gh-137317: fix inspect signature of class with descriptor wrapper

Lib/inspect.py

@@ -1917,17 +1917,21 @@ def _signature_get_user_defined_method(cls, method_name, *, follow_wrapper_chain
     if meth is None:
         return None
 
+    # NOTE: The meth may wraps a non-user-defined callable.
+    # In this case, we treat the meth as non-user-defined callable too.
+    # (e.g. cls.__new__ generated by @warnings.deprecated)
+    unwrapped_meth = None
     if follow_wrapper_chains:
-        meth = unwrap(meth, stop=(lambda m: hasattr(m, "__signature__")
+        unwrapped_meth = unwrap(meth, stop=(lambda m: hasattr(m, "__signature__")
                                   or _signature_is_builtin(m)))
-    if isinstance(meth, _NonUserDefinedCallables):
+
+    if (isinstance(meth, _NonUserDefinedCallables)
+          or isinstance(unwrapped_meth, _NonUserDefinedCallables)):
         # Once '__signature__' will be added to 'C'-level
         # callables, this check won't be necessary
         return None
     if method_name != '__new__':
         meth = _descriptor_get(meth, cls)
-        if follow_wrapper_chains:
-            meth = unwrap(meth, stop=lambda m: hasattr(m, "__signature__"))
     return meth
 
 

Lib/test/test_inspect/test_inspect.py

@@ -148,6 +148,21 @@ def meth_self_o(self, object, /): pass
 def meth_type_noargs(type, /): pass
 def meth_type_o(type, object, /): pass
 
+# Simple decorator for test cases that using decorated functions
+def identity(func):
+    @functools.wraps(func)
+    def wrapped(*args, **kwargs):
+        return func(*args, **kwargs)
+    return wrapped
+
+# Simple descriptor that decorates a function
+class DescWithDeco:
+    def __init__(self, func):
+        self.func = identity(func)
+
+    def __get__(self, instance, owner):
+        return self.func.__get__(instance, owner)
+
 
 class TestPredicates(IsTestBase):
 
@@ -4027,44 +4042,201 @@ def __init__(self, b):
                            ('bar', 2, ..., "keyword_only")),
                           ...))
 
+    def test_signature_on_class_with_decorated_init(self):
+        class C:
+            @identity
+            def __init__(self, b):
+                pass
+
+        C(1)  # does not raise
+        self.assertEqual(self.signature(C),
+                        ((('b', ..., ..., "positional_or_keyword"),),
+                        ...))
+
+        with self.subTest('classmethod'):
+            class C:
+                @classmethod
+                @identity
+                def __init__(cls, b):
+                    pass
+
+            C(1)  # does not raise
+            self.assertEqual(self.signature(C),
+                            ((('b', ..., ..., "positional_or_keyword"),),
+                            ...))
+
+        with self.subTest('staticmethod'):
+            class C:
+                @staticmethod
+                @identity
+                def __init__(b):
+                    pass
+
+            C(1)  # does not raise
+            self.assertEqual(self.signature(C),
+                            ((('b', ..., ..., "positional_or_keyword"),),
+                            ...))
+
+        with self.subTest('MethodType'):
+            class A:
+                @identity
+                def call(self, a):
+                    pass
+
+            class C:
+                __init__ = A().call
+
+            C(1)  # does not raise
+            self.assertEqual(self.signature(C),
+                            ((('a', ..., ..., "positional_or_keyword"),),
+                            ...))
+
+        with self.subTest('partial'):
+            class C:
+                __init__ = functools.partial(identity(lambda x, a, b: None), 2)
+
+            C(1)  # does not raise
+            self.assertEqual(self.signature(C),
+                            ((('b', ..., ..., "positional_or_keyword"),),
+                            ...))
+
+        with self.subTest('partialmethod'):
+            class C:
+                @identity
+                def _init(self, x, a):
+                    self.a = (x, a)
+                __init__ = functools.partialmethod(_init, 2)
+
+            self.assertEqual(C(1).a, (2, 1))
+            self.assertEqual(self.signature(C),
+                            ((('a', ..., ..., "positional_or_keyword"),),
+                            ...))
+
+        with self.subTest('descriptor'):
+            class C:
+                @DescWithDeco
+                def __init__(self, a):
+                    pass
+
+            C(1)  # does not raise
+            self.assertEqual(self.signature(C),
+                            ((('a', ..., ..., "positional_or_keyword"),),
+                            ...))
+
+            self.assertEqual(self.signature(C.__init__),
+                            ((('self', ..., ..., "positional_or_keyword"),
+                            ('a', ..., ..., "positional_or_keyword")),
+                            ...))
+
+            varargs_signature = (
+                (('args', ..., ..., 'var_positional'),
+                ('kwargs', ..., ..., 'var_keyword')),
+                ...,
+            )
+            self.assertEqual(self.signature(C, follow_wrapped=False),
+                             varargs_signature)
+            self.assertEqual(self.signature(C.__new__, follow_wrapped=False),
+                             varargs_signature)
+
+
     def test_signature_on_class_with_decorated_new(self):
         def identity(func):
             @functools.wraps(func)
             def wrapped(*args, **kwargs):
                 return func(*args, **kwargs)
             return wrapped
 
