leetcode Roman to Integer python

class Solution(object):
def romanToInt(self, s):
"""
:type s: str
:rtype: int
"""
numerals = { "M": 1000, "D": 500, "C": 100, "L": 50, "X": 10, "V": 5, "I": 1 }

sums=0
s=s[::-1]
last=None

for x in s:
if last and numerals[x] < last:
sums-=2*numerals[x]
sums+=numerals[x]
last=numerals[x]
return sums