Whenever I start a new project that is likely to have dependencies, I set up virtualenv and create a requirememnts.txt. I am also a big fan of Makefiles. Here is a small snippet that combines these tools:

# Makefile
venv: venv/bin/activate
venv/bin/activate: requirements.txt
    test -d venv || virtualenv venv
    venv/bin/pip install -Ur requirements.txt
    touch venv/bin/activate

So, what does it do? The venv target builds your virtual environment and keeps it in sync with your requirements.txt. If everything is up-to-date, nothing happens. You can use the venv target as a dependency for other targets or call make venv to build or update the virtual environment manually. Just as an example:

devbuild: venv
    venv/bin/python setup.py install

test: devbuild
    venv/bin/python test/runtests.py

Nothing special, but very handy. Especially if your users are new to python and don't know much about virtual environments.