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

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.

1 files changed, 9 insertions, 2 deletions

diff --git a/Lib/test/test_inspect.py b/Lib/test/test_inspect.py
index 6c9f529..7148dfa 100644
--- a/Lib/test/test_inspect.py
+++ b/Lib/test/test_inspect.py
@@ -3153,14 +3153,21 @@ 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():