在virtualenv下的Python 2.7：破坏了`site.py`

$virtualenv testenv -p python2.7 --no-site-packages
Running virtualenv with interpreter /usr/bin/python2.7
New python executable in testenv/bin/python2.7
Also creating executable in testenv/bin/python
Installing setuptools............done.
Installing pip...............done.
$cd testenv/
$source bin/activate
(testenv)$python
Python 2.7.1+ (r271:86832,Apr 11 2011,18:05:24) 
[GCC 4.5.2] on linux2
Type "help","copyright","credits" or "license" for more information.
>>> import site
>>> site.getsitepackages()
Traceback (most recent call last):
  File "<stdin>",line 1,in <module>

AttributeError: 'module' object has no attribute 'getsitepackages'

似乎site.py没有Python 2.7中应该有的新功能.

有什么建议？

编辑：即使我不使用–no-site-packages问题仍然存在：

$virtualenv testenv -p python2.7
Running virtualenv with interpreter /usr/bin/python2.7
New python executable in testenv/bin/python2.7
Also creating executable in testenv/bin/python
Installing setuptools............done.
Installing pip...............done.
$cd testenv/
$source bin/activate
(testenv)$python
Python 2.7.1+ (r271:86832,in <module>
AttributeError: 'module' object has no attribute 'getsitepackages'