uv or Poetry: 'The current project's supported Python range (>=3.8) is not compatible with some of the required packages Python requirement' / 'requires-python' resolution failure. How do I fix it?

Cause: the project declares which Python versions it supports (requires-python in pyproject.toml, or the python key under tool.poetry.dependencies). uv and Poetry resolve dependencies for that whole range, not only for the interpreter you happen to run. If a dependency's releases need a newer Python than the low end of your range (or have an upper cap that your range exceeds), no single version fits the full range and resolution fails. uv's docs describe this as your requires-python needing to be a subset of each dependency's supported range. In Poetry, if you set both requires-python and tool.poetry.dependencies.python, the Poetry value is used for locking and must be a subset of requires-python. Fix (best first): 1. Read the error: it names the package and the Python range it needs. Decide whether you really need to support the low versions. 2. Narrow your supported range to what the dependencies allow, for example requires-python = ">=3.10" (and add an upper bound only if the dependency demands it). Then run uv lock or poetry lock again. 3. If you must keep supporting older Pythons, allow an older release of the dependency for those versions using an environment marker, for example "pkg>=2; python_version >= '3.10'" and "pkg<2; python_version < '3.10'" (in Poetry, a per-dependency python key does the same). 4. Check your own constraint for mistakes: in Poetry, requires-python = ">=3.8,<4.0" together with python = ">=3.8" is invalid because the Poetry range reaches beyond requires-python. 5. Make sure the interpreter used for the environment is inside the range (uv python install 3.12, or the Poetry env use command), and recreate the venv after changing the range. 6. Do not bypass the check by forcing an install of the package with pip --ignore-requires-python; it may import but break at runtime. Verify: uv lock (or poetry lock) finishes without errors, uv sync / poetry install builds the environment, and your tests pass on the lowest Python version you declare. Sources: https://docs.astral.sh/uv/concepts/resolution/ https://python-poetry.org/docs/pyproject/ https://python-poetry.org/docs/dependency-specification/