leetcode Letter Combinations of a Phone Number python

class Solution(object):
def letterCombinations(self, digits):
"""
:type digits: str
:rtype: List[str]
"""
if len(digits) <= 0:
return list()
alpha = ["","1","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"]
res=[]
word=[]
def dfs(cur):
if cur >= len(digits):
res.append(''.join(word))
else:
for x in alpha[int(digits[cur])-(int)('0')]:
word.append(x)
dfs(cur+1)
word.pop()
dfs(0)

return res