When you install a package for the first time using pip, it will download the package into a build directory, unzip it there and install from there. Even if you uninstall it, the copy in the build directory remains intact. The next time you reinstall, pip checks if there is already a version in the build. If its there, then it directly installs that without downloading it again.
This is usually fine, unless you uninstalled in order to install a different version. In that case you might be mystified as to why the same old version is installed again.
pip install pycrypto==2.3The solution to this is to simply delete the package copy from the build directory.
pip uninstall pycrypto
pip install pycrypto==2.0.1 # still installs pycrypto 2.3!!
If you are using virtualenv, then the build directory will be a top-level folder inside the virtualenv.