[mtai84]

You can explicitly specify which version of a package you want to install. For example, if you wanted to install an older version of Tornado,

pip install tornado==1.1

OR if you know you ONLY want tornado 2.0

pip install tornado==2.0

From my experience, pip has been able to fetch PYTHON dependencies if the author of the package wrote their setup.py correctly. I agree that things could be better with regards to C dependencies (MySQL-Python requiring libmysql5dev being particularly annoying) but this is hard to do as every environment has their own way of naming dependent libs.