foo.py

convertion = [
    (1024 ** 5, 'P'),
    (1024 ** 4, 'T'),
    (1024 ** 3, 'G'),
    (1024 ** 2, 'M'),
    (1024 ** 1, 'K'),
    (1024 ** 0, 'B'),
]

def size_suffix(bytes):
    bytes = int(bytes)
    for factor, suffix in convertion:
        if bytes >= factor:
            break
    total = bytes / factor
    return str(total) + suffix