This week someone pushed a nice commit to a Python repository I contribute to, which sped up Travis builds by upgrading pip and using the cache to store requirements. I thought "that's neat, I should try that trick in my own (R)Python projects". And that, friends, is how I entered a whole new world of pain and suffering.
It seems that if you install py.test using pip and cache pip packages in Travis, like this:
I think because PyPy packages its own version of py.test and the two get confused.
On the other hand, if you strip py and py.test from PyPy then you can't translate your interpreter: