile():
    with open('music.mp3', 'rb') as in_file:
        data = len(in_file.read())
        with open('output.mp3', 'wb') as out_file:
            ndata = newBytes(data)
            out_file.write(in_file.read()[:ndata])

def newBytes(bytes):
    newLength = (bytes/100) * 30
    return int(newLength)

loadFile()

Answer 1

You are trying to read your file a second time which will result in no data, e.g. len(in_file.read(). Instead read the whole file into a variable and then calculate the length of that. The variable can then be used a second time.

def newBytes(bytes):
    return (bytes * 70) / 100

def loadFile():
    with open('music.mp3', 'rb') as in_file:
        data = in_file.read()

    with open('output.mp3', 'wb') as out_file:
        ndata = newBytes(len(data))
        out_file.write(data[:ndata])

Also it is better to multiply first and then divide to avoid having to work with floating point numbers.