Python SI prefix formatting

format_si.py

def format_si(number, significant_digits=3):
    """Format number using SI prefixes
    
    The prefix is chosen such that the number before the prefix is 1<x<1000."""
    if number == 0:
        return '0 '
    prefixes = list('afpnμm') + [''] + list('kMGTPE')
    multipliers = 10.**np.arange(-18, 19, 3)
    inv_multipliers = 10.**(-np.arange(-18, 19, 3))
    abs_number = abs(number)
    index = np.searchsorted(multipliers, abs_number) - 1
    prefix = prefixes[index]
    number *= inv_multipliers[index]
    abs_number = inv_multipliers[index] * abs_number
    if (abs_number > 1000 or abs_number < 1) and (
            index == -1 or index == len(multipliers)-1):
        format_ = '{:.3e} '
        number *= multipliers[index]
    else:
        if abs_number < 10:
            digits_before_comma = 1
        elif abs_number < 100:
            digits_before_comma = 2
        else:
            digits_before_comma = 3
        decimals = max(0, significant_digits - digits_before_comma)
        format_ = '{:.' + str(decimals) + 'f} {}'
    return format_.format(number, prefix)