找不到模块lxml

Question

我在El Capitan 10.11.6上运行，当我尝试运行我的program.py文件时，我不断收到此错误：

XXX-Air:test darthvader$ ./program.py 
Traceback (most recent call last):
File "./program.py", line 6, in <module>
from lxml import html
ImportError: No module named lxml

当我尝试pip安装lxml时，我继续这样做：

XXX-Air:test darthvader$ pip install lxml
Requirement already satisfied: lxml in /Library/Python/2.7/site-packages

为什么我仍然会收到导入错误？