feat: add helper to get the Python listing

docs/config.rst

@@ -466,6 +466,13 @@ class.
     :members:
     :undoc-members:
 
+The project helpers
+-------------------
+
+Nox provides helpers for ``pyproject.toml`` projects in the ``nox.project`` namespace.
+
+.. automodule:: nox.project
+   :members:
 
 Modifying Nox's behavior in the Noxfile
 ---------------------------------------

docs/tutorial.rst

@@ -218,6 +218,9 @@ is provided:
        session.install_and_run_script("peps.py")
 
 
+Other helpers for ``pyproject.toml`` based projects are also available in
+``nox.project``.
+
 Running commands
 ----------------
 

nox/project.py

@@ -6,6 +6,8 @@
 from pathlib import Path
 from typing import TYPE_CHECKING
 
+import packaging.specifiers
+
 if TYPE_CHECKING:
     from typing import Any
 
@@ -15,7 +17,7 @@
     import tomllib
 
 
-__all__ = ["load_toml"]
+__all__ = ["load_toml", "python_list"]
 
 
 def __dir__() -> list[str]:
@@ -37,6 +39,15 @@ def load_toml(filename: os.PathLike[str] | str) -> dict[str, Any]:
     The file must have a ``.toml`` extension to be considered a toml file or a
     ``.py`` extension / no extension to be considered a script. Other file
     extensions are not valid in this function.
+
+    Example:
+
+    .. code-block:: python
+
+        @nox.session
+        def myscript(session):
+            myscript_options = nox.project.load_toml("myscript.py")
+            session.install(*myscript_options["dependencies"])
     """
     filepath = Path(filename)
     if filepath.suffix == ".toml":
@@ -67,3 +78,52 @@ def _load_script_block(filepath: Path) -> dict[str, Any]:
         for line in matches[0].group("content").splitlines(keepends=True)
     )
     return tomllib.loads(content)
+
+
+def python_list(
+    pyproject: dict[str, Any], *, max_version: str | None = None
+) -> list[str]:
+    """
+    Read a list of supported Python versions. Without ``max_version``, this
+    will read the trove classifiers (recommended). With a ``max_version``, it
+    will read the requires-python setting for a lower bound, and will use the
+    value of ``max_version`` as the upper bound. (Reminder: you should never
+    set an upper bound in ``requires-python``).
+
+    Example:
+
+    .. code-block:: python
+
+        import nox
+
+        PYPROJECT = nox.project.load_toml("pyproject.toml")
+        # From classifiers
+        PYTHON_VERSIONS = nox.project.python_list(PYPROJECT)
+        # Or from requires-python
+        PYTHON_VERSIONS = nox.project.python_list(PYPROJECT, max_version="3.13")
+    """
+    if max_version is None:
+        # Classifiers are a list of every Python version
+        from_classifiers = [
+            c.split()[-1]
+            for c in pyproject.get("project", {}).get("classifiers", [])
+            if c.startswith("Programming Language :: Python :: 3.")
+        ]
+        if from_classifiers:
+            return from_classifiers
+        raise ValueError('No Python version classifiers found in "project.classifiers"')
+
+    requires_python_str = pyproject.get("project", {}).get("requires-python", "")
+    if not requires_python_str:
+        raise ValueError('No "project.requires-python" value set')
+
+    for spec in packaging.specifiers.SpecifierSet(requires_python_str):
+        if spec.operator in {">", ">=", "~="}:
+            min_minor_version = int(spec.version.split(".")[1])
+            break
+    else:
+        raise ValueError('No minimum version found in "project.requires-python"')
+
+    max_minor_version = int(max_version.split(".")[1])
+
+    return [f"3.{v}" for v in range(min_minor_version, max_minor_version + 1)]

tests/test_project.py

@@ -0,0 +1,65 @@
+import pytest
+
+from nox.project import python_list
+
+
+def test_classifiers():
+    pyproject = {
+        "project": {
+            "classifiers": [
+                "Programming Language :: Python :: 3.7",
+                "Programming Language :: Python :: 3.9",
+                "Programming Language :: Python :: 3.12",
+                "Programming Language :: Python",
+                "Programming Language :: Python :: 3 :: Only",
+                "Topic :: Software Development :: Testing",
+            ],
+            "requires-python": ">=3.10",
+        }
+    }
+
+    assert python_list(pyproject) == ["3.7", "3.9", "3.12"]
+
+
+def test_no_classifiers():
+    pyproject = {"project": {"requires-python": ">=3.9"}}
+    with pytest.raises(ValueError, match="No Python version classifiers"):
+        python_list(pyproject)
+
+
+def test_no_requires_python():
+    pyproject = {"project": {"classifiers": ["Programming Language :: Python :: 3.12"]}}
+    with pytest.raises(ValueError, match='No "project.requires-python" value set'):
+        python_list(pyproject, max_version="3.13")
+
+
+def test_python_range():
+    pyproject = {
+        "project": {
+            "classifiers": [
+                "Programming Language :: Python :: 3.7",
+                "Programming Language :: Python :: 3.9",
+                "Programming Language :: Python :: 3.12",
+                "Programming Language :: Python",
+                "Programming Language :: Python :: 3 :: Only",
+                "Topic :: Software Development :: Testing",
+            ],
+            "requires-python": ">=3.10",
+        }
+    }
+
+    assert python_list(pyproject, max_version="3.12") == ["3.10", "3.11", "3.12"]
+    assert python_list(pyproject, max_version="3.11") == ["3.10", "3.11"]
+
+
+def test_python_range_gt():
+    pyproject = {"project": {"requires-python": ">3.2.1,<3.3"}}
+
+    assert python_list(pyproject, max_version="3.4") == ["3.2", "3.3", "3.4"]
+
+
+def test_python_range_no_min():
+    pyproject = {"project": {"requires-python": "==3.3.1"}}
+
+    with pytest.raises(ValueError, match="No minimum version found"):
+        python_list(pyproject, max_version="3.5")