python环境下 help命令失效

如题，敲入help，提示如下：

help(os)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: name 'help' is not defined

但是dir命令是ok的，求助，谢谢各位！

Python code

>>> help(os)
Traceback (innermost last):
  File "<stdin>", line 1, in <module>
NameError: name 'os' is not defined

>>> import os
>>> help(os)
Help on module os:

NAME