def format_bytes(nbytes):
    """Format ``nbytes`` as a human-readable string with units"""
    if nbytes > 2 ** 50:
        return '%0.2f PiB' % (nbytes / 2 ** 50)
    if nbytes > 2 ** 40:
        return '%0.2f TiB' % (nbytes / 2 ** 40)
    if nbytes > 2 ** 30:
        return '%0.2f GiB' % (nbytes / 2 ** 30)
    if nbytes > 2 ** 20:
        return '%0.2f MiB' % (nbytes / 2 ** 20)
    if nbytes > 2 ** 10:
        return '%0.2f KiB' % (nbytes / 2 ** 10)
    return '%d B' % nbytes
