PATH issue with pytest 'ImportError: No module named ...'

I used easy_install to install pytest on a Mac and started writing tests for a project with a file structure likes so:

repo/
   |--app.py
   |--settings.py
   |--models.py
   |--tests/
          |--test_app.py

Run py.test while in the repo directory, and everything behaves as you would expect.

But when I try that same thing on either Linux or Windows (both have pytest 2.2.3 on them), it barks whenever it hits its first import of something from my application path. For instance, from app import some_def_in_app.

Do I need to be editing my PATH to run py.test on these systems?