Prevent deprecated license classifiers from being written to core metadata (pypa#4833)

Author: abravalheri

newsfragments/4833.feature.rst

@@ -0,0 +1,2 @@
+Added exception (or warning) when deprecated license classifiers are used,
+according to `PEP 639 <https://peps.python.org/pep-0639/#deprecate-license-classifiers>`_.

setuptools/dist.py

@@ -28,6 +28,7 @@
 from ._reqs import _StrOrIter
 from .config import pyprojecttoml, setupcfg
 from .discovery import ConfigDiscovery
+from .errors import InvalidConfigError
 from .monkey import get_unpatched
 from .warnings import InformationOnly, SetuptoolsDeprecationWarning
 
@@ -406,22 +407,32 @@ def _normalize_requires(self):
 
     def _finalize_license_expression(self) -> None:
         """Normalize license and license_expression."""
+        classifiers = self.metadata.get_classifiers()
+        license_classifiers = [cl for cl in classifiers if cl.startswith("License :: ")]
+
         license_expr = self.metadata.license_expression
         if license_expr:
             normalized = canonicalize_license_expression(license_expr)
             if license_expr != normalized:
                 InformationOnly.emit(f"Normalizing '{license_expr}' to '{normalized}'")
                 self.metadata.license_expression = normalized
-
-            for cl in self.metadata.get_classifiers():
-                if not cl.startswith("License :: "):
-                    continue
-                SetuptoolsDeprecationWarning.emit(
-                    "License classifier are deprecated in favor of the license expression.",
-                    f"Please remove the '{cl}' classifier.",
-                    see_url="https://peps.python.org/pep-0639/",
-                    due_date=(2027, 2, 17),  # Introduced 2025-02-17
+            if license_classifiers:
+                raise InvalidConfigError(
+                    "License classifiers have been superseded by license expressions "
+                    "(see https://peps.python.org/pep-0639/). Please remove:\n\n"
+                    + "\n".join(license_classifiers),
                 )
+        elif license_classifiers:
+            pypa_guides = "guides/writing-pyproject-toml/#license"
+            SetuptoolsDeprecationWarning.emit(
+                "License classifiers are deprecated.",
+                "Please consider removing the following classifiers in favor of a "
+                "SPDX license expression:\n\n" + "\n".join(license_classifiers),
+                see_url=f"https://packaging.python.org/en/latest/{pypa_guides}",
+                # Warning introduced on 2025-02-17
+                # TODO: Should we add a due date? It may affect old/unmaintained
+                #       packages in the ecosystem and cause problems...
+            )
 
     def _finalize_license_files(self) -> None:
         """Compute names of all license files which should be included."""

setuptools/tests/config/test_apply_pyprojecttoml.py

@@ -9,7 +9,6 @@
 import io
 import re
 import tarfile
-import warnings
 from inspect import cleandoc
 from pathlib import Path
 from unittest.mock import Mock
@@ -24,7 +23,7 @@
 from setuptools.config import expand, pyprojecttoml, setupcfg
 from setuptools.config._apply_pyprojecttoml import _MissingDynamic, _some_attrgetter
 from setuptools.dist import Distribution
-from setuptools.errors import RemovedConfigError
+from setuptools.errors import InvalidConfigError, RemovedConfigError
 from setuptools.warnings import SetuptoolsDeprecationWarning
 
 from .downloads import retrieve_file, urls_from_file
@@ -331,26 +330,36 @@ def test_license_in_metadata(
     assert not_content_str not in content
 
 
-def test_license_expression_with_bad_classifier(tmp_path):
+def test_license_classifier_with_license_expression(tmp_path):
     text = PEP639_LICENSE_EXPRESSION.rsplit("\n", 2)[0]
     pyproject = _pep621_example_project(
         tmp_path,
         "README",
         f"{text}\n    \"License :: OSI Approved :: MIT License\"\n]",
     )
-    msg = "License classifier are deprecated(?:.|\n)*'License :: OSI Approved :: MIT License'"
-    with pytest.raises(SetuptoolsDeprecationWarning, match=msg):
+    msg = "License classifiers have been superseded by license expressions"
+    with pytest.raises(InvalidConfigError, match=msg) as exc:
         pyprojecttoml.apply_configuration(makedist(tmp_path), pyproject)
 
-    with warnings.catch_warnings():
-        warnings.simplefilter("ignore", SetuptoolsDeprecationWarning)
+    assert "License :: OSI Approved :: MIT License" in str(exc.value)
+
+
+def test_license_classifier_without_license_expression(tmp_path):
+    text = """\
+    [project]
+    name = "spam"
+    version = "2020.0.0"
+    license = {text = "mit or apache-2.0"}
+    classifiers = ["License :: OSI Approved :: MIT License"]
+    """
+    pyproject = _pep621_example_project(tmp_path, "README", text)
+
+    msg = "License classifiers are deprecated(?:.|\n)*MIT License"
+    with pytest.warns(SetuptoolsDeprecationWarning, match=msg):
         dist = pyprojecttoml.apply_configuration(makedist(tmp_path), pyproject)
-        # Check license classifier is still included
-        assert dist.metadata.get_classifiers() == [
-            "Development Status :: 5 - Production/Stable",
-            "Programming Language :: Python",
-            "License :: OSI Approved :: MIT License",
-        ]
+
+    # Check license classifier is still included
+    assert dist.metadata.get_classifiers() == ["License :: OSI Approved :: MIT License"]
 
 
 class TestLicenseFiles: