From 771a2ce6108f5e2da3293a9ed9f0fc64e05af4e0 Mon Sep 17 00:00:00 2001
From: Anderson Bravalheri <andersonbravalheri@gmail.com>
Date: Fri, 11 Jul 2025 16:30:01 +0100
Subject: [PATCH 1/9] Remove unused type ignore mark

---
 distutils/compilers/C/base.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/distutils/compilers/C/base.py b/distutils/compilers/C/base.py
index 93385e13..6412dc34 100644
--- a/distutils/compilers/C/base.py
+++ b/distutils/compilers/C/base.py
@@ -70,7 +70,7 @@ class Compiler:
     # dictionary (see below -- used by the 'new_compiler()' factory
     # function) -- authors of new compiler interface classes are
     # responsible for updating 'compiler_class'!
-    compiler_type: ClassVar[str] = None  # type: ignore[assignment]
+    compiler_type: ClassVar[str] = None
 
     # XXX things not handled by this compiler abstraction model:
     #   * client can't provide additional options for a compiler,

From 4bed279a1800033b34878744f76e7e84a0793814 Mon Sep 17 00:00:00 2001
From: Anderson Bravalheri <andersonbravalheri@gmail.com>
Date: Fri, 11 Jul 2025 13:43:10 +0100
Subject: [PATCH 2/9] Use dataclass for typing Extension

---
 distutils/core.py                 |  20 +--
 distutils/extension.py            | 274 ++++++++++++++++--------------
 distutils/tests/test_extension.py |   2 +-
 3 files changed, 151 insertions(+), 145 deletions(-)

diff --git a/distutils/core.py b/distutils/core.py
index bd62546b..ef39600b 100644
--- a/distutils/core.py
+++ b/distutils/core.py
@@ -25,6 +25,7 @@
     DistutilsSetupError,
 )
 from .extension import Extension
+from .extension import _safe as extension_keywords  # noqa  # backwards compatibility
 
 __all__ = ['Distribution', 'Command', 'Extension', 'setup']
 
@@ -74,25 +75,6 @@ def gen_usage(script_name):
     'obsoletes',
 )
 
-# Legal keyword arguments for the Extension constructor
-extension_keywords = (
-    'name',
-    'sources',
-    'include_dirs',
-    'define_macros',
-    'undef_macros',
-    'library_dirs',
-    'libraries',
-    'runtime_library_dirs',
-    'extra_objects',
-    'extra_compile_args',
-    'extra_link_args',
-    'swig_opts',
-    'export_symbols',
-    'depends',
-    'language',
-)
-
 
 def setup(**attrs):  # noqa: C901
     """The gateway to the Distutils: do everything your setup script needs
diff --git a/distutils/extension.py b/distutils/extension.py
index f5141126..b12a94a1 100644
--- a/distutils/extension.py
+++ b/distutils/extension.py
@@ -8,6 +8,8 @@
 import os
 import warnings
 from collections.abc import Iterable
+from dataclasses import dataclass, field, fields
+from typing import TYPE_CHECKING
 
 # This class is really only used by the "build_ext" command, so it might
 # make sense to put it in distutils.command.build_ext.  However, that
@@ -20,137 +22,159 @@
 # order to do anything.
 
