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 5b9ff7a) Co-authored-by: Gregory P. Smith <greg@krypto.org>

Author: 2 people

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 @@ 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():

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.