@classmethod
def readable_size(C, bytes, suffix='B', decimals=1, sep='\xa0'):
    """given a number of bytes, return the file size in readable units"""
    if bytes is None:
        return
    size = float(bytes)
    for unit in C.SIZE_UNITS:
        if abs(size) < 1024 or unit == C.SIZE_UNITS[-1]:
            return '{size:.{decimals}f}{sep}{unit}{suffix}'.format(size=
                size, unit=unit, suffix=suffix, sep=sep, decimals=C.
                SIZE_UNITS.index(unit) > 0 and decimals or 0)
        size /= 1024
