Why I cannot Exit python shell ??

[Jia Lu]

Hi all.

  After using python shell (IDLE) for a while, I typed commands below
to exit . But error ocurred.

>>> raise SystemExit

Traceback (most recent call last):
  File "<pyshell#10>", line 1, in <module>
    raise SystemExit
SystemExit
>>> sys.exit(0)

Traceback (most recent call last):
  File "<pyshell#11>", line 1, in <module>
    sys.exit(0)
SystemExit: 0
>>> dir()
['__builtins__', '__doc__', '__name__', 'os', 'sys', 'time']

Why??