gh-74044: inspect.signature for wrappers around decorated bound methods (GH-736)

(cherry picked from commit dbf2faf)

Co-authored-by: Anton Ryzhov <[email protected]>

Author: miss-islington

Lib/inspect.py

@@ -2457,7 +2457,10 @@ def _signature_from_callable(obj, *,
 
     # Was this function wrapped by a decorator?
     if follow_wrapper_chains:
-        obj = unwrap(obj, stop=(lambda f: hasattr(f, "__signature__")))
+        # Unwrap until we find an explicit signature or a MethodType (which will be
+        # handled explicitly below).
+        obj = unwrap(obj, stop=(lambda f: hasattr(f, "__signature__")
+                                or isinstance(f, types.MethodType)))
         if isinstance(obj, types.MethodType):
             # If the unwrapped object is a *method*, we might want to
             # skip its first parameter (self).

Lib/test/test_inspect.py

@@ -2960,8 +2960,6 @@ def foo(a): pass
         self.assertEqual(str(inspect.signature(foo)), '(a)')
 
     def test_signature_on_decorated(self):
-        import functools
-
         def decorator(func):
             @functools.wraps(func)
             def wrapper(*args, **kwargs) -> int:
@@ -2973,6 +2971,8 @@ class Foo:
             def bar(self, a, b):
                 pass
 
+        bar = decorator(Foo().bar)
+
         self.assertEqual(self.signature(Foo.bar),
                          ((('self', ..., ..., "positional_or_keyword"),
                            ('a', ..., ..., "positional_or_keyword"),
@@ -2991,6 +2991,11 @@ def bar(self, a, b):
                                 # from "func" to "wrapper", hence no
                                 # return_annotation
 
+        self.assertEqual(self.signature(bar),
+                         ((('a', ..., ..., "positional_or_keyword"),
+                           ('b', ..., ..., "positional_or_keyword")),
+                          ...))
+
         # Test that we handle method wrappers correctly
         def decorator(func):
             @functools.wraps(func)

Misc/NEWS.d/next/Library/2022-11-09-20-48-42.gh-issue-74044.zBj26K.rst

@@ -0,0 +1 @@
+Fixed bug where :func:`inspect.signature` reported incorrect arguments for decorated methods.