error when installing nltk ImportError: no module named yaml

1.Download PyYAML

 http://pyyaml.org/download/pyyaml/PyYAML-3.10.tar.gz

 

2.Unpack the archive and install the package by executing

  $python setup.py install

 

3.The source distribution includes a comprehensive test suite. To run the tests, type  

  $python setup.py test

Reference: http://pyyaml.org/wiki/PyYAML