Python: Format a number of bytes into a human readable format

format_bytes_0.py

def format_bytes(n):  # Convert a number of bytes into a human readable format
    symbols = ('KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB')
    prefix = {}
    for i, s in enumerate(symbols):
        prefix[s] = 1 << (i + 1) * 10
    for s in reversed(symbols):
        if n >= prefix[s]:
            value = float(n) / prefix[s]
            return '%.3f%s' % (value, s)  # 3 digits
    return "%sB" % n