a question about unicode in python

[hzqij]

i have a python source code test.py

# -*- coding: UTF-8 -*-

# s is a unicode string, include chinese
s = u'张三'

then i run

$ python test.py
UnicodeDecodeError: 'utf8' codec can't decode bytes in position 0-1:
invalid data

by in python interactive, it is right

>>> s = u'张三'

why?