It would be nice if it were possible to run tests from outside of the current package when running py.test on the current package. A possible use case for this functionality would be to have a common set of tests for many packages implementing a certain protocol or standard. Another use case is for testing pluggable features (this was the the origin of the idea; see this SO question).
Two things are necessary for this to happen. First, you must get a set of tests contained in another package. Second, you must add that set of tests to the current package.
For getting access to external tests, you can do something similar to what occurs when doing
py.test --collect-only and providing the path to the external package. However, it should additionally be possible to disable all reporting, and to get programmatic access to the tests collected by that run.
For adding external tests to the current collection, one possibility is
pytest_collection_modifyitems. If the tests can be collected programmatically they can just be added to the items list. However, a more explicit hook like
pytest_collection_addexternalitems or something might be preferable.
Since I'd like this for one of my projects, I'm motivated to implement this, but it'd be best to discuss a nice API for doing this before starting an implementation.