Pretty print with unit and physical suffix

# -*- coding: utf-8 -*-

import math
def eng_format(x, unit=''):
    e =(math.floor(math.log10(x))//3)*3
    sL = ['f', 'p', 'n', 'u', 'm', '', 'k', 'M', 'G']

    idx = int(e/3) + sL.index('')

    if idx=len(sL):
        s = '*10^%d '%e
    else:
        s = sL[idx]

    nx = x * 10**(-e)
    return '%.2f%s%s'%(nx, s, unit)

if __name__ == '__main__':
    print eng_format(0.002, 'V')
    print eng_format(0.000003221, 'A')
    print eng_format(30589, 'A')
    print eng_format(60000000, 'W')

2.00mV
3.22uA
30.59kA
60.00MW

• Use better variable naming, with proper vocabulary. In particular, "significand" is the term to use instead of nx.

• You multiplied some intermediate result by 3 to obtain e. Then you immediately do idx = int(e/3) + …, which represents wasted work. More importantly, clarity suffers. I suggest replacing e with power_of_1000 = int(math.floor(math.log10(x) // 3)), whose intent is much clearer.

• You can take advantage of the fact that negative indexes count backwards from the end of a list.

• There should be a Space in front of each prefix for consistency with the '*10^%d ' format.