我的2.7版本的python怎么貌似用不了join函数

>>> list=['a','b','c']
>>> sep='|'
>>> join(list,sep)
Traceback (most recent call last):
   File "<pyshell#7>", line 1, in <module>
     join(list,sep)
NameError: name 'join' is not defined
>>> list=['a','b','c']
>>> join(list)
Traceback (most recent call last):
   File "<pyshell#9>", line 1, in <module>
     join(list)
NameError: name 'join' is not defined
不明白为什么 求指点 谢了