Python对含有数字的字符串进行排序

def _sort(list,b,a):
    '''
    list :待排列数组
    b:数字前一个字符
    a;数字后一个字符
    '''
    list.sort(key = lambda x:int(x.split(a)[0].split(b)[1]))
    return list

x = ["py1.py", "py2.py", "py100.py", "py20.py"]
y = _sort(x,'y','.')