si.py

import bisect

from operator import itemgetter

units = [
    (1e-24, 'y'),
    (1e-21, 'z'),
    (1e-18, 'a'),
    (1e-15, 'f'),
    (1e-12, 'p'),
    (1e-9, 'n'),
    (1e-6, 'u'),
    (1e-3, 'm'),
    (1e-2, 'c'),
    (1e-1, 'd'),
    (1e3, 'k'),
    (1e6, 'M'),
    (1e9, 'G'),
    (1e12, 'T'),
    (1e15, 'P'),
    (1e18, 'E'),
    (1e21, 'Z'),
    (1e24, 'Y'),
]

idx = bisect.bisect(map(itemgetter(0), units), 42e6) - 1
magnitude, postfix = units[idx]
print 42e6 / magnitude, postfix