[Investigation] Reduce the amount of patching in setuptools.build_meta

bootstrap.egg-info/entry_points.txt

@@ -1,5 +1,6 @@
 [distutils.commands]
 egg_info = setuptools.command.egg_info:egg_info
+dist_info = setuptools.command.dist_info:dist_info
 build_py = setuptools.command.build_py:build_py
 sdist = setuptools.command.sdist:sdist
 editable_wheel = setuptools.command.editable_wheel:editable_wheel

setuptools/__init__.py

@@ -11,8 +11,7 @@
 import os
 import sys
 from abc import abstractmethod
-from collections.abc import Mapping
-from typing import TYPE_CHECKING, TypeVar, overload
+from typing import TYPE_CHECKING, Any, TypeVar, overload
 
 sys.path.extend(((vendor_path := os.path.join(os.path.dirname(os.path.dirname(__file__)), 'setuptools', '_vendor')) not in sys.path) * [vendor_path])  # fmt: skip
 # workaround for #4476
@@ -21,6 +20,7 @@
 import _distutils_hack.override  # noqa: F401
 
 from . import logging, monkey
+from .compat import py310
 from .depends import Require
 from .discovery import PackageFinder, PEP420PackageFinder
 from .dist import Distribution
@@ -49,40 +49,19 @@
 find_namespace_packages = PEP420PackageFinder.find
 
 
-def _install_setup_requires(attrs):
-    # Note: do not use `setuptools.Distribution` directly, as
-    # our PEP 517 backend patch `distutils.core.Distribution`.
-    class MinimalDistribution(distutils.core.Distribution):
-        """
-        A minimal version of a distribution for supporting the
-        fetch_build_eggs interface.
-        """
+def _expand_setupcfg(attrs: dict[str, Any]) -> Distribution:
+    """Bare minimum setup.cfg parsing so that we can extract setup_requires"""
+    from setuptools.config.setupcfg import _apply
+
+    dist = Distribution(attrs)
+    dist.set_defaults._disable()
+    if os.path.exists("setup.cfg"):  # Assumes no other config contains setup_requires
+        _apply(dist, "setup.cfg", ignore_option_errors=True)
+    return dist
+
 
-        def __init__(self, attrs: Mapping[str, object]) -> None:
-            _incl = 'dependency_links', 'setup_requires'
-            filtered = {k: attrs[k] for k in set(_incl) & set(attrs)}
-            super().__init__(filtered)
-            # Prevent accidentally triggering discovery with incomplete set of attrs
-            self.set_defaults._disable()
-
-        def _get_project_config_files(self, filenames=None):
-            """Ignore ``pyproject.toml``, they are not related to setup_requires"""
-            try:
-                cfg, _toml = super()._split_standard_project_metadata(filenames)
-            except Exception:
-                return filenames, ()
-            return cfg, ()
-
-        def finalize_options(self):
-            """
-            Disable finalize_options to avoid building the working set.
-            Ref #2158.
-            """
-
-    dist = MinimalDistribution(attrs)
-
-    # Honor setup.cfg's options.
-    dist.parse_config_files(ignore_option_errors=True)
+def _install_setup_requires(attrs: dict[str, Any]) -> None:
+    dist = _expand_setupcfg(attrs)
     if dist.setup_requires:
         _fetch_build_eggs(dist)
 
@@ -101,17 +80,21 @@ def _fetch_build_eggs(dist: Distribution):
         please contact that package's maintainers or distributors.
         """
         if "InvalidVersion" in ex.__class__.__name__:
-            if hasattr(ex, "add_note"):
-                ex.add_note(msg)  # PEP 678
-            else:
-                dist.announce(f"\n{msg}\n")
+            py310.add_note(ex, msg)
         raise
 
 
 def setup(**attrs):
+    if "--private-interrupt-setuppy" in sys.argv:
+        raise _SetupPyInterruption(_expand_setupcfg(attrs))
+
     logging.configure()
-    # Make sure we have any requirements needed to interpret 'attrs'.
-    _install_setup_requires(attrs)
+
+    if "--private-skip-setup-requires" in sys.argv:
+        sys.argv.remove("--private-skip-setup-requires")
+    else:
+        # Make sure we have any requirements needed to interpret 'attrs'.
+        _install_setup_requires(attrs)
     return distutils.core.setup(**attrs)
 
 
@@ -244,5 +227,10 @@ class sic(str):
     """Treat this string as-is (https://en.wikipedia.org/wiki/Sic)"""
 