 
-class Extension:
+@dataclass
+class _Extension:
     """Just a collection of attributes that describes an extension
     module and everything needed to build it (hopefully in a portable
     way, but there are hooks that let you be as unportable as you need).
+    """
+
+    # The use of a parent class as a "trick":
+    # - We need to modify __init__ so to achieve backwards compatibility
+    # - But we don't want to throw away the dataclass-generated __init__
+    # - We also want to fool the typechecker to consider the same type
+    #   signature as the dataclass-generated __init__
+
+    name: str
+    """
+    the full name of the extension, including any packages -- ie.
+    *not* a filename or pathname, but Python dotted name
+    """
+
+    sources: Iterable[str | os.PathLike[str]]
+    """
+    iterable of source filenames (except strings, which could be misinterpreted
+    as a single filename), relative to the distribution root (where the setup
+    script lives), in Unix form (slash-separated) for portability. Can be any
+    non-string iterable (list, tuple, set, etc.) containing strings or
+    PathLike objects. Source files may be C, C++, SWIG (.i), platform-specific
+    resource files, or whatever else is recognized by the "build_ext" command
+    as source for a Python extension.
+    """
+
+    include_dirs: list[str] = field(default_factory=list)
+    """
+    list of directories to search for C/C++ header files (in Unix
+    form for portability)
+    """
+
+    define_macros: list[tuple[str, str | None]] = field(default_factory=list)
+    """
+    list of macros to define; each macro is defined using a 2-tuple,
+    where 'value' is either the string to define it to or None to
+    define it without a particular value (equivalent of "#define
+    FOO" in source or -DFOO on Unix C compiler command line)
+    """
+
+    undef_macros: list[str] = field(default_factory=list)
+    """list of macros to undefine explicitly"""
+
+    library_dirs: list[str] = field(default_factory=list)
+    """list of directories to search for C/C++ libraries at link time"""
+
+    libraries: list[str] = field(default_factory=list)
+    """list of library names (not filenames or paths) to link against"""
+
+    runtime_library_dirs: list[str] = field(default_factory=list)
+    """
+    list of directories to search for C/C++ libraries at run time
+    (for shared extensions, this is when the extension is loaded)
+    """
+
+    extra_objects: list[str] = field(default_factory=list)
+    """
+    list of extra files to link with (eg. object files not implied
+    by 'sources', static library that must be explicitly specified,
+    binary resource files, etc.)
+    """
+
+    extra_compile_args: list[str] = field(default_factory=list)
+    """
+    any extra platform- and compiler-specific information to use
+    when compiling the source files in 'sources'.  For platforms and
+    compilers where "command line" makes sense, this is typically a
+    list of command-line arguments, but for other platforms it could
+    be anything.
+    """
+
+    extra_link_args: list[str] = field(default_factory=list)
+    """
+    any extra platform- and compiler-specific information to use
+    when linking object files together to create the extension (or
+    to create a new static Python interpreter).  Similar
+    interpretation as for 'extra_compile_args'.
+    """
+
+    export_symbols: list[str] = field(default_factory=list)
+    """
+    list of symbols to be exported from a shared extension.  Not
+    used on all platforms, and not generally necessary for Python
+    extensions, which typically export exactly one symbol: "init" +
+    extension_name.
+    """
 
-    Instance attributes:
-      name : string
-        the full name of the extension, including any packages -- ie.
-        *not* a filename or pathname, but Python dotted name
-      sources : Iterable[string | os.PathLike]
-        iterable of source filenames (except strings, which could be misinterpreted
-        as a single filename), relative to the distribution root (where the setup
-        script lives), in Unix form (slash-separated) for portability. Can be any
-        non-string iterable (list, tuple, set, etc.) containing strings or
-        PathLike objects. Source files may be C, C++, SWIG (.i), platform-specific
-        resource files, or whatever else is recognized by the "build_ext" command
-        as source for a Python extension.
-      include_dirs : [string]
-        list of directories to search for C/C++ header files (in Unix
-        form for portability)
-      define_macros : [(name : string, value : string|None)]
-        list of macros to define; each macro is defined using a 2-tuple,
-        where 'value' is either the string to define it to or None to
-        define it without a particular value (equivalent of "#define
-        FOO" in source or -DFOO on Unix C compiler command line)
-      undef_macros : [string]
-        list of macros to undefine explicitly
-      library_dirs : [string]
-        list of directories to search for C/C++ libraries at link time
-      libraries : [string]
-        list of library names (not filenames or paths) to link against
-      runtime_library_dirs : [string]
-        list of directories to search for C/C++ libraries at run time
-        (for shared extensions, this is when the extension is loaded)
-      extra_objects : [string]
-        list of extra files to link with (eg. object files not implied
-        by 'sources', static library that must be explicitly specified,
-        binary resource files, etc.)
-      extra_compile_args : [string]
-        any extra platform- and compiler-specific information to use
-        when compiling the source files in 'sources'.  For platforms and
-        compilers where "command line" makes sense, this is typically a
-        list of command-line arguments, but for other platforms it could
-        be anything.
-      extra_link_args : [string]
-        any extra platform- and compiler-specific information to use
-        when linking object files together to create the extension (or
-        to create a new static Python interpreter).  Similar
-        interpretation as for 'extra_compile_args'.
-      export_symbols : [string]
-        list of symbols to be exported from a shared extension.  Not
-        used on all platforms, and not generally necessary for Python
-        extensions, which typically export exactly one symbol: "init" +
-        extension_name.
-      swig_opts : [string]
-        any extra options to pass to SWIG if a source file has the .i
-        extension.
-      depends : [string]
-        list of files that the extension depends on
-      language : string
-        extension language (i.e. "c", "c++", "objc"). Will be detected
-        from the source extensions if not provided.
-      optional : boolean
-        specifies that a build failure in the extension should not abort the
-        build process, but simply not install the failing extension.
+    swig_opts: list[str] = field(default_factory=list)
     """
+    any extra options to pass to SWIG if a source file has the .i
+    extension.
+    """
+
+    depends: list[str] = field(default_factory=list)
+    """list of files that the extension depends on"""
+
+    language: str | None = None
+    """
+    extension language (i.e. "c", "c++", "objc"). Will be detected
+    from the source extensions if not provided.
+    """
+
+    optional: bool = False
+    """
+    specifies that a build failure in the extension should not abort the
+    build process, but simply not install the failing extension.
+    """
+
+
+# Legal keyword arguments for the Extension constructor
+_safe = tuple(f.name for f in fields(_Extension))
+
+
+if TYPE_CHECKING:
+
+    @dataclass
+    class Extension(_Extension):
+        pass
+
+else:
+
+    @dataclass(init=False)
+    class Extension(_Extension):
+        def __init__(self, name, sources, *args, **kwargs):
+            if not isinstance(name, str):
+                raise TypeError("'name' must be a string")
+
+            # handle the string case first; since strings are iterable, disallow them
+            if isinstance(sources, str):
+                raise TypeError(
+                    "'sources' must be an iterable of strings or PathLike objects, not a string"
+                )
+
+            # now we check if it's iterable and contains valid types
+            try:
+                sources = list(map(os.fspath, sources))
+            except TypeError:
+                raise TypeError(
+                    "'sources' must be an iterable of strings or PathLike objects"
+                )
+
+            extra = {repr(k): kwargs.pop(k) for k in tuple(kwargs) if k not in _safe}
+            if extra:
+                warnings.warn(f"Unknown Extension options: {','.join(extra)}")
 
