No module named "yaml"

edit

I installed ROS on Ubuntu 11.10 using the complete desktop install from ros.org. However, when I try to run roscore, I get the following error:

    `Traceback (most recent call last):
  File "/opt/ros/electric/ros/bin/roscore", line 34, in <module>
    from ros import roslaunch
  File "/opt/ros/electric/ros/core/roslib/src/ros/__init__.py", line 57, in __getattr__
    return __import__(name)
  File "/opt/ros/electric/stacks/ros_comm/tools/roslaunch/src/roslaunch/__init__.py", line 43, in <module>
    import roslaunch.param_dump
  File "/opt/ros/electric/stacks/ros_comm/tools/roslaunch/src/roslaunch/param_dump.py", line 43, in <module>
    import yaml
ImportError: No module named yaml`

I found someone with the same problem here: http://answers.ros.org/question/12862/nao-error-no-module-named-yaml

I also tried to get the yaml package using, "sudo apt-get install build-essential python-yaml cmake subversion wget python-setuptools mercurial git-core", but terminal then says that python-yaml is already updated. How can I get this yaml module working properly?