[Python-checkins] bpo-34706: Preserve subclassing in inspect.Signature.from_callable (GH-16108)

[Gregory P. Smith]

https://github.com/python/cpython/commit/5b9ff7a0dcb16d6f5c3cd4f1f52e0ca6a4bde586
commit: 5b9ff7a0dcb16d6f5c3cd4f1f52e0ca6a4bde586
branch: master
author: Gregory P. Smith <greg at krypto.org>
committer: GitHub <noreply at github.com>
date: 2019-09-13T17:13:51+01:00
summary:

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.

files:
A Misc/NEWS.d/next/Library/2019-09-13-14-54-33.bpo-34706.HWVpOY.rst
M Lib/inspect.py
M Lib/test/test_inspect.py

diff --git a/Lib/inspect.py b/Lib/inspect.py
index 0a57749ccdd4..c2a1ed4148e7 100644
--- a/Lib/inspect.py
+++ b/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))
diff --git a/Lib/test/test_inspect.py b/Lib/test/test_inspect.py
index 6c9f52977807..7148dfa0bab9 100644
--- a/Lib/test/test_inspect.py
+++ b/Lib/test/test_inspect.py
@@ -3153,14 +3153,21 @@ def test_signature_from_callable_python_obj(self):
         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():
diff --git a/Misc/NEWS.d/next/Library/2019-09-13-14-54-33.bpo-34706.HWVpOY.rst b/Misc/NEWS.d/next/Library/2019-09-13-14-54-33.bpo-34706.HWVpOY.rst
new file mode 100644
index 000000000000..1df3742abd27
--- /dev/null
+++ b/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.