-    # When adding arguments to this constructor, be sure to update
-    # setup_keywords in core.py.
-    def __init__(
-        self,
-        name: str,
-        sources: Iterable[str | os.PathLike[str]],
-        include_dirs: list[str] | None = None,
-        define_macros: list[tuple[str, str | None]] | None = None,
-        undef_macros: list[str] | None = None,
-        library_dirs: list[str] | None = None,
-        libraries: list[str] | None = None,
-        runtime_library_dirs: list[str] | None = None,
-        extra_objects: list[str] | None = None,
-        extra_compile_args: list[str] | None = None,
-        extra_link_args: list[str] | None = None,
-        export_symbols: list[str] | None = None,
-        swig_opts: list[str] | None = None,
-        depends: list[str] | None = None,
-        language: str | None = None,
-        optional: bool | None = None,
-        **kw,  # To catch unknown keywords
-    ):
-        if not isinstance(name, str):
-            raise TypeError("'name' must be a string")
-
-        # handle the string case first; since strings are iterable, disallow them
-        if isinstance(sources, str):
-            raise TypeError(
-                "'sources' must be an iterable of strings or PathLike objects, not a string"
-            )
-
-        # now we check if it's iterable and contains valid types
-        try:
-            self.sources = list(map(os.fspath, sources))
-        except TypeError:
-            raise TypeError(
-                "'sources' must be an iterable of strings or PathLike objects"
-            )
-
-        self.name = name
-        self.include_dirs = include_dirs or []
-        self.define_macros = define_macros or []
-        self.undef_macros = undef_macros or []
-        self.library_dirs = library_dirs or []
-        self.libraries = libraries or []
-        self.runtime_library_dirs = runtime_library_dirs or []
-        self.extra_objects = extra_objects or []
-        self.extra_compile_args = extra_compile_args or []
-        self.extra_link_args = extra_link_args or []
-        self.export_symbols = export_symbols or []
-        self.swig_opts = swig_opts or []
-        self.depends = depends or []
-        self.language = language
-        self.optional = optional
-
-        # If there are unknown keyword options, warn about them
-        if len(kw) > 0:
-            options = [repr(option) for option in kw]
-            options = ', '.join(sorted(options))
-            msg = f"Unknown Extension options: {options}"
-            warnings.warn(msg)
-
-    def __repr__(self):
-        return f'<{self.__class__.__module__}.{self.__class__.__qualname__}({self.name!r}) at {id(self):#x}>'
+            # Ensure default values (e.g. []) are used instead of None:
+            positional = {k: v for k, v in zip(_safe[2:], args) if v is not None}
+            keywords = {k: v for k, v in kwargs.items() if v is not None}
+            super().__init__(name, sources, **positional, **keywords)
 
 
 def read_setup_file(filename):  # noqa: C901
diff --git a/distutils/tests/test_extension.py b/distutils/tests/test_extension.py
index 5e8e7682..17d52579 100644
--- a/distutils/tests/test_extension.py
+++ b/distutils/tests/test_extension.py
@@ -106,7 +106,7 @@ def test_extension_init(self):
             assert getattr(ext, attr) == []
 
         assert ext.language is None
-        assert ext.optional is None
+        assert ext.optional is False
 
