Add deprecation messages for `namespace_packages`.

The docs in
https://setuptools.pypa.io/en/latest/userguide/package_discovery.html
and
https://packaging.python.org/en/latest/guides/packaging-namespace-packages/
suggest that this field is deprecated.

2 files changed, 20 insertions, 3 deletions

diff --git a/setuptools/config/_apply_pyprojecttoml.py b/setuptools/config/_apply_pyprojecttoml.py
index a580b63f..069ceb62 100644
--- a/setuptools/config/_apply_pyprojecttoml.py
+++ b/setuptools/config/_apply_pyprojecttoml.py
@@ -16,6 +16,8 @@ from types import MappingProxyType
 from typing import (TYPE_CHECKING, Any, Callable, Dict, List, Optional, Set, Tuple,
                     Type, Union)
 
+from setuptools._deprecation_warning import SetuptoolsDeprecationWarning
+
 if TYPE_CHECKING:
     from setuptools._importlib import metadata  # noqa
     from setuptools.dist import Distribution  # noqa
@@ -75,6 +77,12 @@ def _apply_tool_table(dist: "Distribution", config: dict, filename: _Path):
 
     for field, value in tool_table.items():
         norm_key = json_compatible_key(field)
+
+        if norm_key in TOOL_TABLE_DEPRECATIONS:
+            suggestion = TOOL_TABLE_DEPRECATIONS[norm_key]
+            msg = f"The parameter `{norm_key}` is deprecated, {suggestion}"
+            warnings.warn(msg, SetuptoolsDeprecationWarning)
+
         norm_key = TOOL_TABLE_RENAMES.get(norm_key, norm_key)
         _set_config(dist, norm_key, value)
 
@@ -305,6 +313,9 @@ PYPROJECT_CORRESPONDENCE: Dict[str, _Correspondence] = {
 }
 
 TOOL_TABLE_RENAMES = {"script_files": "scripts"}
+TOOL_TABLE_DEPRECATIONS = {
+    "namespace_packages": "consider using implicit namespaces instead (PEP 420)."
+}
 
 SETUPTOOLS_PATCHES = {"long_description_content_type", "project_urls",
                       "provides_extras", "license_file", "license_files"}
diff --git a/setuptools/config/setupcfg.py b/setuptools/config/setupcfg.py
index d485a8bb..b2d5c346 100644
--- a/setuptools/config/setupcfg.py
+++ b/setuptools/config/setupcfg.py
@@ -12,6 +12,7 @@ from typing import (TYPE_CHECKING, Callable, Any, Dict, Generic, Iterable, List,
 from distutils.errors import DistutilsOptionError, DistutilsFileError
 from setuptools.extern.packaging.version import Version, InvalidVersion
 from setuptools.extern.packaging.specifiers import SpecifierSet
+from setuptools._deprecation_warning import SetuptoolsDeprecationWarning
 
 from . import expand
 
@@ -507,7 +508,7 @@ class ConfigMetadataHandler(ConfigHandler["DistributionMetadata"]):
                 parse_list,
                 "The requires parameter is deprecated, please use "
                 "install_requires for runtime dependencies.",
-                DeprecationWarning,
+                SetuptoolsDeprecationWarning,
             ),
             'obsoletes': parse_list,
             'classifiers': self._get_parser_compound(parse_file, parse_list),
@@ -516,7 +517,7 @@ class ConfigMetadataHandler(ConfigHandler["DistributionMetadata"]):
                 exclude_files_parser('license_file'),
                 "The license_file parameter is deprecated, "
                 "use license_files instead.",
-                DeprecationWarning,
+                SetuptoolsDeprecationWarning,
             ),
             'license_files': parse_list,
             'description': parse_file,
@@ -584,7 +585,12 @@ class ConfigOptionsHandler(ConfigHandler["Distribution"]):
             'scripts': parse_list,
             'eager_resources': parse_list,
             'dependency_links': parse_list,
-            'namespace_packages': parse_list,
+            'namespace_packages': self._deprecated_config_handler(
+                parse_list,
+                "The namespace_packages parameter is deprecated, "
+                "consider using implicit namespaces instead (PEP 420).",
+                SetuptoolsDeprecationWarning,
+            ),
             'install_requires': parse_list_semicolon,
             'setup_requires': parse_list_semicolon,
             'tests_require': parse_list_semicolon,