ImportError: No module named rospkg Reinstalled still not work

edit

I installed the simulation environment of Nao Players in RobCup. Everything is okay but cannot find Rospkg. I reinstalled with many methods but still not work. In fact, the pkg is there exactly and if I import rospkg in python client, the error occurs. I wonder if there is some problem about Python. Hope someone can help me. Thank you very much

Traceback (most recent call last):
  File "/opt/ros/indigo/share/xacro/xacro.py", line 60, in <module>
    import xacro
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 45, in <module>
    from roslaunch import substitution_args
  File "/opt/ros/indigo/lib/python2.7/dist-packages/roslaunch/__init__.py", line 48, in <module>
    import rospkg

**ImportError: No module named rospkg**

while processing /opt/ros/indigo/share/nao_description/launch/upload_nao.launch:
Invalid <param> tag: Cannot load command parameter [robot_description]: command [/opt/ros/indigo/share/xacro/xacro.py /opt/ros/indigo/share/nao_description/urdf/naoV40_generated_urdf/nao_robot.xacro] returned with code [1]. 

Param xml is <param command="$(find xacro)/xacro.py $(find nao_description)/urdf/nao$(arg version)_generated_urdf/nao_robot.xacro" name="robot_description"/>
The traceback for the exception was written to the log file