         # if there are unknown keyword options, warn about them
         with check_warnings() as w:

From 37447a402c09c9acc6332057e60dd57c1389fa93 Mon Sep 17 00:00:00 2001
From: Anderson Bravalheri <andersonbravalheri@gmail.com>
Date: Fri, 11 Jul 2025 16:13:53 +0100
Subject: [PATCH 3/9] Separate __post_init__

---
 distutils/extension.py            | 65 ++++++++++++++++---------------
 distutils/tests/test_extension.py | 47 ++++++++++++++++++++--
 2 files changed, 76 insertions(+), 36 deletions(-)

diff --git a/distutils/extension.py b/distutils/extension.py
index b12a94a1..8a3b156b 100644
--- a/distutils/extension.py
+++ b/distutils/extension.py
@@ -31,9 +31,10 @@ class _Extension:
 
     # The use of a parent class as a "trick":
     # - We need to modify __init__ so to achieve backwards compatibility
+    #   and keep allowing arbitrary keywords to be ignored
     # - But we don't want to throw away the dataclass-generated __init__
-    # - We also want to fool the typechecker to consider the same type
-    #   signature as the dataclass-generated __init__
+    #   specially because we don't want to have to redefine all the typing
+    #   for the function arguments
 
     name: str
     """
@@ -139,42 +140,42 @@ class _Extension:
 _safe = tuple(f.name for f in fields(_Extension))
 
 
-if TYPE_CHECKING:
-
-    @dataclass
-    class Extension(_Extension):
-        pass
-
-else:
-
-    @dataclass(init=False)
-    class Extension(_Extension):
-        def __init__(self, name, sources, *args, **kwargs):
-            if not isinstance(name, str):
-                raise TypeError("'name' must be a string")
-
-            # handle the string case first; since strings are iterable, disallow them
-            if isinstance(sources, str):
-                raise TypeError(
-                    "'sources' must be an iterable of strings or PathLike objects, not a string"
-                )
-
-            # now we check if it's iterable and contains valid types
-            try:
-                sources = list(map(os.fspath, sources))
-            except TypeError:
-                raise TypeError(
-                    "'sources' must be an iterable of strings or PathLike objects"
-                )
+@dataclass(init=True if TYPE_CHECKING else False)  # type: ignore[literal-required]
+class Extension(_Extension):
+    if not TYPE_CHECKING:
 
+        def __init__(self, *args, **kwargs):
             extra = {repr(k): kwargs.pop(k) for k in tuple(kwargs) if k not in _safe}
             if extra:
-                warnings.warn(f"Unknown Extension options: {','.join(extra)}")
+                msg = f"""
+                Please remove unknown `Extension` options: {','.join(extra)}
+                this kind of usage is deprecated and may cause errors in the future.
+                """
+                warnings.warn(msg)
 
             # Ensure default values (e.g. []) are used instead of None:
-            positional = {k: v for k, v in zip(_safe[2:], args) if v is not None}
+            positional = {k: v for k, v in zip(_safe, args) if v is not None}
             keywords = {k: v for k, v in kwargs.items() if v is not None}
-            super().__init__(name, sources, **positional, **keywords)
+            super().__init__(**positional, **keywords)
+            self.__post_init__()  # does not seem to be called when customizing __init__
+
+    def __post_init__(self):
+        if not isinstance(self.name, str):
+            raise TypeError("'name' must be a string")
+
+        # handle the string case first; since strings are iterable, disallow them
+        if isinstance(self.sources, str):
+            raise TypeError(
+                "'sources' must be an iterable of strings or PathLike objects, not a string"
+            )
+
+        # now we check if it's iterable and contains valid types
+        try:
+            self.sources = list(map(os.fspath, self.sources))
+        except TypeError:
+            raise TypeError(
+                "'sources' must be an iterable of strings or PathLike objects"
+            )
 
 
 def read_setup_file(filename):  # noqa: C901
diff --git a/distutils/tests/test_extension.py b/distutils/tests/test_extension.py
index 17d52579..4042d547 100644
--- a/distutils/tests/test_extension.py
+++ b/distutils/tests/test_extension.py
@@ -2,11 +2,13 @@
 
 import os
 import pathlib
+import re
 import warnings
+from dataclasses import dataclass, field
 from distutils.extension import Extension, read_setup_file
+from typing import TYPE_CHECKING
 
 import pytest
-from test.support.warnings_helper import check_warnings
 
 
 class TestExtension:
@@ -109,9 +111,46 @@ def test_extension_init(self):
         assert ext.optional is False
 
