How to run tests without installing package?

I know this question has been already closed, but a simple way I often use is to call pytest via python -m, from the root (the parent of the package).

$ python -m pytest tests

This works because -m option adds the current directory to the python path, and hence mypkg is detected as a local package (not as the installed).

See: https://docs.pytest.org/en/latest/usage.html#calling-pytest-through-python-m-pytest

The normal approach for development is to use a virtualenv and use pip install -e . in the virtualenv (this is almost equivalent to python setup.py develop). Now your source directory is used as installed package on sys.path.

There are of course a bunch of other ways to get your package on sys.path for testing, see Ensuring py.test includes the application directory in sys.path for a question with a more complete answer for this exact same problem.