Import error: No module named 'glob'

edit

Dear all, I got an import error from executing roslaunch pr2_gazebo pr2_no_controllers.launch:

ros@ros-K401UB:~/research/gps$ roslaunch pr2_gazebo pr2_no_controllers.launch 
... logging to /home/ros/.ros/log/751a586e-bd85-11e7-a646-9c5c8ed99f00/roslaunch-ros-K401UB-4556.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

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 36, in <module>
    import glob
ModuleNotFoundError: No module named 'glob'

while processing /opt/ros/indigo/share/pr2_description/robots/upload_pr2.launch:
Invalid <param> tag: Cannot load command parameter [robot_description]: command [/opt/ros/indigo/share/xacro/xacro.py '/opt/ros/indigo/share/pr2_description/robots/pr2.urdf.xacro'] returned with code [1]. 

Param xml is <param command="$(find xacro)/xacro.py '$(find pr2_description)/robots/pr2.urdf.xacro'" name="robot_description"/>
The traceback for the exception was written to the log file

However, I can import glob in python2.7:

ros@ros-K401UB:~/research/gps$ python2.7
Python 2.7.6 (default, Oct 26 2016, 20:30:19) 
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import glob
>>>

Also, I can locate it under /usr/lib/python2.7:

ros@ros-K401UB:/usr/lib/python2.7$ ls | grep glo
glob.py
glob.pyc

Thus, I guess that the problem occurs because the python executable which runs xacro.py can't locate the module glob in its path. So I tried to append /usr/lib/python2.7 to my PYTHONPATH, but this doesn't work.

Does anyone know how to solve this issue? Thanks for any suggestion : )

P.S. I am using Ubuntu 14.04+ROS Indigo, and my python -m site output this:

ros@ros-K401UB:~/research/gps$ python -m site
sys.path = [
    '/home/ros/research/gps',
    '/home/ros/research/tool/rllab',
    '/opt/ros/indigo/lib/python2.7/dist-packages',
    '/usr/lib/python2.7/dist-packages',
    '/home/ros/miniconda3/lib/python36.zip',
    '/home/ros/miniconda3/lib/python3.6',
    '/home/ros/miniconda3/lib/python3.6/lib-dynload',
    '/home/ros/miniconda3/lib/python3.6/site-packages',
    '/home/ros/code/gym',
    '/home/ros/miniconda3/lib/python3.6/site-packages/setuptools-27.2.0-py3.6.egg',
]
USER_BASE: '/home/ros/.local' (exists)
USER_SITE: '/home/ros/.local/lib/python3.6/site-packages' (doesn't exist)
ENABLE_USER_SITE: True

Reference:

1. http://forum.erlerobotics.com/t/solve...
2. https://stackoverflow.com/questions/4...