six module

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hello ; i have a problem during executing this command in the terminal """ rosws init ~/fuerte_workspace /opt/ros/fuerte """ the output is

Traceback (most recent call last): File "/usr/local/bin/rosws", line 52, in <module> import rosinstall.rosws_cli File "/usr/local/lib/python2.7/dist-packages/rosinstall/__init__.py", line 33, in <module> import wstool.helpers File "/usr/local/lib/python2.7/dist-packages/wstool/helpers.py", line 37, in <module> from wstool.config_elements import SetupConfigElement File "/usr/local/lib/python2.7/dist-packages/wstool/config_elements.py", line 38, in <module> from vcstools.vcs_abstraction import get_vcs_client File "/usr/local/lib/python2.7/dist-packages/vcstools/__init__.py", line 44, in <module> from vcstools.svn import SvnClient File "/usr/local/lib/python2.7/dist-packages/vcstools/svn.py", line 44, in <module> import dateutil.parser # For parsing date strings File "/usr/local/lib/python2.7/dist-packages/dateutil/parser.py", line 24, in <module> from six import text_type, binary_type, integer_types ImportError: No module named six

plz ; how can i fix this error ????