Python: 'ImportError: attempted relative import with no known parent package' (or 'ModuleNotFoundError' for my own package) when running a script directly. How do I fix it?

Cause: relative imports (from . import utils, from ..core import x) only work when Python knows which package the module belongs to. When you run a file directly (python mypkg/cli.py), Python loads it as the top-level module __main__ with no parent package, so any relative import fails. Running from a different working directory, an IDE run button, or a test runner configured for the wrong root cause the same problem, and the ModuleNotFoundError variant appears when the project root isn't on sys.path. Fix (best first): 1. Run it as a module from the project root: python -m mypkg.cli (no .py, dots instead of slashes). Python then sets the package context and relative imports work. This is the documented way to run modules inside packages. 2. Make the project installable and install it in editable mode: add a pyproject.toml, then pip install -e . in your virtual environment. Imports then work from any directory, in tests too. Define a console script entry point in pyproject.toml if you want a command to run. 3. Use absolute imports inside the package (from mypkg import utils) once the package is importable via 1 or 2; they're clearer and work the same everywhere. 4. For pytest, run it from the project root and use a src/ or package layout with the package installed (pip install -e .), or set pythonpath in the pytest configuration. 5. Avoid sys.path.append hacks at the top of files; they break when files move and hide the real layout problem. Verify: python -m mypkg.cli runs from the project root, and python -c "import mypkg; print(mypkg.__file__)" shows the expected location. Sources: https://docs.python.org/3/reference/import.html https://docs.python.org/3/using/cmdline.html https://peps.python.org/pep-0366/