Add nox.needs_version to specify Nox version requirements

docs/config.rst

@@ -339,7 +339,6 @@ Produces these sessions when running ``nox --list``:
     * tests(mysql, new)
 
 
-
 The session object
 ------------------
 
@@ -394,3 +393,29 @@ The following options can be specified in the Noxfile:
 
 
 When invoking ``nox``, any options specified on the command line take precedence over the options specified in the Noxfile. If either ``--sessions`` or ``--keywords`` is specified on the command line, *both* options specified in the Noxfile will be ignored.
+
+
+Nox version requirements
+------------------------
+
+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:
+
+.. code-block:: python
+
+    import nox
+
+    nox.needs_version = ">=2019.5.30"
+
+    @nox.session(name="test")  # name argument was added in 2019.5.30
+    def pytest(session):
+        session.run("pytest")
+
+Any of the version specifiers defined in `PEP 440`_ can be used.
+
+.. warning:: 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/__init__.py

@@ -12,10 +12,14 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import Optional
+
 from nox._options import noxfile_options as options
 from nox._parametrize import Param as param
 from nox._parametrize import parametrize_decorator as parametrize
 from nox.registry import session_decorator as session
 from nox.sessions import Session
 
-__all__ = ["parametrize", "param", "session", "options", "Session"]
+needs_version: Optional[str] = None
+
+__all__ = ["needs_version", "parametrize", "param", "session", "options", "Session"]

nox/__main__.py

@@ -22,13 +22,9 @@
 import sys
 
 from nox import _options, tasks, workflow
+from nox._version import get_nox_version
 from nox.logger import setup_logging
 
-try:
-    import importlib.metadata as metadata
-except ImportError:  # pragma: no cover
-    import importlib_metadata as metadata
-
 
 def main() -> None:
     args = _options.options.parse_args()
@@ -38,7 +34,7 @@ def main() -> None:
         return
 
     if args.version:
-        print(metadata.version("nox"), file=sys.stderr)
+        print(get_nox_version(), file=sys.stderr)
         return
 
     setup_logging(

nox/_version.py

@@ -0,0 +1,113 @@
+# Copyright 2021 Alethea Katherine Flowers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import ast
+import contextlib
+import sys
+from typing import Optional
+
+from packaging.specifiers import InvalidSpecifier, SpecifierSet
+from packaging.version import InvalidVersion, Version
+
+try:
+    import importlib.metadata as metadata
+except ImportError:  # pragma: no cover
+    import importlib_metadata as metadata
+
+
+class VersionCheckFailed(Exception):
+    """The Nox version does not satisfy what ``nox.needs_version`` specifies."""
+
+
+class InvalidVersionSpecifier(Exception):
+    """The ``nox.needs_version`` specifier cannot be parsed."""
+
+
+def get_nox_version() -> str:
+    """Return the version of the installed Nox package."""
+    return metadata.version("nox")
+
+
+def _parse_string_constant(node: ast.AST) -> Optional[str]:  # pragma: no cover
+    """Return the value of a string constant."""
+    if sys.version_info < (3, 8):
+        if isinstance(node, ast.Str) and isinstance(node.s, str):
+            return node.s
+    elif isinstance(node, ast.Constant) and isinstance(node.value, str):
+        return node.value
+    return None
+
+
+def _parse_needs_version(source: str, filename: str = "<unknown>") -> Optional[str]:
+    """Parse ``nox.needs_version`` from the user's noxfile."""
+    value: Optional[str] = None
+    module: ast.Module = ast.parse(source, filename=filename)
+    for statement in module.body:
+        if isinstance(statement, ast.Assign):
+            for target in statement.targets:
+                if (
+                    isinstance(target, ast.Attribute)
+                    and isinstance(target.value, ast.Name)
+                    and target.value.id == "nox"
+                    and target.attr == "needs_version"
+                ):
+                    value = _parse_string_constant(statement.value)
+    return value
+
+
+def _read_needs_version(filename: str) -> Optional[str]:
+    """Read ``nox.needs_version`` from the user's noxfile."""
+    with open(filename) as io:
+        source = io.read()
+
+    return _parse_needs_version(source, filename=filename)
+
+
+def _check_nox_version_satisfies(needs_version: str) -> None:
+    """Check if the Nox version satisfies the given specifiers."""
+    version = Version(get_nox_version())
+
+    try:
+        specifiers = SpecifierSet(needs_version)
+    except InvalidSpecifier as error:
+        message = f"Cannot parse `nox.needs_version`: {error}"
+        with contextlib.suppress(InvalidVersion):
+            Version(needs_version)
+            message += f", did you mean '>= {needs_version}'?"
+        raise InvalidVersionSpecifier(message)
+
+    if not specifiers.contains(version, prereleases=True):
+        raise VersionCheckFailed(
+            f"The Noxfile requires Nox {specifiers}, you have {version}"
+        )
+
+
+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. ``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.
+    """
+    needs_version = _read_needs_version(filename)
+
+    if needs_version is not None:
+        _check_nox_version_satisfies(needs_version)

nox/tasks.py

@@ -23,6 +23,7 @@
 import nox
 from colorlog.escape_codes import parse_colors
 from nox import _options, registry
+from nox._version import InvalidVersionSpecifier, VersionCheckFailed, check_nox_version
 from nox.logger import logger
 from nox.manifest import WARN_PYTHONS_IGNORED, Manifest
 from nox.sessions import Result
@@ -51,6 +52,9 @@ def load_nox_module(global_config: Namespace) -> Union[types.ModuleType, int]:
             os.path.expandvars(global_config.noxfile)
         )
 
