bpo-34706: Preserve subclassing in inspect.Signature.from_callable (GH-16108) (GH-16113)

https://bugs.python.org/issue34706

Specifically in the case of a class that does not override its
constructor signature inherited from object.

These are Buck Evan @bukzor's changes cherrypicked from GH-9344.
(cherry picked from commit 5b9ff7a0dc)

Co-authored-by: Gregory P. Smith <greg@krypto.org>

Lib/inspect.py

@@ -2367,7 +2367,7 @@ def _signature_from_callable(obj, *,
                 if (obj.__init__ is object.__init__ and
                     obj.__new__ is object.__new__):
                     # Return a signature of 'object' builtin.
-                    return signature(object)
+                    return sigcls.from_callable(object)
                 else:
                     raise ValueError(
                         'no signature found for builtin type {!r}'.format(obj))

Lib/test/test_inspect.py

@@ -3157,14 +3157,21 @@ class TestSignatureObject(unittest.TestCase):
         class MySignature(inspect.Signature): pass
         def foo(a, *, b:1): pass
         foo_sig = MySignature.from_callable(foo)
-        self.assertTrue(isinstance(foo_sig, MySignature))
+        self.assertIsInstance(foo_sig, MySignature)
+
+    def test_signature_from_callable_class(self):
+        # A regression test for a class inheriting its signature from `object`.
+        class MySignature(inspect.Signature): pass
+        class foo: pass
+        foo_sig = MySignature.from_callable(foo)
+        self.assertIsInstance(foo_sig, MySignature)
 
     @unittest.skipIf(MISSING_C_DOCSTRINGS,
                      "Signature information for builtins requires docstrings")
     def test_signature_from_callable_builtin_obj(self):
         class MySignature(inspect.Signature): pass
         sig = MySignature.from_callable(_pickle.Pickler)
-        self.assertTrue(isinstance(sig, MySignature))
+        self.assertIsInstance(sig, MySignature)
 
     def test_signature_definition_order_preserved_on_kwonly(self):
         for fn in signatures_with_lexicographic_keyword_only_parameters():

Misc/NEWS.d/next/Library/2019-09-13-14-54-33.bpo-34706.HWVpOY.rst

@@ -0,0 +1 @@
+Preserve subclassing in inspect.Signature.from_callable.