How to specify version in only one place when using pyproject.toml?

After you have installed your project – either in editable mode by poetry install or from the wheel – you can access several metadata via importlib.metadata (importlib_metadata for python < 3.8).

So keep the version only in the pyproject.toml and use this in your python code:

import importlib.metadata

__version__ = importlib.metadata.version("mypackage")