To elaborate on Ignacio Vazquez-Abrams’s answer:
The Python import mechanism works relative to the __name__ of the current file. When you execute a file directly, it doesn’t have its usual name, but has “__main__” as its name instead. So relative imports don’t work.
You can, as Igancio suggested, execute it using the -m option. If you have a part of your package that is meant to be run as a script, you can also use the __package__ attribute to tell that file what name it’s supposed to have in the package hierarchy.
See http://www.python.org/dev/peps/pep-0366/ for details.
Yes. You’re not using it as a package.
python -m pkg.tests.core_test