-        class Foo:
-            @identity
-            def __new__(cls, a, b):
-                pass
+        with self.subTest('FunctionType'):
+            class C:
+                @identity
+                def __new__(cls, a):
+                    return a
 
-        self.assertEqual(self.signature(Foo),
-                         ((('a', ..., ..., "positional_or_keyword"),
-                           ('b', ..., ..., "positional_or_keyword")),
-                          ...))
+            self.assertEqual(C(1), 1)
+            self.assertEqual(self.signature(C),
+                            ((('a', ..., ..., "positional_or_keyword"),),
+                            ...))
 
-        self.assertEqual(self.signature(Foo.__new__),
-                         ((('cls', ..., ..., "positional_or_keyword"),
-                           ('a', ..., ..., "positional_or_keyword"),
-                           ('b', ..., ..., "positional_or_keyword")),
-                          ...))
+        with self.subTest('classmethod'):
+            class C:
+                @classmethod
+                @identity
+                def __new__(cls, cls2, a):
+                    return a
 
-        class Bar:
-            __new__ = identity(object.__new__)
+            self.assertEqual(C(1), 1)
+            self.assertEqual(self.signature(C),
+                            ((('a', ..., ..., "positional_or_keyword"),),
+                            ...))
 
-        varargs_signature = (
-            (('args', ..., ..., 'var_positional'),
-             ('kwargs', ..., ..., 'var_keyword')),
-            ...,
-        )
+        with self.subTest('staticmethod'):
+            class C:
+                @staticmethod
+                @identity
+                def __new__(cls, a):
+                    return a
+
+            self.assertEqual(C(1), 1)
+            self.assertEqual(self.signature(C),
+                            ((('a', ..., ..., "positional_or_keyword"),),
+                            ...))
 
-        self.assertEqual(self.signature(Bar), ((), ...))
-        self.assertEqual(self.signature(Bar.__new__), varargs_signature)
-        self.assertEqual(self.signature(Bar, follow_wrapped=False),
-                         varargs_signature)
-        self.assertEqual(self.signature(Bar.__new__, follow_wrapped=False),
-                         varargs_signature)
+        with self.subTest('MethodType'):
+            class A:
+                @identity
+                def call(self, cls, a):
+                    return a
+            class C:
+                __new__ = A().call
+
+            self.assertEqual(C(1), 1)
+            self.assertEqual(self.signature(C),
+                            ((('a', ..., ..., "positional_or_keyword"),),
+                            ...))
+
+        with self.subTest('partial'):
+            class C:
+                __new__ = functools.partial(identity(lambda x, cls, a: (x, a)), 2)
+
+            self.assertEqual(C(1), (2, 1))
+            self.assertEqual(self.signature(C),
+                            ((('a', ..., ..., "positional_or_keyword"),),
+                            ...))
+
+        with self.subTest('partialmethod'):
+            class C:
+                __new__ = functools.partialmethod(identity(lambda cls, x, a: (x, a)), 2)
+
+            self.assertEqual(C(1), (2, 1))
+            self.assertEqual(self.signature(C),
+                            ((('a', ..., ..., "positional_or_keyword"),),
+                            ...))
+
+        with self.subTest('descriptor'):
+            class C:
+                @DescWithDeco
+                def __new__(cls, a):
+                    return a
+
+            self.assertEqual(C(1), 1)
+            self.assertEqual(self.signature(C),
+                            ((('a', ..., ..., "positional_or_keyword"),),
+                            ...))
+
+            self.assertEqual(self.signature(C.__new__),
+                            ((('cls', ..., ..., "positional_or_keyword"),
+                            ('a', ..., ..., "positional_or_keyword")),
+                            ...))
+
+            varargs_signature = (
+                (('args', ..., ..., 'var_positional'),
+                ('kwargs', ..., ..., 'var_keyword')),
+                ...,
+            )
+            self.assertEqual(self.signature(C, follow_wrapped=False),
+                             varargs_signature)
+            self.assertEqual(self.signature(C.__new__, follow_wrapped=False),
+                             varargs_signature)
 
     def test_signature_on_class_with_init(self):
         class C:

Misc/NEWS.d/next/Library/2025-08-16-16-04-15.gh-issue-137317.Dl13B5.rst

@@ -0,0 +1,3 @@
+:func:`inspect.signature` now correctly handles classes that use a descriptor
+on a wrapped :meth:`!__init__` or :meth:`!__new__` method.
+Contributed by Yongyu Yan.