Convert strings with SI prefixes to a floating point number

si_parse.py

import re

def si_parse(s):
    """
    Convert a string `s` with SI prefix scaling to a float.

    Example:
         >>> si_parse('1k')
         1000.0
         >>> si_parse('1000 mm')
         1.0
         >>> si_parse('1000 m') # m is interpreted as prefix, not unit!
         1.0
         >>> si_parse('2TB')
         2000000000000.0
         >>> si_parse('-1.5μm')
         -1.5e-06
    """
    si_str_regex = r'(?P<value>[-+]?[0-9]*\.?[0-9]+)(?:\s*(?P<si_prefix>[yzafpnμumkMGTPEZY])(?P<unit>[A-z]*))?'
    si_prefixes = ['y', 'z', 'a', 'f', 'p', 'n', 'u', 'm', None, 'k', 'M', 'G', 'T', 'P', 'E', 'Z', 'Y']
    # index of the (empty) unity scaling prefix (encoded as None)
    # indices to the left scale with <1, indices to the right with >1
    index_offset = 8 
    # handle unicode \mu
    s = s.replace('μ', 'u')
    match = re.match(si_str_regex, s).groupdict()
    value = float(match['value'])
    # if there is no SI prefix in the string, match['si_prefix'] is None,
    #  which is found in si_prefixes at the correct index, leading to scaling = 1.
    # factor of 3 because the steps between SI prefixes scale by 1000 (instead of 10)
    scaling = 10 ** (3 * (si_prefixes.index(match['si_prefix']) - index_offset))
    return scaling * value