+        # Check ``nox.needs_version`` by parsing the AST.
+        check_nox_version(global_config.noxfile)
+
         # Move to the path where the Noxfile is.
         # This will ensure that the Noxfile's path is on sys.path, and that
         # import-time path resolutions work the way the Noxfile author would
@@ -60,6 +64,9 @@ def load_nox_module(global_config: Namespace) -> Union[types.ModuleType, int]:
             "user_nox_module", global_config.noxfile
         ).load_module()  # type: ignore
 
+    except (VersionCheckFailed, InvalidVersionSpecifier) as error:
+        logger.error(str(error))
+        return 2
     except (IOError, OSError):
         logger.exception("Failed to load Noxfile {}".format(global_config.noxfile))
         return 2

setup.py

@@ -52,6 +52,7 @@
     install_requires=[
         "argcomplete>=1.9.4,<2.0",
         "colorlog>=2.6.1,<5.0.0",
+        "packaging>=20.9",
         "py>=1.4.0,<2.0.0",
         "virtualenv>=14.0.0",
         "importlib_metadata; python_version < '3.8'",

tests/test__version.py

@@ -0,0 +1,135 @@
+# Copyright 2021 Alethea Katherine Flowers
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from pathlib import Path
+from textwrap import dedent
+from typing import Optional
+
+import pytest
+from nox import needs_version
+from nox._version import (
+    InvalidVersionSpecifier,
+    VersionCheckFailed,
+    _parse_needs_version,
+    check_nox_version,
+    get_nox_version,
+)
+
+
+@pytest.fixture
+def temp_noxfile(tmp_path: Path):
+    def make_temp_noxfile(content: str) -> str:
+        path = tmp_path / "noxfile.py"
+        path.write_text(content)
+        return str(path)
+
+    return make_temp_noxfile
+
+
+def test_needs_version_default() -> None:
+    """It is None by default."""
+    assert needs_version is None
+
+
+def test_get_nox_version() -> None:
+    """It returns something that looks like a Nox version."""
+    result = get_nox_version()
+    year, month, day = [int(part) for part in result.split(".")[:3]]
+    assert year >= 2020
+
+
+@pytest.mark.parametrize(
+    "text,expected",
+    [
+        ("", None),
+        (
+            dedent(
+                """
+                import nox
+                nox.needs_version = '>=2020.12.31'
+                """
+            ),
+            ">=2020.12.31",
+        ),
+        (
+            dedent(
+                """
+                import nox
+                nox.needs_version = 'bogus'
+                nox.needs_version = '>=2020.12.31'
+                """
+            ),
+            ">=2020.12.31",
+        ),
+        (
+            dedent(
+                """
+                import nox.sessions
+                nox.needs_version = '>=2020.12.31'
+                """
+            ),
+            ">=2020.12.31",
+        ),
+        (
+            dedent(
+                """
+                import nox as _nox
+                _nox.needs_version = '>=2020.12.31'
+                """
+            ),
+            None,
+        ),
+    ],
+)
+def test_parse_needs_version(text: str, expected: Optional[str]) -> None:
+    """It is parsed successfully."""
+    assert expected == _parse_needs_version(text)
+
+
+@pytest.mark.parametrize("specifiers", ["", ">=2020.12.31", ">=2020.12.31,<9999.99.99"])
+def test_check_nox_version_succeeds(temp_noxfile, specifiers: str) -> None:
+    """It does not raise if the version specifiers are satisfied."""
+    text = dedent(
+        f"""
+        import nox
+        nox.needs_version = "{specifiers}"
+        """
+    )
+    check_nox_version(temp_noxfile(text))
+
+
+@pytest.mark.parametrize("specifiers", [">=9999.99.99"])
+def test_check_nox_version_fails(temp_noxfile, specifiers: str) -> None:
+    """It raises an exception if the version specifiers are not satisfied."""
+    text = dedent(
+        f"""
+        import nox
+        nox.needs_version = "{specifiers}"
+        """
+    )
+    with pytest.raises(VersionCheckFailed):
+        check_nox_version(temp_noxfile(text))
+
+
+@pytest.mark.parametrize("specifiers", ["invalid", "2020.12.31"])
+def test_check_nox_version_invalid(temp_noxfile, specifiers: str) -> None:
+    """It raises an exception if the version specifiers cannot be parsed."""
+    text = dedent(
+        f"""
+        import nox
+        nox.needs_version = "{specifiers}"
+        """
+    )
+    with pytest.raises(InvalidVersionSpecifier):
+        check_nox_version(temp_noxfile(text))