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Thank for @dgyx's solution. I also have this problem when I run the roscore command after I have installed Matlab MCR on my Ubuntu 1404 32bit system.

Traceback (most recent call last): File "/opt/ros/indigo/bin/roscore", line 69, in <module> import roslaunch File "/opt/ros/indigo/lib/python2.7/dist-packages/roslaunch/__init__.py", line 48, in <module> import rospkg File "/usr/lib/python2.7/dist-packages/rospkg/__init__.py", line 43, in <module> from .rospack import RosPack, RosStack, \ File "/usr/lib/python2.7/dist-packages/rospkg/rospack.py", line 35, in <module> from xml.etree.cElementTree import ElementTree File "/usr/lib/python2.7/xml/etree/cElementTree.py", line 3, in <module> from _elementtree import * ImportError: PyCapsule_Import could not import module "pyexpat"

I add the library path of my system - "/lib/i386-linux-gnu" - in $LD_LIBRARY_PATH in front of Matlab's lib paths. Then the problem is solved. By the way, only adding system library path in front of Matlab's paths works.

Thank for @dgyx's solution. I also have this problem when I run the roscore command after I have installed Matlab MCR on my Ubuntu 1404 32bit system.

Traceback (most recent call last):
  File "/opt/ros/indigo/bin/roscore", line 69, in <module>
    import roslaunch
  File "/opt/ros/indigo/lib/python2.7/dist-packages/roslaunch/__init__.py", line 48, in <module>
    import rospkg
  File "/usr/lib/python2.7/dist-packages/rospkg/__init__.py", line 43, in <module>
    from .rospack import RosPack, RosStack, \
  File "/usr/lib/python2.7/dist-packages/rospkg/rospack.py", line 35, in <module>
    from xml.etree.cElementTree import ElementTree
  File "/usr/lib/python2.7/xml/etree/cElementTree.py", line 3, in <module>
    from _elementtree import *
ImportError: PyCapsule_Import could not import module "pyexpat"
"pyexpat"