 
+class _SetupPyInterruption(Exception):
+    def __init__(self, dist: Distribution):
+        self.dist = dist
+
+
 # Apply monkey patches
 monkey.patch_all()

setuptools/build_meta.py

@@ -48,7 +48,6 @@
 from ._reqs import parse_strings
 from .warnings import SetuptoolsDeprecationWarning
 
-import distutils
 from distutils.util import strtobool
 
 if TYPE_CHECKING:
@@ -64,53 +63,9 @@
     'prepare_metadata_for_build_editable',
     'build_editable',
     '__legacy__',
-    'SetupRequirementsError',
 ]
 
 
-class SetupRequirementsError(BaseException):
-    def __init__(self, specifiers) -> None:
-        self.specifiers = specifiers
-
-
-class Distribution(setuptools.dist.Distribution):
-    def fetch_build_eggs(self, specifiers):
-        specifier_list = list(parse_strings(specifiers))
-
-        raise SetupRequirementsError(specifier_list)
-
-    @classmethod
-    @contextlib.contextmanager
-    def patch(cls):
-        """
-        Replace
-        distutils.dist.Distribution with this class
-        for the duration of this context.
-        """
-        orig = distutils.core.Distribution
-        distutils.core.Distribution = cls  # type: ignore[misc] # monkeypatching
-        try:
-            yield
-        finally:
-            distutils.core.Distribution = orig  # type: ignore[misc] # monkeypatching
-
-
-@contextlib.contextmanager
-def no_install_setup_requires():
-    """Temporarily disable installing setup_requires
-
-    Under PEP 517, the backend reports build dependencies to the frontend,
-    and the frontend is responsible for ensuring they're installed.
-    So setuptools (acting as a backend) should not try to install them.
-    """
-    orig = setuptools._install_setup_requires
-    setuptools._install_setup_requires = lambda attrs: None
-    try:
-        yield
-    finally:
-        setuptools._install_setup_requires = orig
-
-
 def _get_immediate_subdirectories(a_dir):
     return [
         name for name in os.listdir(a_dir) if os.path.isdir(os.path.join(a_dir, name))
@@ -291,16 +246,14 @@ class _BuildMetaBackend(_ConfigSettingsTranslator):
     def _get_build_requires(
         self, config_settings: _ConfigSettings, requirements: list[str]
     ):
-        sys.argv = [
-            *sys.argv[:1],
-            *self._global_args(config_settings),
-            "egg_info",
-        ]
+        sys.argv = [*sys.argv[:1], "--private-interrupt-setuppy"]
         try:
-            with Distribution.patch():
-                self.run_setup()
-        except SetupRequirementsError as e:
-            requirements += e.specifiers
+            self.run_setup()
+        except setuptools._SetupPyInterruption as ex:
+            setup_requires = parse_strings(ex.dist.setup_requires or "")
+            return requirements + list(setup_requires)
+        except Exception:
+            pass  # Ignore other arbitrary exceptions from setup.py, e.g. SystemExit
 
         return requirements
 
@@ -313,6 +266,8 @@ def run_setup(self, setup_script: str = 'setup.py'):
         with _open_setup_script(__file__) as f:
             code = f.read().replace(r'\r\n', r'\n')
 
+        sys.argv.append("--private-skip-setup-requires")
+
         try:
             exec(code, locals())
         except SystemExit as e:
@@ -370,8 +325,7 @@ def prepare_metadata_for_build_wheel(
             str(metadata_directory),
             "--keep-egg-info",
         ]
-        with no_install_setup_requires():
-            self.run_setup()
+        self.run_setup()
 
         self._bubble_up_info_directory(metadata_directory, ".egg-info")
         return self._bubble_up_info_directory(metadata_directory, ".dist-info")
@@ -400,8 +354,7 @@ def _build_with_temp_dir(
                 tmp_dist_dir,
                 *arbitrary_args,
             ]
-            with no_install_setup_requires():
-                self.run_setup()
+            self.run_setup()
 
             result_basename = _file_with_extension(tmp_dist_dir, result_extension)
             result_path = os.path.join(result_directory, result_basename)
@@ -546,3 +499,20 @@ class _IncompatibleBdistWheel(SetuptoolsDeprecationWarning):
 
 # The legacy backend
 __legacy__ = _BuildMetaLegacyBackend()
+
+
+def __getattr__(name):
+    if name == "SetupRequirementsError":
+        SetuptoolsDeprecationWarning.emit(
+            "SetupRequirementsError is no longer part of the public API.",
+            "Please do not import SetupRequirementsError.",
+            due_date=(2026, 5, 12),
+        )
+
+        class SetupRequirementsError(BaseException):
+            def __init__(self, specifiers) -> None:
+                self.specifiers = specifiers
+
+        return SetupRequirementsError
+
+    raise AttributeError(name)