I used pip to install all python packages, and the path is:
 PYTHONPATH="/usr/local/lib/python2.7/site-packages"
I found all the packages I tried to install were installed under this path, but when I tried to import them, it always said module not found.
MacBook-Air:~ User$ pip install tweepy
Requirement already satisfied (use --upgrade to upgrade): tweepy in /usr/local/lib/python2.7/site-packages
import tweepy
Traceback (most recent call last):
  File "", line 1, in 
ImportError: No module named tweepy
I tried with tweepy, httplib2, oauth and some others, none of these can work.
Can anyone tell how can I solve this problem?
Thanks!!!!
Feel like comment has too many things..
So as @Zahir Jacobs said, this problem is because pip is installing all packages in different path. After I moved all the packages to $which python path, I can import these modules now.
But the following question is, if I still want to use pip installation in the future, I have to move it by manually again. is there anyway to change the path for pip? 
I tried to move pip package, but it returned:
MacBook-Air:~ User$ pip install tweepy
Traceback (most recent call last):
  File "/usr/local/bin/pip", line 5, in 
    from pkg_resources import load_entry_point
ImportError: No module named pkg_resources
Python2.7 package installed in right path but not found