humansize.py

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
    size_of_unit = 1
    for unit_suffix in SUFFIXES[multiple]:
        size_of_unit *= multiple
        number_of_units = float(size) / size_of_unit
        if number_of_units < multiple:
            return "{0:.1f} {1}".format(number_of_units, unit_suffix)

    """
        Can someone tell me how the above for loop selects the correct suffix? It does it as if by magic.
        I understand how everything else works, but don't understand how the right suffix within the list
        is selected.
    """

    raise ValueError("Number too large")

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

a little, thanks :)