学python时我遇到了''.join()失效的情况，求大佬指点

import time as t
class timer:

    def __init__(self):
        print('这是一个计时器')
        self.unitlist=['年','月','日','时','分','秒']
        
    def __str__(self):
        return self.thepast
    
    __repr__=__str__
    

    
    def start(self):
        self.begin=t.localtime()
        self.thepast=[]
        self.numlist=[]
        
    def stop(self):
        self.end=t.localtime()
        for i in range(6):
            self.numlist.append(str(self.end[i]-self.begin[i]))
            self.numlist.append(self.unitlist[i])
        
        self.thepast='经过了'+(''.join(str(self.numlist)))

"".join(列表)
# 不是字符串

''.join(self.numlist)