在虚拟环境中通过PIP安装后，并发期货导入失败

Question

在python 2.7.13上安装了成功期货，有人想知道为什么不能导入它？

[user@server development]$ source app/bin/activate
(app) [user@server development]$ pip install futures
Requirement already satisfied: futures in ./app/lib/python2.7/site-packages
(app) [user@server development]$ python
Python 2.7.13 (default, Jan 18 2017, 18:22:53)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-55)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import futures
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named futures

提前感谢。