A quick and dirty way to format SI units.

prefix.py

import math

prefixes = [
    ("yotta",24),
    ("zetta",21),
    ("exa",18),
    ("peta",15),
    ("tera",12),
    ("giga",9),
    ("mega",6),
    ("kilo",3),
    ("",0),
    ("milli",-3),
    ("micro",-6),
    ("nano",-9),
    ("pico",-12),
    ("femto",-15),
    ("atto",-18),
    ("zepto",-21),
    ("yocto",-24)
]

def print_prefix(num,unit,sig_figs):
    if num == 0:
        return "0 " + unit
    dec = int(math.log10(num))-1;
    prefix = [i for i in prefixes if i[1]<=dec]
    if len(prefix) == 0:
        prefix = ("yocto",-24)
    else:
        prefix = prefix[0]
    unrounded_mantissa = num/10**(prefix[1])
    mantissa = round(unrounded_mantissa, sig_figs - int(math.floor(math.log10(abs(unrounded_mantissa)))) - 1)
    return str(mantissa) + " " + prefix[0] + unit
    

#[print(print_prefix(2**(-i),"meters",3)) for i in range(40)]