def filesize(value, format='decimal', digits=2):
    '''
    Convert a file size into natural readable format. Multiple formats are
    supported.

    :param value: size
    :param format: default ``decimal``, choices ``binary``, ``decimal`` or
                   ``gnu``
    :param digits: default ``2``

    >>> print(filesize(123))
    123.00 B
    >>> print(filesize(123456))
    120.56 kB
    >>> print(filesize(1234567890))
    1.15 GB
    '''

    if format not in FILESIZE_SUFFIX:
        raise TypeError

    base = FILESIZE_BASE[format]
    size = int(value)
    sign = size < 0 and u'-' or ''
    size = abs(size)

    for i, suffix in enumerate(FILESIZE_SUFFIX[format]):
        unit = base ** (i + 1)
        if size < unit:
            result = u''.join([
                sign,
                _format(base * size / float(unit), digits),
                u' ',
                suffix,
            ])
            if format == 'gnu':
                result = result.replace(' ', '')
            return result

    raise OverflowError