If you're doing some sort of math with the numbers you might also want to know the units. Given your input restrictions (that the input string contains unit and value only), this should correctly return both (you'll just need to figure out how to convert units into common units for your math).
def unit_value(str):
m = re.match(r'([^\d]*)(\d*\.?\d+)([^\d]*)', str)
if m:
g = m.groups()
return ' '.join((g[0], g[2])).strip(), float(g[1])
else:
return int(str)
