pip install -e . fails with 'ERROR: file:///... does not appear to be a Python project: neither setup.py nor pyproject.toml found' (or 'Project ... has a pyproject.toml but no build backend / editable mode not supported'). How do I set up an editable install?

Cause: pip install -e . (editable or 'development' install) needs the directory you point it at to be an installable Python project: it must contain a pyproject.toml (modern) or setup.py (legacy). The error means pip didn't find either there: you're in the wrong directory (a subfolder or the repo root of a monorepo), the project has only requirements.txt, or the pyproject.toml has no [build-system] / [project] configuration. Some old build setups also don't support editable installs under PEP 660. Fix: 1. Run it from the folder that contains pyproject.toml (or setup.py), or pass that path: pip install -e path/to/project. 2. If the project has no packaging config, add a minimal pyproject.toml, as the Python Packaging User Guide describes: a [build-system] table (for example requires = ["setuptools>=61"] and build-backend = "setuptools.build_meta", or hatchling) and a [project] table with name, version and dependencies. Then pip install -e . works. 3. If you only need the dependencies, not the project itself, use pip install -r requirements.txt instead. 4. Upgrade pip and the build backend before retrying (python -m pip install -U pip setuptools); old setuptools versions lack modern editable support. 5. With a src/ layout, make sure the package directory is configured (setuptools finds src/ packages automatically in recent versions; otherwise configure package discovery). Verify: python -m pip show <name> lists the project with an editable location, and python -c "import yourpackage" picks up changes without reinstalling. Sources: https://pip.pypa.io/en/stable/topics/local-project-installs/ https://packaging.python.org/en/latest/guides/writing-pyproject-toml/