"""Convert file sizes to human-readable form.

Available functions:
approximate_size(size, a_kilobyte_is_1024_bytes)
    takes a file size and returns a human-readable string

Examples:
>>> approximate_size(1024)
'1.0 KiB'
>>> approximate_size(1000, False)
'1.0 KB'

"""

SUFFIXES = {1000: ('KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'),
            1024: ('KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB')}

def approximate_size(size, a_kilobyte_is_1024_bytes=True):
    """Convert a file size to human-readable form.

    Keyword arguments:
    size -- file size in bytes
    a_kilobyte_is_1024_bytes -- if True (default), use multiples of 1024
                                if False, use multiples of 1000

    Returns: string

    """
    if size < 0:
        raise ValueError('number must be non-negative')

    multiple = 1024 if a_kilobyte_is_1024_bytes else 1000
    for suffix in SUFFIXES[multiple]:
        size /= multiple
        if size < multiple:
            return "{0:.1f} {1}".format(size, suffix)

    raise ValueError('number too large')

if __name__ == "__main__":
    print(approximate_size(1000000000000, False))
    print(approximate_size(1000000000000))