Issue #23934: Fix inspect.signature to fail correctly for builtin types.

Initial patch by James Powell.

Lib/inspect.py

@@ -2255,9 +2255,13 @@ def _signature_from_callable(obj, *,
             if type not in obj.__mro__:
                 # We have a class (not metaclass), but no user-defined
                 # __init__ or __new__ for it
-                if obj.__init__ is object.__init__:
+                if (obj.__init__ is object.__init__ and
+                    obj.__new__ is object.__new__):
                     # Return a signature of 'object' builtin.
                     return signature(object)
+                else:
+                    raise ValueError(
+                        'no signature found for builtin type {!r}'.format(obj))
 
     elif not isinstance(obj, _NonUserDefinedCallables):
         # An object with __call__

Lib/test/test_inspect.py

@@ -1980,9 +1980,14 @@ class TestSignatureObject(unittest.TestCase):
     @cpython_only
     def test_signature_on_builtins_no_signature(self):
         import _testcapi
-        with self.assertRaisesRegex(ValueError, 'no signature found for builtin'):
+        with self.assertRaisesRegex(ValueError,
+                                    'no signature found for builtin'):
             inspect.signature(_testcapi.docstring_no_signature)
 
+        with self.assertRaisesRegex(ValueError,
+                                    'no signature found for builtin'):
+            inspect.signature(str)
+
     def test_signature_on_non_function(self):
         with self.assertRaisesRegex(TypeError, 'is not a callable object'):
             inspect.signature(42)

Misc/NEWS

@@ -31,6 +31,9 @@ Library
 
 - Issue #16991: Add a C implementation of OrderedDict.
 
+- Issue #23934: Fix inspect.signature to fail correctly for builtin types
+  lacking signature information.  Initial patch by James Powell.
+
 
 What's New in Python 3.5.0 beta 1?
 ==================================