         # if there are unknown keyword options, warn about them
-        with check_warnings() as w:
+        msg = re.escape("unknown `Extension` options: 'chic'")
+        with pytest.warns(UserWarning, match=msg) as w:
             warnings.simplefilter('always')
             ext = Extension('name', ['file1', 'file2'], chic=True)
 
-        assert len(w.warnings) == 1
-        assert str(w.warnings[0].message) == "Unknown Extension options: 'chic'"
+        assert len(w) == 1
+
+
+def test_can_be_extended_by_setuptools() -> None:
+    # Emulate how it could be extended in setuptools
+
+    @dataclass(init=True if TYPE_CHECKING else False)  # type: ignore[literal-required]
+    class setuptools_Extension(Extension):
+        py_limited_api: bool = False
+        _full_name: str = field(init=False, repr=False)
+
+        if not TYPE_CHECKING:
+            # Custom __init__ is only needed for backwards compatibility
+            # (to ignore arbitrary keywords)
+
+            def __init__(self, *args, py_limited_api=False, **kwargs):
+                self.py_limited_api = py_limited_api
+                super().__init__(*args, **kwargs)
+
+    ext1 = setuptools_Extension("name", ["hello.c"], py_limited_api=True)
+    assert ext1.py_limited_api is True
+    assert ext1.define_macros == []
+
+    msg = re.escape("unknown `Extension` options: 'world'")
+    with pytest.warns(UserWarning, match=msg):
+        ext2 = setuptools_Extension("name", ["hello.c"], world=True)  # type: ignore[call-arg]
+
+    assert "world" not in ext2.__dict__
+    assert ext2.py_limited_api is False
+
+    # Without __init__ customization the following warning would be an error:
+    msg = re.escape("unknown `Extension` options: '_full_name'")
+    with pytest.warns(UserWarning, match=msg):
+        ext3 = setuptools_Extension("name", ["hello.c"], _full_name="hello")  # type: ignore[call-arg]
+
+    assert "_full_name" not in ext3.__dict__
+    ext3._full_name = "hello world"  # can still be set in build_ext
+    assert ext3._full_name == "hello world"

From 0bb738e9e37ebd310808b6e91945043ab5e44ae8 Mon Sep 17 00:00:00 2001
From: Anderson Bravalheri <andersonbravalheri@gmail.com>
Date: Wed, 28 Jan 2026 17:16:41 +0000
Subject: [PATCH 4/9] Add compat.py310.dataclass_transform fallback

---
 distutils/compat/py310.py | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)
 create mode 100644 distutils/compat/py310.py

diff --git a/distutils/compat/py310.py b/distutils/compat/py310.py
new file mode 100644
index 00000000..f5865962
--- /dev/null
+++ b/distutils/compat/py310.py
@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+import sys
+from typing import TYPE_CHECKING, Any, Callable, TypeVar
+
+_T = TypeVar("_T")
+
+if sys.version_info >= (3, 11):
+    from typing import dataclass_transform
+else:
+    if TYPE_CHECKING:
+        # typing_extensions usually "exist" when type-checking,
+        # without the need for extra runtime dependencies
+        from typing_extensions import dataclass_transform
+    else:
+        # Runtime no-op
+        def dataclass_transform(  # type: ignore[misc]
+            *,
+            eq_default: bool | None = None,
+            order_default: bool | None = None,
+            kw_only_default: bool | None = None,
+            field_specifiers: tuple[type[Any], ...] = (),
+            **_: Any,
+        ) -> Callable[[_T], _T]:
+            def _decorator(obj: _T) -> _T:
+                return obj
+
+            return _decorator

From 15630428790f5ce2eb8e769d9ab28eabe2e6c9d7 Mon Sep 17 00:00:00 2001
From: Anderson Bravalheri <andersonbravalheri@gmail.com>
Date: Wed, 28 Jan 2026 17:17:21 +0000
Subject: [PATCH 5/9] Add a dataclass-like decorator that is lenient towards
 extra arguments

---
 distutils/_dataclass.py | 51 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)
 create mode 100644 distutils/_dataclass.py

diff --git a/distutils/_dataclass.py b/distutils/_dataclass.py
new file mode 100644
index 00000000..0f1d00fa
--- /dev/null
+++ b/distutils/_dataclass.py
@@ -0,0 +1,51 @@
+# This is a private module, but setuptools has the explicit permission to use it.
+from __future__ import annotations
+
+import warnings
+from dataclasses import dataclass, fields
+from functools import wraps
+from typing import TypeVar
+
+from .compat.py310 import dataclass_transform
+
+_T = TypeVar("_T", bound=type)
+
+
+@dataclass_transform()
+def lenient_dataclass(**dc_kwargs):
+    """
+    Problem this class intends to solve:
+    - We need to modify __init__ so to achieve backwards compatibility
+      and keep allowing arbitrary keywords to be ignored
+    - But we don't want to throw away the dataclass-generated __init__
+      specially because we don't want to have to redefine all the typing
+      for the function arguments
+    """
+
+    @wraps(dataclass)
+    def _wrap(cls: _T) -> _T:
+        cls = dataclass(**dc_kwargs)(cls)  # type: ignore[misc]
+        # Allowed field names in order
+        safe = tuple(f.name for f in fields(cls))
+        orig_init = cls.__init__
+
+        @wraps(orig_init)
+        def _wrapped_init(self, *args, **kwargs):
+            extra = {repr(k): kwargs.pop(k) for k in tuple(kwargs) if k not in safe}
+            if extra:
+                msg = f"""
+                Please remove unknown `{cls.__name__}` options: {','.join(extra)}
+                this kind of usage is deprecated and may cause errors in the future.
+                """
+                warnings.warn(msg)
+
+            # Ensure default values (e.g. []) are used instead of None:
+            positional = {k: v for k, v in zip(safe, args) if v is not None}
+            keywords = {k: v for k, v in kwargs.items() if v is not None}
+            orig_init(self, **positional, **keywords)
+            self.__post_init__()  # does not seem to be called when customizing __init__
+
+        cls.__init__ = _wrapped_init
+        return cls
+
+    return _wrap

From dab5c11f054ca01227821f6a40ec1ff3a1f563ac Mon Sep 17 00:00:00 2001
From: Anderson Bravalheri <andersonbravalheri@gmail.com>
Date: Wed, 28 Jan 2026 17:18:17 +0000
Subject: [PATCH 6/9] Use lenient_dataclass to simplify Extension class

---
 distutils/extension.py            | 45 +++++++------------------------
 distutils/tests/test_extension.py | 29 ++++++--------------
 2 files changed, 17 insertions(+), 57 deletions(-)

diff --git a/distutils/extension.py b/distutils/extension.py
index 8a3b156b..c9cf2b23 100644
--- a/distutils/extension.py
+++ b/distutils/extension.py
@@ -6,10 +6,10 @@
 from __future__ import annotations
 
 import os
-import warnings
 from collections.abc import Iterable
-from dataclasses import dataclass, field, fields
-from typing import TYPE_CHECKING
+from dataclasses import field, fields
+
+from ._dataclass import lenient_dataclass
 
 # This class is really only used by the "build_ext" command, so it might
 # make sense to put it in distutils.command.build_ext.  However, that
@@ -22,20 +22,13 @@
 # order to do anything.
 
 
-@dataclass
-class _Extension:
+@lenient_dataclass()
+class Extension:
     """Just a collection of attributes that describes an extension
     module and everything needed to build it (hopefully in a portable
     way, but there are hooks that let you be as unportable as you need).
     """
 
-    # The use of a parent class as a "trick":
-    # - We need to modify __init__ so to achieve backwards compatibility
-    #   and keep allowing arbitrary keywords to be ignored
-    # - But we don't want to throw away the dataclass-generated __init__
-    #   specially because we don't want to have to redefine all the typing
-    #   for the function arguments
-
     name: str
     """
     the full name of the extension, including any packages -- ie.
@@ -135,30 +128,6 @@ class _Extension:
     build process, but simply not install the failing extension.
     """
 
-
-# Legal keyword arguments for the Extension constructor
-_safe = tuple(f.name for f in fields(_Extension))
-
-
-@dataclass(init=True if TYPE_CHECKING else False)  # type: ignore[literal-required]
-class Extension(_Extension):
-    if not TYPE_CHECKING:
-
-        def __init__(self, *args, **kwargs):
-            extra = {repr(k): kwargs.pop(k) for k in tuple(kwargs) if k not in _safe}
-            if extra:
-                msg = f"""
-                Please remove unknown `Extension` options: {','.join(extra)}
-                this kind of usage is deprecated and may cause errors in the future.
-                """
-                warnings.warn(msg)
-
-            # Ensure default values (e.g. []) are used instead of None:
-            positional = {k: v for k, v in zip(_safe, args) if v is not None}
-            keywords = {k: v for k, v in kwargs.items() if v is not None}
-            super().__init__(**positional, **keywords)
-            self.__post_init__()  # does not seem to be called when customizing __init__
-
     def __post_init__(self):
         if not isinstance(self.name, str):
             raise TypeError("'name' must be a string")
@@ -178,6 +147,10 @@ def __post_init__(self):
             )
 
 
+# Legal keyword arguments for the Extension constructor
+_safe = tuple(f.name for f in fields(Extension))
+
+
 def read_setup_file(filename):  # noqa: C901
     """Reads a Setup file and returns Extension instances."""
     from distutils.sysconfig import _variable_rx, expand_makefile_vars, parse_makefile
diff --git a/distutils/tests/test_extension.py b/distutils/tests/test_extension.py
index 4042d547..804bb3a7 100644
--- a/distutils/tests/test_extension.py
+++ b/distutils/tests/test_extension.py
@@ -4,9 +4,9 @@
 import pathlib
 import re
 import warnings
-from dataclasses import dataclass, field
+from dataclasses import field
+from distutils._dataclass import lenient_dataclass
 from distutils.extension import Extension, read_setup_file
-from typing import TYPE_CHECKING
 
 import pytest
 
@@ -122,35 +122,22 @@ def test_extension_init(self):
 def test_can_be_extended_by_setuptools() -> None:
     # Emulate how it could be extended in setuptools
 
-    @dataclass(init=True if TYPE_CHECKING else False)  # type: ignore[literal-required]
+    @lenient_dataclass()
     class setuptools_Extension(Extension):
         py_limited_api: bool = False
         _full_name: str = field(init=False, repr=False)
 
-        if not TYPE_CHECKING:
-            # Custom __init__ is only needed for backwards compatibility
-            # (to ignore arbitrary keywords)
-
-            def __init__(self, *args, py_limited_api=False, **kwargs):
-                self.py_limited_api = py_limited_api
-                super().__init__(*args, **kwargs)
-
     ext1 = setuptools_Extension("name", ["hello.c"], py_limited_api=True)
     assert ext1.py_limited_api is True
     assert ext1.define_macros == []
 
-    msg = re.escape("unknown `Extension` options: 'world'")
+    # Without __init__ customization the following warning would be an error:
+    msg = re.escape("unknown `setuptools_Extension` options: 'world'")
     with pytest.warns(UserWarning, match=msg):
         ext2 = setuptools_Extension("name", ["hello.c"], world=True)  # type: ignore[call-arg]
 
     assert "world" not in ext2.__dict__
     assert ext2.py_limited_api is False
-
-    # Without __init__ customization the following warning would be an error:
-    msg = re.escape("unknown `Extension` options: '_full_name'")
-    with pytest.warns(UserWarning, match=msg):
-        ext3 = setuptools_Extension("name", ["hello.c"], _full_name="hello")  # type: ignore[call-arg]
-
-    assert "_full_name" not in ext3.__dict__
-    ext3._full_name = "hello world"  # can still be set in build_ext
-    assert ext3._full_name == "hello world"
+    assert "_full_name" not in ext2.__dict__  # not initialized by default
+    ext2._full_name = "hello world"  # can still be set in build_ext
+    assert ext2._full_name == "hello world"

From e5c817be7ba4af4c2cadc08c636d95252e42fb43 Mon Sep 17 00:00:00 2001
From: Anderson Bravalheri <andersonbravalheri@gmail.com>
Date: Wed, 28 Jan 2026 17:29:07 +0000
Subject: [PATCH 7/9] Remove the now unecessary explicit call to __post_init__

---
 distutils/_dataclass.py           | 5 ++---
 distutils/tests/test_extension.py | 4 ++--
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/distutils/_dataclass.py b/distutils/_dataclass.py
index 0f1d00fa..b9aeb988 100644
--- a/distutils/_dataclass.py
+++ b/distutils/_dataclass.py
@@ -24,7 +24,7 @@ def lenient_dataclass(**dc_kwargs):
 
     @wraps(dataclass)
     def _wrap(cls: _T) -> _T:
-        cls = dataclass(**dc_kwargs)(cls)  # type: ignore[misc]
+        cls = dataclass(**dc_kwargs)(cls)
         # Allowed field names in order
         safe = tuple(f.name for f in fields(cls))
         orig_init = cls.__init__
@@ -42,8 +42,7 @@ def _wrapped_init(self, *args, **kwargs):
             # Ensure default values (e.g. []) are used instead of None:
             positional = {k: v for k, v in zip(safe, args) if v is not None}
             keywords = {k: v for k, v in kwargs.items() if v is not None}
-            orig_init(self, **positional, **keywords)
-            self.__post_init__()  # does not seem to be called when customizing __init__
+            return orig_init(self, **positional, **keywords)
 
         cls.__init__ = _wrapped_init
         return cls
diff --git a/distutils/tests/test_extension.py b/distutils/tests/test_extension.py
index 804bb3a7..b0868e14 100644
--- a/distutils/tests/test_extension.py
+++ b/distutils/tests/test_extension.py
@@ -64,14 +64,14 @@ def test_read_setup_file(self):
 
     def test_extension_init(self):
         # the first argument, which is the name, must be a string
-        with pytest.raises(TypeError):
+        with pytest.raises(TypeError, match="'name' must be a string"):
             Extension(1, [])
         ext = Extension('name', [])
         assert ext.name == 'name'
 
         # the second argument, which is the list of files, must
         # be an iterable of strings or PathLike objects, and not a string
-        with pytest.raises(TypeError):
+        with pytest.raises(TypeError, match="'sources' must be an iterable"):
             Extension('name', 'file')
         with pytest.raises(TypeError):
             Extension('name', ['file', 1])

From a24af9205a932a5cad618bf1ea5374ff2f7115b0 Mon Sep 17 00:00:00 2001
From: Anderson Bravalheri <andersonbravalheri+github@gmail.com>
Date: Wed, 28 Jan 2026 17:37:13 +0000
Subject: [PATCH 8/9] Add comment about what to do when backward compatibility
 is not needed.

---
 distutils/_dataclass.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/distutils/_dataclass.py b/distutils/_dataclass.py
index b9aeb988..6a870d60 100644
--- a/distutils/_dataclass.py
+++ b/distutils/_dataclass.py
@@ -20,6 +20,10 @@ def lenient_dataclass(**dc_kwargs):
     - But we don't want to throw away the dataclass-generated __init__
       specially because we don't want to have to redefine all the typing
       for the function arguments
+
+    If/when lenient behaviour and backward compatibility are no longer needed,
+    the whole customization can be removed.
+    A regular ``dataclass`` could be used instead.
     """
 
     @wraps(dataclass)

From 7a083daba92a7c5cbb35615036c9f21b0a8b1ffd Mon Sep 17 00:00:00 2001
From: Anderson Bravalheri <andersonbravalheri@gmail.com>
Date: Thu, 29 Jan 2026 00:50:23 +0000
Subject: [PATCH 9/9] Add sanity check for type inference on Extension

---
 distutils/tests/test_extension.py | 59 +++++++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)

diff --git a/distutils/tests/test_extension.py b/distutils/tests/test_extension.py
index b0868e14..44ce3118 100644
--- a/distutils/tests/test_extension.py
+++ b/distutils/tests/test_extension.py
@@ -1,5 +1,7 @@
 """Tests for distutils.extension."""
 
+from __future__ import annotations
+
 import os
 import pathlib
 import re
@@ -7,6 +9,7 @@
 from dataclasses import field
 from distutils._dataclass import lenient_dataclass
 from distutils.extension import Extension, read_setup_file
+from inspect import cleandoc
 
 import pytest
 
@@ -141,3 +144,59 @@ class setuptools_Extension(Extension):
     assert "_full_name" not in ext2.__dict__  # not initialized by default
     ext2._full_name = "hello world"  # can still be set in build_ext
     assert ext2._full_name == "hello world"
+
+
+TYPE_INFERENCE = {
+    # Simple example
+    """
+    from distutils.extension import Extension
+
+    reveal_type(Extension.__init__)
+    """: [
+        "name: builtins.str",
+        "sources: typing.Iterable[builtins.str | os.PathLike[builtins.str]]",
+        "include_dirs: builtins.list[builtins.str]",
+    ],
+    # Inheritance example
+    """
+    from dataclasses import field
+    from distutils._dataclass import lenient_dataclass
+    from distutils.extension import Extension
+
+    @lenient_dataclass()
+    class setuptools_Extension(Extension):
+        py_limited_api: bool = False
+        _full_name: str = field(init=False, repr=False)
+
+    reveal_type(setuptools_Extension.__init__)
+    """: [
+        "libraries: builtins.list[builtins.str]",
+        "swig_opts: builtins.list[builtins.str]",
+        "py_limited_api: builtins.bool",
+        "_full_name: builtins.str",
+    ],
+}
+
+
+@pytest.mark.filterwarnings("ignore::EncodingWarning")  # mypy.api.run
+@pytest.mark.parametrize("example,expectations", TYPE_INFERENCE.items())
+def test_inference_sanity_check(
+    tmp_path: pathlib.Path, example: str, expectations: list[str]
+) -> None:
+    """Ensure type inference is working well for Extension and subclasses"""
+    from mypy import api
+
+    f = tmp_path / "typecheck_file.py"
+    f.write_text(cleandoc(example), encoding="utf-8")
+
+    # Use an empty config file to avoid interference with test
+    empty = tmp_path / "empty"
+    empty.touch()
+    result = api.run([os.fspath(f), "--config-file", os.fspath(empty)])
+
+    separator = 'note: Revealed type is "def (self:'
+    assert separator in result[0]
+    _, _, note = result[0].partition(separator)
+
+    for expectation in expectations:
+        assert expectation in note