>>> size_as_bytes(7.5, 'T')
    8246337208320

def size_as_bytes(size_, prefix):
    """
    >>> size_as_bytes(7.5, 'T')
    8246337208320
    """
    prefix = prefix.upper()

    assert prefix in si_prefixes

    exponent = si_prefixes.index(prefix) + 1

    return int(size_ * (1024.0 ** exponent))