为什么python-c“…”加载的dateutil与python file.py不同

为什么会这样:

$ python -c "import dateutil; print dateutil.__version__"
2.6.0

但这会失败,test.py:

import dateutil; print dateutil.__version__

当运行方式

$ python test.py 
Traceback (most recent call last):
  File "test.py ", line 3, in <module>
    import dateutil; print dateutil.__version__
AttributeError: 'module' object has no attribute '__version__'

相同的python解释器、相同的bash会话和环境都用于:

$ python
Python 2.7.6 (default, Jun 22 2015, 17:58:13) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>