Format numbers with the appropriate SI prefix

si_format.py

def si_format(number, unit=''):
    """
    Formats a number with the appropriate SI prefix.

    Example:
         >>> si_format(500000, unit='B')
         '500 KB'
         >>> si_format(2000000, unit='B')
         '2 MB'
         >>> si_format(1e-6, unit='m')
         '1 μm'
    """
    si_prefixes = ['', 'k', 'M', 'G', 'T', 'P', 'E', 'Z', 'Y', 'y', 'z', 'a', 'f', 'p', 'n', 'μ', 'm']
    try:
        # max(min(...)) to clip list indices to the valid range.
        # floor() yields *max.* 3 places before the decimal separator
        # each SI prefixes increases by a factor of 1000 = 10^3
        # scaling factors for numbers < 1 are achieved with Python's fancy negative indexing
        scaling_index = max(min(math.floor(math.log10(abs(number)) / 3), 8), -8)
    except ValueError:
        # log10(0) is undefined
        scaling_index = 0
    # Formatting 4g = 4 *valid* digits (before or after the decimal separator)
    return f'{number / (1000 ** scaling_index):.4g} {si_prefixes[scaling_index]}{unit}'.rstrip()