Require nox.needs_version to be specified statically

Drop the two-phase approach and support only version requirements that can be
determined by parsing the AST of the user's Noxfile.

docs/config.rst

@@ -398,7 +398,7 @@ When invoking ``nox``, any options specified on the command line take precedence
 Nox version requirements
 ------------------------
 
-Nox version requirements can be specified in your ``noxfile.py`` by setting
+Nox version requirements can be specified in your Noxfile by setting
 ``nox.needs_version``. If the Nox version does not satisfy the requirements, Nox
 exits with a friendly error message. For example:
 
@@ -412,9 +412,10 @@ exits with a friendly error message. For example:
     def pytest(session):
         session.run("pytest")
 
+Any of the version specifiers defined in `PEP 440`_ can be used.
 
-Any of the version specifiers defined in `PEP 440`_ can be used. If you assign a
-string literal like in the example above, Nox is able to check the version
-without importing the Noxfile.
+**Important**: Version requirements *must* be specified as a string literal,
+using a simple assignment to ``nox.needs_version`` at the module level. This
+allows Nox to check the version without importing the Noxfile.
 
 .. _PEP 440: https://www.python.org/dev/peps/pep-0440/

nox/_version.py

@@ -93,25 +93,21 @@ def _check_nox_version_satisfies(needs_version: str) -> None:
         )
 
 
-def check_nox_version(filename: Optional[str] = None) -> None:
+def check_nox_version(filename: str) -> None:
     """Check if ``nox.needs_version`` in the user's noxfile is satisfied.
 
     Args:
-        filename: The location of the user's noxfile. When specified, read
-            ``nox.needs_version`` from the noxfile by parsing the AST.
-            Otherwise, assume that the noxfile was already imported, and
-            use ``nox.needs_version`` directly.
+
+        filename: The location of the user's noxfile. ``nox.needs_version`` is
+            read from the noxfile by parsing the AST.
 
     Raises:
         VersionCheckFailed: The Nox version does not satisfy what
             ``nox.needs_version`` specifies.
         InvalidVersionSpecifier: The ``nox.needs_version`` specifier cannot be
             parsed.
     """
-    from nox import needs_version
-
-    if filename is not None:
-        needs_version = _read_needs_version(filename)  # noqa: F811
+    needs_version = _read_needs_version(filename)
 
     if needs_version is not None:
         _check_nox_version_satisfies(needs_version)

nox/tasks.py

@@ -60,15 +60,10 @@ def load_nox_module(global_config: Namespace) -> Union[types.ModuleType, int]:
         # import-time path resolutions work the way the Noxfile author would
         # guess.
         os.chdir(os.path.realpath(os.path.dirname(global_config.noxfile)))
-        module = importlib.machinery.SourceFileLoader(
+        return importlib.machinery.SourceFileLoader(
             "user_nox_module", global_config.noxfile
         ).load_module()  # type: ignore
 
-        # Check ``nox.needs_version`` as set by the Noxfile.
-        check_nox_version()
-
-        return module
-
     except (VersionCheckFailed, InvalidVersionSpecifier) as error:
         logger.error(str(error))
         return 2

tests/test_tasks.py

@@ -18,7 +18,6 @@
 import json
 import os
 import platform
-from pathlib import Path
 from textwrap import dedent
 from unittest import mock
 
@@ -81,31 +80,44 @@ def test_load_nox_module_not_found():
     assert tasks.load_nox_module(config) == 2
 
 
-@pytest.mark.parametrize(
-    "text",
-    [
-        dedent(
-            """
-            import nox
-            nox.needs_version = ">=9999.99.99"
-            """
-        ),
-        dedent(
-            """
-            import nox
-            NOX_NEEDS_VERSION = ">=9999.99.99"
-            nox.needs_version = NOX_NEEDS_VERSION
-            """
-        ),
-    ],
-)
-def test_load_nox_module_needs_version(text: str, tmp_path: Path):
+@pytest.fixture
+def reset_needs_version():
+    """Do not leak ``nox.needs_version`` between tests."""
+    try:
+        yield
+    finally:
+        nox.needs_version = None
+
+
+def test_load_nox_module_needs_version_static(reset_needs_version, tmp_path):
+    text = dedent(
+        """
+        import nox
+        nox.needs_version = ">=9999.99.99"
+        """
+    )
     noxfile = tmp_path / "noxfile.py"
     noxfile.write_text(text)
     config = _options.options.namespace(noxfile=str(noxfile))
     assert tasks.load_nox_module(config) == 2
 
 
+def test_load_nox_module_needs_version_dynamic(reset_needs_version, tmp_path):
+    text = dedent(
+        """
+        import nox
+        NOX_NEEDS_VERSION = ">=9999.99.99"
+        nox.needs_version = NOX_NEEDS_VERSION
+        """
+    )
+    noxfile = tmp_path / "noxfile.py"
+    noxfile.write_text(text)
+    config = _options.options.namespace(noxfile=str(noxfile))
+    tasks.load_nox_module(config)
+    # Dynamic version requirements are not checked.
+    assert nox.needs_version == ">=9999.99.99"
+
+
 def test_discover_session_functions_decorator():
     # Define sessions using the decorator.
     @nox.session