>>> seq = [1, 2, 3, 4]  
>>> sep = '+'  
>>> sep.join(seq)  
  
Traceback (most recent call last):  
  File "<pyshell#25>", line 1, in <module>  
    sep.join(seq)  
TypeError: sequence item 0: expected string, int found  

>>> seq = ['1', '2', '3', '4']  
>>> sep = '+'  
>>> sep.join(seq)  
'1+2+3+4'